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 <div class="section" id="logging-cookbook">
<h1>Logging Cookbook<a class="headerlink" href="#logging-cookbook" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">Vinay Sajip &lt;vinay_sajip at red-dove dot com&gt;</td>
</tr>
</tbody>
</table>
This page contains a number of recipes related to logging, which have been found
useful in the past.
<div class="section" id="using-logging-in-multiple-modules">
<h2>Using logging in multiple modules<a class="headerlink" href="#using-logging-in-multiple-modules" title="Permalink to this headline">¶</a></h2>
Multiple calls to <code class="docutils literal notranslate">logging.getLogger('someLogger')</code> return a reference to the
same logger object. This is true not only within the same module, but also
across modules as long as it is in the same Python interpreter process. It is
true for references to the same object; additionally, application code can
define and configure a parent logger in one module and create (but not
configure) a child logger in a separate module, and all logger calls to the
child will pass up to the parent. Here is a main module:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import auxiliary_module

# create logger with &#39;spam_application&#39;
logger = logging.getLogger(&#39;spam_application&#39;)
logger.setLevel(logging.DEBUG)
# create file handler which logs even debug messages
fh = logging.FileHandler(&#39;spam.log&#39;)
fh.setLevel(logging.DEBUG)
# create console handler with a higher log level
ch = logging.StreamHandler()
ch.setLevel(logging.ERROR)
# create formatter and add it to the handlers
formatter = logging.Formatter(&#39;%(asctime)s - %(name)s - %(levelname)s - %(message)s&#39;)
fh.setFormatter(formatter)
ch.setFormatter(formatter)
# add the handlers to the logger
logger.addHandler(fh)
logger.addHandler(ch)

logger.info(&#39;creating an instance of auxiliary_module.Auxiliary&#39;)
a = auxiliary_module.Auxiliary()
logger.info(&#39;created an instance of auxiliary_module.Auxiliary&#39;)
logger.info(&#39;calling auxiliary_module.Auxiliary.do_something&#39;)
a.do_something()
logger.info(&#39;finished auxiliary_module.Auxiliary.do_something&#39;)
logger.info(&#39;calling auxiliary_module.some_function()&#39;)
auxiliary_module.some_function()
logger.info(&#39;done with auxiliary_module.some_function()&#39;)
</pre></div>
</div>
Here is the auxiliary module:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging

# create logger
module_logger = logging.getLogger(&#39;spam_application.auxiliary&#39;)

class Auxiliary:
 def __init__(self):
 self.logger = logging.getLogger(&#39;spam_application.auxiliary.Auxiliary&#39;)
 self.logger.info(&#39;creating an instance of Auxiliary&#39;)

def do_something(self):
 self.logger.info(&#39;doing something&#39;)
 a = 1 + 1
 self.logger.info(&#39;done doing something&#39;)

def some_function():
 module_logger.info(&#39;received a call to &quot;some_function&quot;&#39;)
</pre></div>
</div>
The output looks like this:
<div class="highlight-default notranslate"><div class="highlight"><pre>2005-03-23 23:47:11,663 - spam_application - INFO -
 creating an instance of auxiliary_module.Auxiliary
2005-03-23 23:47:11,665 - spam_application.auxiliary.Auxiliary - INFO -
 creating an instance of Auxiliary
2005-03-23 23:47:11,665 - spam_application - INFO -
 created an instance of auxiliary_module.Auxiliary
2005-03-23 23:47:11,668 - spam_application - INFO -
 calling auxiliary_module.Auxiliary.do_something
2005-03-23 23:47:11,668 - spam_application.auxiliary.Auxiliary - INFO -
 doing something
2005-03-23 23:47:11,669 - spam_application.auxiliary.Auxiliary - INFO -
 done doing something
2005-03-23 23:47:11,670 - spam_application - INFO -
 finished auxiliary_module.Auxiliary.do_something
2005-03-23 23:47:11,671 - spam_application - INFO -
 calling auxiliary_module.some_function()
2005-03-23 23:47:11,672 - spam_application.auxiliary - INFO -
 received a call to &#39;some_function&#39;
2005-03-23 23:47:11,673 - spam_application - INFO -
 done with auxiliary_module.some_function()
</pre></div>
</div>
</div>
<div class="section" id="logging-from-multiple-threads">
<h2>Logging from multiple threads<a class="headerlink" href="#logging-from-multiple-threads" title="Permalink to this headline">¶</a></h2>
Logging from multiple threads requires no special effort. The following example
shows logging from the main (initIal) thread and another thread:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import threading
import time

def worker(arg):
 while not arg[&#39;stop&#39;]:
 logging.debug(&#39;Hi from myfunc&#39;)
 time.sleep(0.5)

def main():
 logging.basicConfig(level=logging.DEBUG, format=&#39;%(relativeCreated)6d %(threadName)s %(message)s&#39;)
 info = {&#39;stop&#39;: False}
 thread = threading.Thread(target=worker, args=(info,))
 thread.start()
 while True:
 try:
 logging.debug(&#39;Hello from main&#39;)
 time.sleep(0.75)
 except KeyboardInterrupt:
 info[&#39;stop&#39;] = True
 break
 thread.join()

logger = logging.getLogger(&#39;simple_example&#39;)
logger.setLevel(logging.DEBUG)
# create file handler which logs even debug messages
fh = logging.FileHandler(&#39;spam.log&#39;)
fh.setLevel(logging.DEBUG)
# create console handler with a higher log level
ch = logging.StreamHandler()
ch.setLevel(logging.ERROR)
# create formatter and add it to the handlers
formatter = logging.Formatter(&#39;%(asctime)s - %(name)s - %(levelname)s - %(message)s&#39;)
ch.setFormatter(formatter)
fh.setFormatter(formatter)
# add the handlers to logger
logger.addHandler(ch)
logger.addHandler(fh)

# &#39;application&#39; code
logger.debug(&#39;debug message&#39;)
logger.info(&#39;info message&#39;)
logger.warn(&#39;warn message&#39;)
logger.error(&#39;error message&#39;)
logger.critical(&#39;critical message&#39;)
</pre></div>
</div>
Notice that the ‘application’ code does not care about multiple handlers. All
that changed was the addition and configuration of a new handler named fh.
The ability to create new handlers with higher- or lower-severity filters can be
very helpful when writing and testing an application. Instead of using many
<code class="docutils literal notranslate">print</code> statements for debugging, use <code class="docutils literal notranslate">logger.debug</code>: Unlike the print
statements, which you will have to delete or comment out later, the logger.debug
statements can remain intact in the source code and remain dormant until you
need them again. At that time, the only change that needs to happen is to
modify the severity level of the logger and/or handler to debug.
</div>
<div class="section" id="logging-to-multiple-destinations">
<h2>Logging to multiple destinations<a class="headerlink" href="#logging-to-multiple-destinations" title="Permalink to this headline">¶</a></h2>
Let’s say you want to log to console and file with different message formats and
in differing circumstances. Say you want to log messages with levels of DEBUG
and higher to file, and those messages at level INFO and higher to the console.
Let’s also assume that the file should contain timestamps, but the console
messages should not. Here’s how you can achieve this:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging

# set up logging to file - see previous section for more details
logging.basicConfig(level=logging.DEBUG,
 format=&#39;%(asctime)s %(name)-12s %(levelname)-8s %(message)s&#39;,
 datefmt=&#39;%m-%d %H:%M&#39;,
 filename=&#39;/temp/myapp.log&#39;,
 filemode=&#39;w&#39;)
# define a Handler which writes INFO messages or higher to the sys.stderr
console = logging.StreamHandler()
console.setLevel(logging.INFO)
# set a format which is simpler for console use
formatter = logging.Formatter(&#39;%(name)-12s: %(levelname)-8s %(message)s&#39;)
# tell the handler to use this format
console.setFormatter(formatter)
# add the handler to the root logger
logging.getLogger(&#39;&#39;).addHandler(console)

