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Python Enhancement Proposals

PEP 282 – A Logging System

Vinay Sajip <vinay_sajip at>, Trent Mick <trentm at>
Standards Track

Table of Contents


This PEP describes a proposed logging package for Python’s standard library.

Basically the system involves the user creating one or more logger objects on which methods are called to log debugging notes, general information, warnings, errors etc. Different logging ‘levels’ can be used to distinguish important messages from less important ones.

A registry of named singleton logger objects is maintained so that

  1. different logical logging streams (or ‘channels’) exist (say, one for ‘zope.zodb’ stuff and another for ‘mywebsite’-specific stuff)
  2. one does not have to pass logger object references around.

The system is configurable at runtime. This configuration mechanism allows one to tune the level and type of logging done while not touching the application itself.


If a single logging mechanism is enshrined in the standard library, 1) logging is more likely to be done ‘well’, and 2) multiple libraries will be able to be integrated into larger applications which can be logged reasonably coherently.


This proposal was put together after having studied the following logging packages:

  • java.util.logging in JDK 1.4 (a.k.a. JSR047) [1]
  • log4j [2]
  • the Syslog package from the Protomatter project [3]
  • MAL’s mx.Log package [4]

Simple Example

This shows a very simple example of how the logging package can be used to generate simple logging output on stderr.

--------- -------------------------------
import logging
log = logging.getLogger("MyModule")

def doIt():
        log.debug("Doin' stuff...")
        #do stuff...
        raise TypeError, "Bogus type error for testing"
--------- ----------------------------------
import mymodule, logging


log = logging.getLogger("MyApp")"Starting my app")
except Exception, e:
        log.exception("There was a problem.")"Ending my app")
$ python

INFO:MyApp: Starting my app
DEBUG:MyModule: Doin' stuff...
ERROR:MyApp: There was a problem.
Traceback (most recent call last):
        File "", line 9, in ?
        File "", line 7, in doIt
                raise TypeError, "Bogus type error for testing"
TypeError: Bogus type error for testing

INFO:MyApp: Ending my app

The above example shows the default output format. All aspects of the output format should be configurable, so that you could have output formatted like this:

2002-04-19 07:56:58,174 MyModule   DEBUG - Doin' stuff...

or just

Doin' stuff...

Control Flow

Applications make logging calls on Logger objects. Loggers are organized in a hierarchical namespace and child Loggers inherit some logging properties from their parents in the namespace.

Logger names fit into a “dotted name” namespace, with dots (periods) indicating sub-namespaces. The namespace of logger objects therefore corresponds to a single tree data structure.

  • "" is the root of the namespace
  • "Zope" would be a child node of the root
  • "Zope.ZODB" would be a child node of "Zope"

These Logger objects create LogRecord objects which are passed to Handler objects for output. Both Loggers and Handlers may use logging levels and (optionally) Filters to decide if they are interested in a particular LogRecord. When it is necessary to output a LogRecord externally, a Handler can (optionally) use a Formatter to localize and format the message before sending it to an I/O stream.

Each Logger keeps track of a set of output Handlers. By default all Loggers also send their output to all Handlers of their ancestor Loggers. Loggers may, however, also be configured to ignore Handlers higher up the tree.

The APIs are structured so that calls on the Logger APIs can be cheap when logging is disabled. If logging is disabled for a given log level, then the Logger can make a cheap comparison test and return. If logging is enabled for a given log level, the Logger is still careful to minimize costs before passing the LogRecord into the Handlers. In particular, localization and formatting (which are relatively expensive) are deferred until the Handler requests them.

The overall Logger hierarchy can also have a level associated with it, which takes precedence over the levels of individual Loggers. This is done through a module-level function:

def disable(lvl):
    Do not generate any LogRecords for requests with a severity less
    than 'lvl'.


The logging levels, in increasing order of importance, are:

  • INFO
  • WARN

The term CRITICAL is used in preference to FATAL, which is used by log4j. The levels are conceptually the same - that of a serious, or very serious, error. However, FATAL implies death, which in Python implies a raised and uncaught exception, traceback, and exit. Since the logging module does not enforce such an outcome from a FATAL-level log entry, it makes sense to use CRITICAL in preference to FATAL.

These are just integer constants, to allow simple comparison of importance. Experience has shown that too many levels can be confusing, as they lead to subjective interpretation of which level should be applied to any particular log request.

Although the above levels are strongly recommended, the logging system should not be prescriptive. Users may define their own levels, as well as the textual representation of any levels. User defined levels must, however, obey the constraints that they are all positive integers and that they increase in order of increasing severity.

User-defined logging levels are supported through two module-level functions:

def getLevelName(lvl):
        """Return the text for level 'lvl'."""

def addLevelName(lvl, lvlName):
        Add the level 'lvl' with associated text 'levelName', or
        set the textual representation of existing level 'lvl' to be


Each Logger object keeps track of a log level (or threshold) that it is interested in, and discards log requests below that level.

A Manager class instance maintains the hierarchical namespace of named Logger objects. Generations are denoted with dot-separated names: Logger “foo” is the parent of Loggers “” and “foo.baz”.

The Manager class instance is a singleton and is not directly exposed to users, who interact with it using various module-level functions.

The general logging method is:

class Logger:
    def log(self, lvl, msg, *args, **kwargs):
        """Log 'str(msg) % args' at logging level 'lvl'."""

However, convenience functions are defined for each logging level:

class Logger:
    def debug(self, msg, *args, **kwargs): ...
    def info(self, msg, *args, **kwargs): ...
    def warn(self, msg, *args, **kwargs): ...
    def error(self, msg, *args, **kwargs): ...
    def critical(self, msg, *args, **kwargs): ...