# Now, we can log to the root logger, or any other logger. First the root...
logging.info(&#39;Jackdaws love my big sphinx of quartz.&#39;)

# Now, define a couple of other loggers which might represent areas in your
# application:

logger1 = logging.getLogger(&#39;myapp.area1&#39;)
logger2 = logging.getLogger(&#39;myapp.area2&#39;)

logger1.debug(&#39;Quick zephyrs blow, vexing daft Jim.&#39;)
logger1.info(&#39;How quickly daft jumping zebras vex.&#39;)
logger2.warning(&#39;Jail zesty vixen who grabbed pay from quack.&#39;)
logger2.error(&#39;The five boxing wizards jump quickly.&#39;)
</pre></div>
</div>
When you run this, on the console you will see
<div class="highlight-default notranslate"><div class="highlight"><pre>root : INFO Jackdaws love my big sphinx of quartz.
myapp.area1 : INFO How quickly daft jumping zebras vex.
myapp.area2 : WARNING Jail zesty vixen who grabbed pay from quack.
myapp.area2 : ERROR The five boxing wizards jump quickly.
</pre></div>
</div>
and in the file you will see something like
<div class="highlight-default notranslate"><div class="highlight"><pre>10-22 22:19 root INFO Jackdaws love my big sphinx of quartz.
10-22 22:19 myapp.area1 DEBUG Quick zephyrs blow, vexing daft Jim.
10-22 22:19 myapp.area1 INFO How quickly daft jumping zebras vex.
10-22 22:19 myapp.area2 WARNING Jail zesty vixen who grabbed pay from quack.
10-22 22:19 myapp.area2 ERROR The five boxing wizards jump quickly.
</pre></div>
</div>
As you can see, the DEBUG message only shows up in the file. The other messages
are sent to both destinations.
This example uses console and file handlers, but you can use any number and
combination of handlers you choose.
</div>
<div class="section" id="configuration-server-example">
<h2>Configuration server example<a class="headerlink" href="#configuration-server-example" title="Permalink to this headline">¶</a></h2>
Here is an example of a module using the logging configuration server:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import logging.config
import time
import os

# read initial config file
logging.config.fileConfig(&#39;logging.conf&#39;)

# create and start listener on port 9999
t = logging.config.listen(9999)
t.start()

try:
 # loop through logging calls to see the difference
 # new configurations make, until Ctrl+C is pressed
 while True:
 logger.debug(&#39;debug message&#39;)
 logger.info(&#39;info message&#39;)
 logger.warn(&#39;warn message&#39;)
 logger.error(&#39;error message&#39;)
 logger.critical(&#39;critical message&#39;)
 time.sleep(5)
except KeyboardInterrupt:
 # cleanup
 logging.config.stopListening()
 t.join()
</pre></div>
</div>
And here is a script that takes a filename and sends that file to the server,
properly preceded with the binary-encoded length, as the new logging
configuration:
<div class="highlight-default notranslate"><div class="highlight"><pre>#!/usr/bin/env python
import socket, sys, struct

with open(sys.argv[1], &#39;rb&#39;) as f:
 data_to_send = f.read()

HOST = &#39;localhost&#39;
PORT = 9999
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
print(&#39;connecting...&#39;)
s.connect((HOST, PORT))
print(&#39;sending config...&#39;)
s.send(struct.pack(&#39;&gt;L&#39;, len(data_to_send)))
s.send(data_to_send)
s.close()
print(&#39;complete&#39;)
</pre></div>
</div>
</div>
<div class="section" id="sending-and-receiving-logging-events-across-a-network">
<h2>Sending and receiving logging events across a network<a class="headerlink" href="#sending-and-receiving-logging-events-across-a-network" title="Permalink to this headline">¶</a></h2>
Let’s say you want to send logging events across a network, and handle them at
the receiving end. A simple way of doing this is attaching a
<code class="xref py py-class docutils literal notranslate">SocketHandler</code> instance to the root logger at the sending end:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging, logging.handlers

rootLogger = logging.getLogger(&#39;&#39;)
rootLogger.setLevel(logging.DEBUG)
socketHandler = logging.handlers.SocketHandler(&#39;localhost&#39;,
 logging.handlers.DEFAULT_TCP_LOGGING_PORT)
# don&#39;t bother with a formatter, since a socket handler sends the event as
# an unformatted pickle
rootLogger.addHandler(socketHandler)

# Now, define a couple of other loggers which might represent areas in your
# application:

class LogRecordStreamHandler(SocketServer.StreamRequestHandler):
 &quot;&quot;&quot;Handler for a streaming logging request.

This basically logs the record using whatever logging policy is
 configured locally.
 &quot;&quot;&quot;

def handle(self):
 &quot;&quot;&quot;
 Handle multiple requests - each expected to be a 4-byte length,
 followed by the LogRecord in pickle format. Logs the record
 according to whatever policy is configured locally.
 &quot;&quot;&quot;
 while True:
 chunk = self.connection.recv(4)
 if len(chunk) &lt; 4:
 break
 slen = struct.unpack(&#39;&gt;L&#39;, chunk)[0]
 chunk = self.connection.recv(slen)
 while len(chunk) &lt; slen:
 chunk = chunk + self.connection.recv(slen - len(chunk))
 obj = self.unPickle(chunk)
 record = logging.makeLogRecord(obj)
 self.handleLogRecord(record)

def unPickle(self, data):
 return pickle.loads(data)

def handleLogRecord(self, record):
 # if a name is specified, we use the named logger rather than the one
 # implied by the record.
 if self.server.logname is not None:
 name = self.server.logname
 else:
 name = record.name
 logger = logging.getLogger(name)
 # N.B. EVERY record gets logged. This is because Logger.handle
 # is normally called AFTER logger-level filtering. If you want
 # to do filtering, do it at the client end to save wasting
 # cycles and network bandwidth!
 logger.handle(record)

class LogRecordSocketReceiver(SocketServer.ThreadingTCPServer):
 &quot;&quot;&quot;
 Simple TCP socket-based logging receiver suitable for testing.
 &quot;&quot;&quot;

allow_reuse_address = 1

def __init__(self, host=&#39;localhost&#39;,
 port=logging.handlers.DEFAULT_TCP_LOGGING_PORT,
 handler=LogRecordStreamHandler):
 SocketServer.ThreadingTCPServer.__init__(self, (host, port), handler)
 self.abort = 0
 self.timeout = 1
 self.logname = None

def serve_until_stopped(self):
 import select
 abort = 0
 while not abort:
 rd, wr, ex = select.select([self.socket.fileno()],
 [], [],
 self.timeout)
 if rd:
 self.handle_request()
 abort = self.abort

if __name__ == &#39;__main__&#39;:
 main()
</pre></div>
</div>
First run the server, and then the client. On the client side, nothing is
printed on the console; on the server side, you should see something like:
<div class="highlight-default notranslate"><div class="highlight"><pre>About to start TCP server...
 59 root INFO Jackdaws love my big sphinx of quartz.
 59 myapp.area1 DEBUG Quick zephyrs blow, vexing daft Jim.
 69 myapp.area1 INFO How quickly daft jumping zebras vex.
 69 myapp.area2 WARNING Jail zesty vixen who grabbed pay from quack.
 69 myapp.area2 ERROR The five boxing wizards jump quickly.
</pre></div>
</div>
Note that there are some security issues with pickle in some scenarios. If
these affect you, you can use an alternative serialization scheme by overriding
the <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler.makePickle" title="logging.handlers.SocketHandler.makePickle"><code class="xref py py-meth docutils literal notranslate">makePickle()</code></a> method and implementing your
alternative there, as well as adapting the above script to use your alternative
serialization.
</div>
<div class="section" id="adding-contextual-information-to-your-logging-output">
<h2>Adding contextual information to your logging output<a class="headerlink" href="#adding-contextual-information-to-your-logging-output" title="Permalink to this headline">¶</a></h2>
Sometimes you want logging output to contain contextual information in
addition to the parameters passed to the logging call. For example, in a
networked application, it may be desirable to log client-specific information
in the log (e.g. remote client’s username, or IP address). Although you could
use the extra parameter to achieve this, it’s not always convenient to pass
the information in this way. While it might be tempting to create
<a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a> instances on a per-connection basis, this is not a good idea
because these instances are not garbage collected. While this is not a problem
in practice, when the number of <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a> instances is dependent on the
level of granularity you want to use in logging an application, it could
be hard to manage if the number of <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a> instances becomes
effectively unbounded.
<div class="section" id="using-loggeradapters-to-impart-contextual-information">
<h3>Using LoggerAdapters to impart contextual information<a class="headerlink" href="#using-loggeradapters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3>
An easy way in which you can pass contextual information to be output along
with logging event information is to use the <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a> class.
This class is designed to look like a <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a>, so that you can call
<a class="reference internal" href="../library/logging.html#logging.debug" title="logging.debug"><code class="xref py py-meth docutils literal notranslate">debug()</code></a>, <a class="reference internal" href="../library/logging.html#logging.info" title="logging.info"><code class="xref py py-meth docutils literal notranslate">info()</code></a>, <a class="reference internal" href="../library/logging.html#logging.warning" title="logging.warning"><code class="xref py py-meth docutils literal notranslate">warning()</code></a>, <a class="reference internal" href="../library/logging.html#logging.error" title="logging.error"><code class="xref py py-meth docutils literal notranslate">error()</code></a>,
<a class="reference internal" href="../library/logging.html#logging.exception" title="logging.exception"><code class="xref py py-meth docutils literal notranslate">exception()</code></a>, <a class="reference internal" href="../library/logging.html#logging.critical" title="logging.critical"><code class="xref py py-meth docutils literal notranslate">critical()</code></a> and <a class="reference internal" href="../library/logging.html#logging.log" title="logging.log"><code class="xref py py-meth docutils literal notranslate">log()</code></a>. These methods have the
same signatures as their counterparts in <a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a>, so you can use the
two types of instances interchangeably.
When you create an instance of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a>, you pass it a
<a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a> instance and a dict-like object which contains your contextual
information. When you call one of the logging methods on an instance of
<a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a>, it delegates the call to the underlying instance of
<a class="reference internal" href="../library/logging.html#logging.Logger" title="logging.Logger"><code class="xref py py-class docutils literal notranslate">Logger</code></a> passed to its constructor, and arranges to pass the contextual
information in the delegated call. Here’s a snippet from the code of
<a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a>:
<div class="highlight-default notranslate"><div class="highlight"><pre>def debug(self, msg, *args, **kwargs):
 &quot;&quot;&quot;
 Delegate a debug call to the underlying logger, after adding
 contextual information from this adapter instance.
 &quot;&quot;&quot;
 msg, kwargs = self.process(msg, kwargs)
 self.logger.debug(msg, *args, **kwargs)
</pre></div>
</div>
The <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter.process" title="logging.LoggerAdapter.process"><code class="xref py py-meth docutils literal notranslate">process()</code></a> method of <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a> is where the
contextual information is added to the logging output. It’s passed the message
and keyword arguments of the logging call, and it passes back (potentially)
modified versions of these to use in the call to the underlying logger. The
default implementation of this method leaves the message alone, but inserts
an ‘extra’ key in the keyword argument whose value is the dict-like object
passed to the constructor. Of course, if you had passed an ‘extra’ keyword
argument in the call to the adapter, it will be silently overwritten.
The advantage of using ‘extra’ is that the values in the dict-like object are
merged into the <a class="reference internal" href="../library/logging.html#logging.LogRecord" title="logging.LogRecord"><code class="xref py py-class docutils literal notranslate">LogRecord</code></a> instance’s __dict__, allowing you to use
customized strings with your <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal notranslate">Formatter</code></a> instances which know about
the keys of the dict-like object. If you need a different method, e.g. if you
want to prepend or append the contextual information to the message string,
you just need to subclass <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a> and override
<a class="reference internal" href="../library/logging.html#logging.LoggerAdapter.process" title="logging.LoggerAdapter.process"><code class="xref py py-meth docutils literal notranslate">process()</code></a> to do what you need. Here is a simple example:
<div class="highlight-default notranslate"><div class="highlight"><pre>class CustomAdapter(logging.LoggerAdapter):
 &quot;&quot;&quot;
 This example adapter expects the passed in dict-like object to have a
 &#39;connid&#39; key, whose value in brackets is prepended to the log message.
 &quot;&quot;&quot;
 def process(self, msg, kwargs):
 return &#39;[%s] %s&#39; % (self.extra[&#39;connid&#39;], msg), kwargs
</pre></div>
</div>
which you can use like this:
<div class="highlight-default notranslate"><div class="highlight"><pre>logger = logging.getLogger(__name__)
adapter = CustomAdapter(logger, {&#39;connid&#39;: some_conn_id})
</pre></div>
</div>
Then any events that you log to the adapter will have the value of
<code class="docutils literal notranslate">some_conn_id</code> prepended to the log messages.
<div class="section" id="using-objects-other-than-dicts-to-pass-contextual-information">
<h4>Using objects other than dicts to pass contextual information<a class="headerlink" href="#using-objects-other-than-dicts-to-pass-contextual-information" title="Permalink to this headline">¶</a></h4>
You don’t need to pass an actual dict to a <a class="reference internal" href="../library/logging.html#logging.LoggerAdapter" title="logging.LoggerAdapter"><code class="xref py py-class docutils literal notranslate">LoggerAdapter</code></a> - you could
pass an instance of a class which implements <code class="docutils literal notranslate">__getitem__</code> and <code class="docutils literal notranslate">__iter__</code> so
that it looks like a dict to logging. This would be useful if you want to
generate values dynamically (whereas the values in a dict would be constant).
</div>
</div>
<div class="section" id="using-filters-to-impart-contextual-information">
<h3>Using Filters to impart contextual information<a class="headerlink" href="#using-filters-to-impart-contextual-information" title="Permalink to this headline">¶</a></h3>
You can also add contextual information to log output using a user-defined
<a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate">Filter</code></a>. <code class="docutils literal notranslate">Filter</code> instances are allowed to modify the <code class="docutils literal notranslate">LogRecords</code>
passed to them, including adding additional attributes which can then be output
using a suitable format string, or if needed a custom <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal notranslate">Formatter</code></a>.
For example in a web application, the request being processed (or at least,
the interesting parts of it) can be stored in a threadlocal
(<a class="reference internal" href="../library/threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate">threading.local</code></a>) variable, and then accessed from a <code class="docutils literal notranslate">Filter</code> to
add, say, information from the request - say, the remote IP address and remote
user’s username - to the <code class="docutils literal notranslate">LogRecord</code>, using the attribute names ‘ip’ and
‘user’ as in the <code class="docutils literal notranslate">LoggerAdapter</code> example above. In that case, the same format
string can be used to get similar output to that shown above. Here’s an example
script:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
from random import choice

class ContextFilter(logging.Filter):
 &quot;&quot;&quot;
 This is a filter which injects contextual information into the log.

Rather than use actual contextual information, we just use random
 data in this demo.
 &quot;&quot;&quot;

USERS = [&#39;jim&#39;, &#39;fred&#39;, &#39;sheila&#39;]
 IPS = [&#39;123.231.231.123&#39;, &#39;127.0.0.1&#39;, &#39;192.168.0.1&#39;]

def filter(self, record):

record.ip = choice(ContextFilter.IPS)
 record.user = choice(ContextFilter.USERS)
 return True

if __name__ == &#39;__main__&#39;:
 levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL)
 logging.basicConfig(level=logging.DEBUG,
 format=&#39;%(asctime)-15s %(name)-5s %(levelname)-8s IP: %(ip)-15s User: %(user)-8s %(message)s&#39;)
 a1 = logging.getLogger(&#39;a.b.c&#39;)
 a2 = logging.getLogger(&#39;d.e.f&#39;)

f = ContextFilter()
 a1.addFilter(f)
 a2.addFilter(f)
 a1.debug(&#39;A debug message&#39;)
 a1.info(&#39;An info message with %s&#39;, &#39;some parameters&#39;)
 for x in range(10):
 lvl = choice(levels)
 lvlname = logging.getLevelName(lvl)
 a2.log(lvl, &#39;A message at %s level with %d %s&#39;, lvlname, 2, &#39;parameters&#39;)
</pre></div>
</div>
which, when run, produces something like:
<div class="highlight-default notranslate"><div class="highlight"><pre>2010-09-06 22:38:15,292 a.b.c DEBUG IP: 123.231.231.123 User: fred A debug message
2010-09-06 22:38:15,300 a.b.c INFO IP: 192.168.0.1 User: sheila An info message with some parameters
2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1 User: sheila A message at CRITICAL level with 2 parameters
2010-09-06 22:38:15,300 d.e.f ERROR IP: 127.0.0.1 User: jim A message at ERROR level with 2 parameters
2010-09-06 22:38:15,300 d.e.f DEBUG IP: 127.0.0.1 User: sheila A message at DEBUG level with 2 parameters
2010-09-06 22:38:15,300 d.e.f ERROR IP: 123.231.231.123 User: fred A message at ERROR level with 2 parameters
2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 192.168.0.1 User: jim A message at CRITICAL level with 2 parameters
2010-09-06 22:38:15,300 d.e.f CRITICAL IP: 127.0.0.1 User: sheila A message at CRITICAL level with 2 parameters
2010-09-06 22:38:15,300 d.e.f DEBUG IP: 192.168.0.1 User: jim A message at DEBUG level with 2 parameters
2010-09-06 22:38:15,301 d.e.f ERROR IP: 127.0.0.1 User: sheila A message at ERROR level with 2 parameters
2010-09-06 22:38:15,301 d.e.f DEBUG IP: 123.231.231.123 User: fred A message at DEBUG level with 2 parameters
2010-09-06 22:38:15,301 d.e.f INFO IP: 123.231.231.123 User: fred A message at INFO level with 2 parameters
</pre></div>
</div>
</div>
</div>
<div class="section" id="logging-to-a-single-file-from-multiple-processes">
<h2>Logging to a single file from multiple processes<a class="headerlink" href="#logging-to-a-single-file-from-multiple-processes" title="Permalink to this headline">¶</a></h2>
Although logging is thread-safe, and logging to a single file from multiple
threads in a single process is supported, logging to a single file from
multiple processes is not supported, because there is no standard way to
serialize access to a single file across multiple processes in Python. If you
need to log to a single file from multiple processes, one way of doing this is
to have all the processes log to a <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SocketHandler" title="logging.handlers.SocketHandler"><code class="xref py py-class docutils literal notranslate">SocketHandler</code></a>, and have a
separate process which implements a socket server which reads from the socket
and logs to file. (If you prefer, you can dedicate one thread in one of the
existing processes to perform this function.)
<a class="reference internal" href="#network-logging">This section</a> documents this approach in more detail and
includes a working socket receiver which can be used as a starting point for you
to adapt in your own applications.
If you are using a recent version of Python which includes the
<a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based &quot;threading&quot; interface."><code class="xref py py-mod docutils literal notranslate">multiprocessing</code></a> module, you could write your own handler which uses the
<a class="reference internal" href="../library/multiprocessing.html#multiprocessing.Lock" title="multiprocessing.Lock"><code class="xref py py-class docutils literal notranslate">Lock</code></a> class from this module to serialize access to the
file from your processes. The existing <a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal notranslate">FileHandler</code></a> and subclasses do
not make use of <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based &quot;threading&quot; interface."><code class="xref py py-mod docutils literal notranslate">multiprocessing</code></a> at present, though they may do so in the
future. Note that at present, the <a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based &quot;threading&quot; interface."><code class="xref py py-mod docutils literal notranslate">multiprocessing</code></a> module does not provide
working lock functionality on all platforms (see
<a class="reference external" href="https://bugs.python.org/issue3770">https://bugs.python.org/issue3770</a>).
</div>
<div class="section" id="using-file-rotation">
<h2>Using file rotation<a class="headerlink" href="#using-file-rotation" title="Permalink to this headline">¶</a></h2>
Sometimes you want to let a log file grow to a certain size, then open a new
file and log to that. You may want to keep a certain number of these files, and
when that many files have been created, rotate the files so that the number of
files and the size of the files both remain bounded. For this usage pattern, the
logging package provides a <a class="reference internal" href="../library/logging.handlers.html#logging.handlers.RotatingFileHandler" title="logging.handlers.RotatingFileHandler"><code class="xref py py-class docutils literal notranslate">RotatingFileHandler</code></a>:
<div class="highlight-default notranslate"><div class="highlight"><pre>import glob
import logging
import logging.handlers

LOG_FILENAME = &#39;logging_rotatingfile_example.out&#39;

# Set up a specific logger with our desired output level
my_logger = logging.getLogger(&#39;MyLogger&#39;)
my_logger.setLevel(logging.DEBUG)

# Add the log message handler to the logger
handler = logging.handlers.RotatingFileHandler(
 LOG_FILENAME, maxBytes=20, backupCount=5)

my_logger.addHandler(handler)

# Log some messages
for i in range(20):
 my_logger.debug(&#39;i = %d&#39; % i)

# See what files are created
logfiles = glob.glob(&#39;%s*&#39; % LOG_FILENAME)

for filename in logfiles:
 print(filename)
</pre></div>
</div>
The result should be 6 separate files, each with part of the log history for the
application:
<div class="highlight-default notranslate"><div class="highlight"><pre>logging_rotatingfile_example.out
logging_rotatingfile_example.out.1
logging_rotatingfile_example.out.2
logging_rotatingfile_example.out.3
logging_rotatingfile_example.out.4
logging_rotatingfile_example.out.5
</pre></div>
</div>
The most current file is always <code class="file docutils literal notranslate">logging_rotatingfile_example.out</code>,
and each time it reaches the size limit it is renamed with the suffix
<code class="docutils literal notranslate">.1</code>. Each of the existing backup files is renamed to increment the suffix
(<code class="docutils literal notranslate">.1</code> becomes <code class="docutils literal notranslate">.2</code>, etc.) and the <code class="docutils literal notranslate">.6</code> file is erased.
Obviously this example sets the log length much too small as an extreme
example. You would want to set maxBytes to an appropriate value.
</div>
<div class="section" id="an-example-dictionary-based-configuration">
<h2>An example dictionary-based configuration<a class="headerlink" href="#an-example-dictionary-based-configuration" title="Permalink to this headline">¶</a></h2>
Below is an example of a logging configuration dictionary - it’s taken from
the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">documentation on the Django project</a>.
This dictionary is passed to <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a> to put the configuration into effect:
<div class="highlight-default notranslate"><div class="highlight"><pre>LOGGING = {
 &#39;version&#39;: 1,
 &#39;disable_existing_loggers&#39;: True,
 &#39;formatters&#39;: {
 &#39;verbose&#39;: {
 &#39;format&#39;: &#39;%(levelname)s %(asctime)s %(module)s %(process)d %(thread)d %(message)s&#39;
 },
 &#39;simple&#39;: {
 &#39;format&#39;: &#39;%(levelname)s %(message)s&#39;
 },
 },
 &#39;filters&#39;: {
 &#39;special&#39;: {
 &#39;()&#39;: &#39;project.logging.SpecialFilter&#39;,
 &#39;foo&#39;: &#39;bar&#39;,
 }
 },
 &#39;handlers&#39;: {
 &#39;null&#39;: {
 &#39;level&#39;:&#39;DEBUG&#39;,
 &#39;class&#39;:&#39;django.utils.log.NullHandler&#39;,
 },
 &#39;console&#39;:{
 &#39;level&#39;:&#39;DEBUG&#39;,
 &#39;class&#39;:&#39;logging.StreamHandler&#39;,
 &#39;formatter&#39;: &#39;simple&#39;
 },
 &#39;mail_admins&#39;: {
 &#39;level&#39;: &#39;ERROR&#39;,
 &#39;class&#39;: &#39;django.utils.log.AdminEmailHandler&#39;,
 &#39;filters&#39;: [&#39;special&#39;]
 }
 },
 &#39;loggers&#39;: {
 &#39;django&#39;: {
 &#39;handlers&#39;:[&#39;null&#39;],
 &#39;propagate&#39;: True,
 &#39;level&#39;:&#39;INFO&#39;,
 },
 &#39;django.request&#39;: {
 &#39;handlers&#39;: [&#39;mail_admins&#39;],
 &#39;level&#39;: &#39;ERROR&#39;,
 &#39;propagate&#39;: False,
 },
 &#39;myproject.custom&#39;: {
 &#39;handlers&#39;: [&#39;console&#39;, &#39;mail_admins&#39;],
 &#39;level&#39;: &#39;INFO&#39;,
 &#39;filters&#39;: [&#39;special&#39;]
 }
 }
}
</pre></div>
</div>
For more information about this configuration, you can see the <a class="reference external" href="https://docs.djangoproject.com/en/1.9/topics/logging/#configuring-logging">relevant
section</a>
of the Django documentation.
</div>
<div class="section" id="inserting-a-bom-into-messages-sent-to-a-sysloghandler">
<h2>Inserting a BOM into messages sent to a SysLogHandler<a class="headerlink" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler" title="Permalink to this headline">¶</a></h2>
<a class="reference external" href="https://tools.ietf.org/html/rfc5424">RFC 5424</a> requires that a
Unicode message be sent to a syslog daemon as a set of bytes which have the
following structure: an optional pure-ASCII component, followed by a UTF-8 Byte
Order Mark (BOM), followed by Unicode encoded using UTF-8. (See the <a class="reference external" href="https://tools.ietf.org/html/rfc5424#section-6">relevant
section of the specification</a>.)
In Python 2.6 and 2.7, code was added to
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SysLogHandler" title="logging.handlers.SysLogHandler"><code class="xref py py-class docutils literal notranslate">SysLogHandler</code></a> to insert a BOM into the message, but
unfortunately, it was implemented incorrectly, with the BOM appearing at the
beginning of the message and hence not allowing any pure-ASCII component to
appear before it.
As this behaviour is broken, the incorrect BOM insertion code is being removed
from Python 2.7.4 and later. However, it is not being replaced, and if you
want to produce RFC 5424-compliant messages which include a BOM, an optional
pure-ASCII sequence before it and arbitrary Unicode after it, encoded using
UTF-8, then you need to do the following:
<ol class="arabic">
<li>Attach a <a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal notranslate">Formatter</code></a> instance to your
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.SysLogHandler" title="logging.handlers.SysLogHandler"><code class="xref py py-class docutils literal notranslate">SysLogHandler</code></a> instance, with a format string
such as:
<div class="highlight-default notranslate"><div class="highlight"><pre>u&#39;ASCII section\ufeffUnicode section&#39;
</pre></div>
</div>
The Unicode code point <code class="docutils literal notranslate">u'\ufeff'</code>, when encoded using UTF-8, will be
encoded as a UTF-8 BOM – the byte-string <code class="docutils literal notranslate">'\xef\xbb\xbf'</code>.
</li>
<li>Replace the ASCII section with whatever placeholders you like, but make sure
that the data that appears in there after substitution is always ASCII (that
way, it will remain unchanged after UTF-8 encoding).
</li>
<li>Replace the Unicode section with whatever placeholders you like; if the data
which appears there after substitution contains characters outside the ASCII
range, that’s fine – it will be encoded using UTF-8.
</li>
</ol>
If the formatted message is Unicode, it will be encoded using UTF-8 encoding
by <code class="docutils literal notranslate">SysLogHandler</code>. If you follow the above rules, you should be able to
produce RFC 5424-compliant messages. If you don’t, logging may not complain,
but your messages will not be RFC 5424-compliant, and your syslog daemon may
complain.
</div>
<div class="section" id="implementing-structured-logging">
<h2>Implementing structured logging<a class="headerlink" href="#implementing-structured-logging" title="Permalink to this headline">¶</a></h2>
Although most logging messages are intended for reading by humans, and thus not
readily machine-parseable, there might be circumstances where you want to output
messages in a structured format which is capable of being parsed by a program
(without needing complex regular expressions to parse the log message). This is
straightforward to achieve using the logging package. There are a number of
ways in which this could be achieved, but the following is a simple approach
which uses JSON to serialise the event in a machine-parseable manner:
<div class="highlight-default notranslate"><div class="highlight"><pre>import json
import logging

class StructuredMessage(object):
 def __init__(self, message, **kwargs):
 self.message = message
 self.kwargs = kwargs

def __str__(self):
 return &#39;%s &gt;&gt;&gt; %s&#39; % (self.message, json.dumps(self.kwargs))

_ = StructuredMessage # optional, to improve readability

logging.basicConfig(level=logging.INFO, format=&#39;%(message)s&#39;)
logging.info(_(&#39;message 1&#39;, foo=&#39;bar&#39;, bar=&#39;baz&#39;, num=123, fnum=123.456))
</pre></div>
</div>
If the above script is run, it prints:
<div class="highlight-default notranslate"><div class="highlight"><pre>message 1 &gt;&gt;&gt; {&quot;fnum&quot;: 123.456, &quot;num&quot;: 123, &quot;bar&quot;: &quot;baz&quot;, &quot;foo&quot;: &quot;bar&quot;}
</pre></div>
</div>
Note that the order of items might be different according to the version of
Python used.
If you need more specialised processing, you can use a custom JSON encoder,
as in the following complete example:
<div class="highlight-default notranslate"><div class="highlight"><pre>from __future__ import unicode_literals

import json
import logging

# This next bit is to ensure the script runs unchanged on 2.x and 3.x
try:
 unicode
except NameError:
 unicode = str

class Encoder(json.JSONEncoder):
 def default(self, o):
 if isinstance(o, set):
 return tuple(o)
 elif isinstance(o, unicode):
 return o.encode(&#39;unicode_escape&#39;).decode(&#39;ascii&#39;)
 return super(Encoder, self).default(o)

def __str__(self):
 s = Encoder().encode(self.kwargs)
 return &#39;%s &gt;&gt;&gt; %s&#39; % (self.message, s)

_ = StructuredMessage # optional, to improve readability

def owned_file_handler(filename, mode=&#39;a&#39;, encoding=None, owner=None):
 if owner:
 if not os.path.exists(filename):
 open(filename, &#39;a&#39;).close()
 shutil.chown(filename, *owner)
 return logging.FileHandler(filename, mode, encoding)

LOGGING = {
 &#39;version&#39;: 1,
 &#39;disable_existing_loggers&#39;: False,
 &#39;formatters&#39;: {
 &#39;default&#39;: {
 &#39;format&#39;: &#39;%(asctime)s %(levelname)s %(name)s %(message)s&#39;
 },
 },
 &#39;handlers&#39;: {
 &#39;file&#39;:{
 # The values below are popped from this dictionary and
 # used to create the handler, set the handler&#39;s level and
 # its formatter.
 &#39;()&#39;: owned_file_handler,
 &#39;level&#39;:&#39;DEBUG&#39;,
 &#39;formatter&#39;: &#39;default&#39;,
 # The values below are passed to the handler creator callable
 # as keyword arguments.
 &#39;owner&#39;: [&#39;pulse&#39;, &#39;pulse&#39;],
 &#39;filename&#39;: &#39;chowntest.log&#39;,
 &#39;mode&#39;: &#39;w&#39;,
 &#39;encoding&#39;: &#39;utf-8&#39;,
 },
 },
 &#39;root&#39;: {
 &#39;handlers&#39;: [&#39;file&#39;],
 &#39;level&#39;: &#39;DEBUG&#39;,
 },
}

logging.config.dictConfig(LOGGING)
logger = logging.getLogger(&#39;mylogger&#39;)
logger.debug(&#39;A debug message&#39;)
</pre></div>
</div>
To run this, you will probably need to run as <code class="docutils literal notranslate">root</code>:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ sudo python3.3 chowntest.py
$ cat chowntest.log
2013-11-05 09:34:51,128 DEBUG mylogger A debug message
$ ls -l chowntest.log
-rw-r--r-- 1 pulse pulse 55 2013-11-05 09:34 chowntest.log
</pre></div>
</div>
Note that this example uses Python 3.3 because that’s where <code class="xref py py-func docutils literal notranslate">shutil.chown()</code>
makes an appearance. This approach should work with any Python version that
supports <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a> - namely, Python 2.7, 3.2 or later. With pre-3.3
versions, you would need to implement the actual ownership change using e.g.
<a class="reference internal" href="../library/os.html#os.chown" title="os.chown"><code class="xref py py-func docutils literal notranslate">os.chown()</code></a>.
In practice, the handler-creating function may be in a utility module somewhere
in your project. Instead of the line in the configuration:
<div class="highlight-default notranslate"><div class="highlight"><pre>&#39;()&#39;: owned_file_handler,
</pre></div>
</div>
you could use e.g.:
<div class="highlight-default notranslate"><div class="highlight"><pre>&#39;()&#39;: &#39;ext://project.util.owned_file_handler&#39;,
</pre></div>
</div>
where <code class="docutils literal notranslate">project.util</code> can be replaced with the actual name of the package
where the function resides. In the above working script, using
<code class="docutils literal notranslate">'ext://__main__.owned_file_handler'</code> should work. Here, the actual callable
is resolved by <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a> from the <code class="docutils literal notranslate">ext://</code> specification.
This example hopefully also points the way to how you could implement other
types of file change - e.g. setting specific POSIX permission bits - in the
same way, using <a class="reference internal" href="../library/os.html#os.chmod" title="os.chmod"><code class="xref py py-func docutils literal notranslate">os.chmod()</code></a>.
Of course, the approach could also be extended to types of handler other than a
<a class="reference internal" href="../library/logging.handlers.html#logging.FileHandler" title="logging.FileHandler"><code class="xref py py-class docutils literal notranslate">FileHandler</code></a> - for example, one of the rotating file handlers,
or a different type of handler altogether.
</div>
<div class="section" id="configuring-filters-with-dictconfig">
<h2>Configuring filters with <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a><a class="headerlink" href="#configuring-filters-with-dictconfig" title="Permalink to this headline">¶</a></h2>
You can configure filters using <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a>, though it
might not be obvious at first glance how to do it (hence this recipe). Since
<a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate">Filter</code></a> is the only filter class included in the standard
library, and it is unlikely to cater to many requirements (it’s only there as a
base class), you will typically need to define your own <a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate">Filter</code></a>
subclass with an overridden <a class="reference internal" href="../library/logging.html#logging.Filter.filter" title="logging.Filter.filter"><code class="xref py py-meth docutils literal notranslate">filter()</code></a> method. To do this,
specify the <code class="docutils literal notranslate">()</code> key in the configuration dictionary for the filter,
specifying a callable which will be used to create the filter (a class is the
most obvious, but you can provide any callable which returns a
<a class="reference internal" href="../library/logging.html#logging.Filter" title="logging.Filter"><code class="xref py py-class docutils literal notranslate">Filter</code></a> instance). Here is a complete example:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import logging.config
import sys

class MyFilter(logging.Filter):
 def __init__(self, param=None):
 self.param = param

def filter(self, record):
 if self.param is None:
 allow = True
 else:
 allow = self.param not in record.msg
 if allow:
 record.msg = &#39;changed: &#39; + record.msg
 return allow

if __name__ == &#39;__main__&#39;:
 logging.config.dictConfig(LOGGING)
 logging.debug(&#39;hello&#39;)
 logging.debug(&#39;hello - noshow&#39;)
</pre></div>
</div>
This example shows how you can pass configuration data to the callable which
constructs the instance, in the form of keyword parameters. When run, the above
script will print:
<div class="highlight-default notranslate"><div class="highlight"><pre>changed: hello
</pre></div>
</div>
which shows that the filter is working as configured.
A couple of extra points to note:
<ul class="simple">
<li>If you can’t refer to the callable directly in the configuration (e.g. if it
lives in a different module, and you can’t import it directly where the
configuration dictionary is), you can use the form <code class="docutils literal notranslate">ext://...</code> as described
in <a class="reference internal" href="../library/logging.config.html#logging-config-dict-externalobj">Access to external objects</a>. For example, you could have used
the text <code class="docutils literal notranslate">'ext://__main__.MyFilter'</code> instead of <code class="docutils literal notranslate">MyFilter</code> in the above
example.</li>
<li>As well as for filters, this technique can also be used to configure custom
handlers and formatters. See <a class="reference internal" href="../library/logging.config.html#logging-config-dict-userdef">User-defined objects</a> for more
information on how logging supports using user-defined objects in its
configuration, and see the other cookbook recipe <a class="reference internal" href="#custom-handlers">Customizing handlers with dictConfig()</a> above.</li>
</ul>
</div>
<div class="section" id="customized-exception-formatting">
<h2>Customized exception formatting<a class="headerlink" href="#customized-exception-formatting" title="Permalink to this headline">¶</a></h2>
There might be times when you want to do customized exception formatting - for
argument’s sake, let’s say you want exactly one line per logged event, even
when exception information is present. You can do this with a custom formatter
class, as shown in the following example:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging

class OneLineExceptionFormatter(logging.Formatter):
 def formatException(self, exc_info):
 &quot;&quot;&quot;
 Format an exception so that it prints on a single line.
 &quot;&quot;&quot;
 result = super(OneLineExceptionFormatter, self).formatException(exc_info)
 return repr(result) # or format into one line however you want to

def format(self, record):
 s = super(OneLineExceptionFormatter, self).format(record)
 if record.exc_text:
 s = s.replace(&#39;\n&#39;, &#39;&#39;) + &#39;|&#39;
 return s

def configure_logging():
 fh = logging.FileHandler(&#39;output.txt&#39;, &#39;w&#39;)
 f = OneLineExceptionFormatter(&#39;%(asctime)s|%(levelname)s|%(message)s|&#39;,
 &#39;%d/%m/%Y %H:%M:%S&#39;)
 fh.setFormatter(f)
 root = logging.getLogger()
 root.setLevel(logging.DEBUG)
 root.addHandler(fh)

def main():
 configure_logging()
 logging.info(&#39;Sample message&#39;)
 try:
 x = 1 / 0
 except ZeroDivisionError as e:
 logging.exception(&#39;ZeroDivisionError: %s&#39;, e)

class TTSHandler(logging.Handler):
 def emit(self, record):
 msg = self.format(record)
 # Speak slowly in a female English voice
 cmd = [&#39;espeak&#39;, &#39;-s150&#39;, &#39;-ven+f3&#39;, msg]
 p = subprocess.Popen(cmd, stdout=subprocess.PIPE,
 stderr=subprocess.STDOUT)
 # wait for the program to finish
 p.communicate()

def configure_logging():
 h = TTSHandler()
 root = logging.getLogger()
 root.addHandler(h)
 # the default formatter just returns the message
 root.setLevel(logging.DEBUG)

def main():
 logging.info(&#39;Hello&#39;)
 logging.debug(&#39;Goodbye&#39;)

if __name__ == &#39;__main__&#39;:
 configure_logging()
 sys.exit(main())
</pre></div>
</div>
When run, this script should say “Hello” and then “Goodbye” in a female voice.
The above approach can, of course, be adapted to other TTS systems and even
other systems altogether which can process messages via external programs run
from a command line.
</div>
<div class="section" id="buffering-logging-messages-and-outputting-them-conditionally">
<h2>Buffering logging messages and outputting them conditionally<a class="headerlink" href="#buffering-logging-messages-and-outputting-them-conditionally" title="Permalink to this headline">¶</a></h2>
There might be situations where you want to log messages in a temporary area
and only output them if a certain condition occurs. For example, you may want to
start logging debug events in a function, and if the function completes without
errors, you don’t want to clutter the log with the collected debug information,
but if there is an error, you want all the debug information to be output as well
as the error.
Here is an example which shows how you could do this using a decorator for your
functions where you want logging to behave this way. It makes use of the
<a class="reference internal" href="../library/logging.handlers.html#logging.handlers.MemoryHandler" title="logging.handlers.MemoryHandler"><code class="xref py py-class docutils literal notranslate">logging.handlers.MemoryHandler</code></a>, which allows buffering of logged events
until some condition occurs, at which point the buffered events are <code class="docutils literal notranslate">flushed</code>
- passed to another handler (the <code class="docutils literal notranslate">target</code> handler) for processing. By default,
the <code class="docutils literal notranslate">MemoryHandler</code> flushed when its buffer gets filled up or an event whose
level is greater than or equal to a specified threshold is seen. You can use this
recipe with a more specialised subclass of <code class="docutils literal notranslate">MemoryHandler</code> if you want custom
flushing behavior.
The example script has a simple function, <code class="docutils literal notranslate">foo</code>, which just cycles through
all the logging levels, writing to <code class="docutils literal notranslate">sys.stderr</code> to say what level it’s about
to log at, and then actually logging a message at that level. You can pass a
parameter to <code class="docutils literal notranslate">foo</code> which, if true, will log at ERROR and CRITICAL levels -
otherwise, it only logs at DEBUG, INFO and WARNING levels.
The script just arranges to decorate <code class="docutils literal notranslate">foo</code> with a decorator which will do the
conditional logging that’s required. The decorator takes a logger as a parameter
and attaches a memory handler for the duration of the call to the decorated
function. The decorator can be additionally parameterised using a target handler,
a level at which flushing should occur, and a capacity for the buffer. These
default to a <a class="reference internal" href="../library/logging.handlers.html#logging.StreamHandler" title="logging.StreamHandler"><code class="xref py py-class docutils literal notranslate">StreamHandler</code></a> which writes to <code class="docutils literal notranslate">sys.stderr</code>,
<code class="docutils literal notranslate">logging.ERROR</code> and <code class="docutils literal notranslate">100</code> respectively.
Here’s the script:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
from logging.handlers import MemoryHandler
import sys

def log_if_errors(logger, target_handler=None, flush_level=None, capacity=None):
 if target_handler is None:
 target_handler = logging.StreamHandler()
 if flush_level is None:
 flush_level = logging.ERROR
 if capacity is None:
 capacity = 100
 handler = MemoryHandler(capacity, flushLevel=flush_level, target=target_handler)

def decorator(fn):
 def wrapper(*args, **kwargs):
 logger.addHandler(handler)
 try:
 return fn(*args, **kwargs)
 except Exception:
 logger.exception(&#39;call failed&#39;)
 raise
 finally:
 super(MemoryHandler, handler).flush()
 logger.removeHandler(handler)
 return wrapper

return decorator

def write_line(s):
 sys.stderr.write(&#39;%s\n&#39; % s)

def foo(fail=False):
 write_line(&#39;about to log at DEBUG ...&#39;)
 logger.debug(&#39;Actually logged at DEBUG&#39;)
 write_line(&#39;about to log at INFO ...&#39;)
 logger.info(&#39;Actually logged at INFO&#39;)
 write_line(&#39;about to log at WARNING ...&#39;)
 logger.warning(&#39;Actually logged at WARNING&#39;)
 if fail:
 write_line(&#39;about to log at ERROR ...&#39;)
 logger.error(&#39;Actually logged at ERROR&#39;)
 write_line(&#39;about to log at CRITICAL ...&#39;)
 logger.critical(&#39;Actually logged at CRITICAL&#39;)
 return fail

decorated_foo = log_if_errors(logger)(foo)

if __name__ == &#39;__main__&#39;:
 logger.setLevel(logging.DEBUG)
 write_line(&#39;Calling undecorated foo with False&#39;)
 assert not foo(False)
 write_line(&#39;Calling undecorated foo with True&#39;)
 assert foo(True)
 write_line(&#39;Calling decorated foo with False&#39;)
 assert not decorated_foo(False)
 write_line(&#39;Calling decorated foo with True&#39;)
 assert decorated_foo(True)
</pre></div>
</div>
When this script is run, the following output should be observed:
<div class="highlight-default notranslate"><div class="highlight"><pre>Calling undecorated foo with False
about to log at DEBUG ...
about to log at INFO ...
about to log at WARNING ...
Calling undecorated foo with True
about to log at DEBUG ...
about to log at INFO ...
about to log at WARNING ...
about to log at ERROR ...
about to log at CRITICAL ...
Calling decorated foo with False
about to log at DEBUG ...
about to log at INFO ...
about to log at WARNING ...
Calling decorated foo with True
about to log at DEBUG ...
about to log at INFO ...
about to log at WARNING ...
about to log at ERROR ...
Actually logged at DEBUG
Actually logged at INFO
Actually logged at WARNING
Actually logged at ERROR
about to log at CRITICAL ...
Actually logged at CRITICAL
</pre></div>
</div>
As you can see, actual logging output only occurs when an event is logged whose
severity is ERROR or greater, but in that case, any previous events at lower
severities are also logged.
You can of course use the conventional means of decoration:
<div class="highlight-default notranslate"><div class="highlight"><pre>@log_if_errors(logger)
def foo(fail=False):
 ...
</pre></div>
</div>
</div>
<div class="section" id="formatting-times-using-utc-gmt-via-configuration">
<h2>Formatting times using UTC (GMT) via configuration<a class="headerlink" href="#formatting-times-using-utc-gmt-via-configuration" title="Permalink to this headline">¶</a></h2>
Sometimes you want to format times using UTC, which can be done using a class
such as <cite>UTCFormatter</cite>, shown below:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import time

class UTCFormatter(logging.Formatter):
 converter = time.gmtime
</pre></div>
</div>
and you can then use the <code class="docutils literal notranslate">UTCFormatter</code> in your code instead of
<a class="reference internal" href="../library/logging.html#logging.Formatter" title="logging.Formatter"><code class="xref py py-class docutils literal notranslate">Formatter</code></a>. If you want to do that via configuration, you can
use the <a class="reference internal" href="../library/logging.config.html#logging.config.dictConfig" title="logging.config.dictConfig"><code class="xref py py-func docutils literal notranslate">dictConfig()</code></a> API with an approach illustrated by
the following complete example:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import logging.config
import time

LOGGING = {
 &#39;version&#39;: 1,
 &#39;disable_existing_loggers&#39;: False,
 &#39;formatters&#39;: {
 &#39;utc&#39;: {
 &#39;()&#39;: UTCFormatter,
 &#39;format&#39;: &#39;%(asctime)s %(message)s&#39;,
 },
 &#39;local&#39;: {
 &#39;format&#39;: &#39;%(asctime)s %(message)s&#39;,
 }
 },
 &#39;handlers&#39;: {
 &#39;console1&#39;: {
 &#39;class&#39;: &#39;logging.StreamHandler&#39;,
 &#39;formatter&#39;: &#39;utc&#39;,
 },
 &#39;console2&#39;: {
 &#39;class&#39;: &#39;logging.StreamHandler&#39;,
 &#39;formatter&#39;: &#39;local&#39;,
 },
 },
 &#39;root&#39;: {
 &#39;handlers&#39;: [&#39;console1&#39;, &#39;console2&#39;],
 }
}

if __name__ == &#39;__main__&#39;:
 logging.config.dictConfig(LOGGING)
 logging.warning(&#39;The local time is %s&#39;, time.asctime())
</pre></div>
</div>
When this script is run, it should print something like:
<div class="highlight-default notranslate"><div class="highlight"><pre>2015-10-17 12:53:29,501 The local time is Sat Oct 17 13:53:29 2015
2015-10-17 13:53:29,501 The local time is Sat Oct 17 13:53:29 2015
</pre></div>
</div>
showing how the time is formatted both as local time and UTC, one for each
handler.
</div>
<div class="section" id="using-a-context-manager-for-selective-logging">
<h2>Using a context manager for selective logging<a class="headerlink" href="#using-a-context-manager-for-selective-logging" title="Permalink to this headline">¶</a></h2>
There are times when it would be useful to temporarily change the logging
configuration and revert it back after doing something. For this, a context
manager is the most obvious way of saving and restoring the logging context.
Here is a simple example of such a context manager, which allows you to
optionally change the logging level and add a logging handler purely in the
scope of the context manager:
<div class="highlight-default notranslate"><div class="highlight"><pre>import logging
import sys

class LoggingContext(object):
 def __init__(self, logger, level=None, handler=None, close=True):
 self.logger = logger
 self.level = level
 self.handler = handler
 self.close = close

def __enter__(self):
 if self.level is not None:
 self.old_level = self.logger.level
 self.logger.setLevel(self.level)
 if self.handler:
 self.logger.addHandler(self.handler)

def __exit__(self, et, ev, tb):
 if self.level is not None:
 self.logger.setLevel(self.old_level)
 if self.handler:
 self.logger.removeHandler(self.handler)
 if self.handler and self.close:
 self.handler.close()
 # implicit return of None =&gt; don&#39;t swallow exceptions
</pre></div>
</div>
If you specify a level value, the logger’s level is set to that value in the
scope of the with block covered by the context manager. If you specify a
handler, it is added to the logger on entry to the block and removed on exit
from the block. You can also ask the manager to close the handler for you on
block exit - you could do this if you don’t need the handler any more.
To illustrate how it works, we can add the following block of code to the
above:
<div class="highlight-default notranslate"><div class="highlight"><pre>if __name__ == &#39;__main__&#39;:
 logger = logging.getLogger(&#39;foo&#39;)
 logger.addHandler(logging.StreamHandler())
 logger.setLevel(logging.INFO)
 logger.info(&#39;1. This should appear just once on stderr.&#39;)
 logger.debug(&#39;2. This should not appear.&#39;)
 with LoggingContext(logger, level=logging.DEBUG):
 logger.debug(&#39;3. This should appear once on stderr.&#39;)
 logger.debug(&#39;4. This should not appear.&#39;)
 h = logging.StreamHandler(sys.stdout)
 with LoggingContext(logger, level=logging.DEBUG, handler=h, close=True):
 logger.debug(&#39;5. This should appear twice - once on stderr and once on stdout.&#39;)
 logger.info(&#39;6. This should appear just once on stderr.&#39;)
 logger.debug(&#39;7. This should not appear.&#39;)
</pre></div>
</div>
We initially set the logger’s level to <code class="docutils literal notranslate">INFO</code>, so message #1 appears and
message #2 doesn’t. We then change the level to <code class="docutils literal notranslate">DEBUG</code> temporarily in the
following <code class="docutils literal notranslate">with</code> block, and so message #3 appears. After the block exits, the
logger’s level is restored to <code class="docutils literal notranslate">INFO</code> and so message #4 doesn’t appear. In the
next <code class="docutils literal notranslate">with</code> block, we set the level to <code class="docutils literal notranslate">DEBUG</code> again but also add a handler
writing to <code class="docutils literal notranslate">sys.stdout</code>. Thus, message #5 appears twice on the console (once
via <code class="docutils literal notranslate">stderr</code> and once via <code class="docutils literal notranslate">stdout</code>). After the <code class="docutils literal notranslate">with</code> statement’s
completion, the status is as it was before so message #6 appears (like message
#1) whereas message #7 doesn’t (just like message #2).
If we run the resulting script, the result is as follows:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python logctx.py
1. This should appear just once on stderr.
3. This should appear once on stderr.
5. This should appear twice - once on stderr and once on stdout.
5. This should appear twice - once on stderr and once on stdout.
6. This should appear just once on stderr.
</pre></div>
</div>
If we run it again, but pipe <code class="docutils literal notranslate">stderr</code> to <code class="docutils literal notranslate">/dev/null</code>, we see the following,
which is the only message written to <code class="docutils literal notranslate">stdout</code>:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python logctx.py 2&gt;/dev/null
5. This should appear twice - once on stderr and once on stdout.
</pre></div>
</div>
Once again, but piping <code class="docutils literal notranslate">stdout</code> to <code class="docutils literal notranslate">/dev/null</code>, we get:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python logctx.py &gt;/dev/null
1. This should appear just once on stderr.
3. This should appear once on stderr.
5. This should appear twice - once on stderr and once on stdout.
6. This should appear just once on stderr.
</pre></div>
</div>
In this case, the message #5 printed to <code class="docutils literal notranslate">stdout</code> doesn’t appear, as expected.
Of course, the approach described here can be generalised, for example to attach
logging filters temporarily. Note that the above code works in Python 2 as well
as Python 3.
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">Logging Cookbook</a><ul>
<li><a class="reference internal" href="#using-logging-in-multiple-modules">Using logging in multiple modules</a></li>
<li><a class="reference internal" href="#logging-from-multiple-threads">Logging from multiple threads</a></li>
<li><a class="reference internal" href="#multiple-handlers-and-formatters">Multiple handlers and formatters</a></li>
<li><a class="reference internal" href="#logging-to-multiple-destinations">Logging to multiple destinations</a></li>
<li><a class="reference internal" href="#configuration-server-example">Configuration server example</a></li>
<li><a class="reference internal" href="#sending-and-receiving-logging-events-across-a-network">Sending and receiving logging events across a network</a></li>
<li><a class="reference internal" href="#adding-contextual-information-to-your-logging-output">Adding contextual information to your logging output</a><ul>
<li><a class="reference internal" href="#using-loggeradapters-to-impart-contextual-information">Using LoggerAdapters to impart contextual information</a><ul>
<li><a class="reference internal" href="#using-objects-other-than-dicts-to-pass-contextual-information">Using objects other than dicts to pass contextual information</a></li>
</ul>
</li>
<li><a class="reference internal" href="#using-filters-to-impart-contextual-information">Using Filters to impart contextual information</a></li>
</ul>
</li>
<li><a class="reference internal" href="#logging-to-a-single-file-from-multiple-processes">Logging to a single file from multiple processes</a></li>
<li><a class="reference internal" href="#using-file-rotation">Using file rotation</a></li>
<li><a class="reference internal" href="#an-example-dictionary-based-configuration">An example dictionary-based configuration</a></li>
<li><a class="reference internal" href="#inserting-a-bom-into-messages-sent-to-a-sysloghandler">Inserting a BOM into messages sent to a SysLogHandler</a></li>
<li><a class="reference internal" href="#implementing-structured-logging">Implementing structured logging</a></li>
<li><a class="reference internal" href="#customizing-handlers-with-dictconfig">Customizing handlers with <code class="docutils literal notranslate">dictConfig()</code></a></li>
<li><a class="reference internal" href="#configuring-filters-with-dictconfig">Configuring filters with <code class="docutils literal notranslate">dictConfig()</code></a></li>
<li><a class="reference internal" href="#customized-exception-formatting">Customized exception formatting</a></li>
<li><a class="reference internal" href="#speaking-logging-messages">Speaking logging messages</a></li>
<li><a class="reference internal" href="#buffering-logging-messages-and-outputting-them-conditionally">Buffering logging messages and outputting them conditionally</a></li>
<li><a class="reference internal" href="#formatting-times-using-utc-gmt-via-configuration">Formatting times using UTC (GMT) via configuration</a></li>
<li><a class="reference internal" href="#using-a-context-manager-for-selective-logging">Using a context manager for selective logging</a></li>
</ul>
</li>
</ul>

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