Only one keyword argument is recognized at present - “exc_info”. If true, the caller wants exception information to be provided in the logging output. This mechanism is only needed if exception information needs to be provided at any logging level. In the more common case, where exception information needs to be added to the log only when errors occur, i.e. at the ERROR level, then another convenience method is provided:

class Logger:
    def exception(self, msg, *args): ...

This should only be called in the context of an exception handler, and is the preferred way of indicating a desire for exception information in the log. The other convenience methods are intended to be called with exc_info only in the unusual situation where you might want to provide exception information in the context of an INFO message, for example.

The “msg” argument shown above will normally be a format string; however, it can be any object x for which str(x) returns the format string. This facilitates, for example, the use of an object which fetches a locale- specific message for an internationalized/localized application, perhaps using the standard gettext module. An outline example:

class Message:
    """Represents a message"""
    def __init__(self, id):
        """Initialize with the message ID"""

    def __str__(self):
        """Return an appropriate localized message text"""

..."abc"), ...)

Gathering and formatting data for a log message may be expensive, and a waste if the logger was going to discard the message anyway. To see if a request will be honoured by the logger, the isEnabledFor() method can be used:

class Logger:
    def isEnabledFor(self, lvl):
        Return true if requests at level 'lvl' will NOT be

so instead of this expensive and possibly wasteful DOM to XML conversion:

hamletStr = hamletDom.toxml()

one can do this:

if log.isEnabledFor(logging.INFO):
    hamletStr = hamletDom.toxml()

When new loggers are created, they are initialized with a level which signifies “no level”. A level can be set explicitly using the setLevel() method:

class Logger:
    def setLevel(self, lvl): ...

If a logger’s level is not set, the system consults all its ancestors, walking up the hierarchy until an explicitly set level is found. That is regarded as the “effective level” of the logger, and can be queried via the getEffectiveLevel() method:

def getEffectiveLevel(self): ...

Loggers are never instantiated directly. Instead, a module-level function is used:

def getLogger(name=None): ...

If no name is specified, the root logger is returned. Otherwise, if a logger with that name exists, it is returned. If not, a new logger is initialized and returned. Here, “name” is synonymous with “channel name”.

Users can specify a custom subclass of Logger to be used by the system when instantiating new loggers:

def setLoggerClass(klass): ...

The passed class should be a subclass of Logger, and its __init__ method should call Logger.__init__.


Handlers are responsible for doing something useful with a given LogRecord. The following core Handlers will be implemented:

  • StreamHandler: A handler for writing to a file-like object.
  • FileHandler: A handler for writing to a single file or set of rotating files.
  • SocketHandler: A handler for writing to remote TCP ports.
  • DatagramHandler: A handler for writing to UDP sockets, for low-cost logging. Jeff Bauer already had such a system [5].
  • MemoryHandler: A handler that buffers log records in memory until the buffer is full or a particular condition occurs [1].
  • SMTPHandler: A handler for sending to email addresses via SMTP.
  • SysLogHandler: A handler for writing to Unix syslog via UDP.
  • NTEventLogHandler: A handler for writing to event logs on Windows NT, 2000 and XP.
  • HTTPHandler: A handler for writing to a Web server with either GET or POST semantics.

Handlers can also have levels set for them using the setLevel() method:

def setLevel(self, lvl): ...

The FileHandler can be set up to create a rotating set of log files. In this case, the file name passed to the constructor is taken as a “base” file name. Additional file names for the rotation are created by appending .1, .2, etc. to the base file name, up to a maximum as specified when rollover is requested. The setRollover method is used to specify a maximum size for a log file and a maximum number of backup files in the rotation.

def setRollover(maxBytes, backupCount): ...

If maxBytes is specified as zero, no rollover ever occurs and the log file grows indefinitely. If a non-zero size is specified, when that size is about to be exceeded, rollover occurs. The rollover method ensures that the base file name is always the most recent, .1 is the next most recent, .2 the next most recent after that, and so on.

There are many additional handlers implemented in the test/example scripts provided with [6] - for example, XMLHandler and SOAPHandler.


A LogRecord acts as a receptacle for information about a logging event. It is little more than a dictionary, though it does define a getMessage method which merges a message with optional runarguments.


A Formatter is responsible for converting a LogRecord to a string representation. A Handler may call its Formatter before writing a record. The following core Formatters will be implemented:

  • Formatter: Provide printf-like formatting, using the % operator.
  • BufferingFormatter: Provide formatting for multiple messages, with header and trailer formatting support.

Formatters are associated with Handlers by calling setFormatter() on a handler:

def setFormatter(self, form): ...

Formatters use the % operator to format the logging message. The format string should contain %(name)x and the attribute dictionary of the LogRecord is used to obtain message-specific data. The following attributes are provided:

%(name)s Name of the logger (logging channel)
%(levelno)s Numeric logging level for the message (DEBUG, INFO, WARN, ERROR, CRITICAL)
%(levelname)s Text logging level for the message (“DEBUG”, “INFO”, “WARN”, “ERROR”, “CRITICAL”)
%(pathname)s Full pathname of the source file where the logging call was issued (if available)
%(filename)s Filename portion of pathname
%(module)s Module from which logging call was made
%(lineno)d Source line number where the logging call was issued (if available)
%(created)f Time when the LogRecord was created (time.time() return value)
%(asctime)s Textual time when the LogRecord was created
%(msecs)d Millisecond portion of the creation time
%(relativeCreated)d Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded (typically at application startup time)
%(thread)d Thread ID (if available)
%(message)s The result of record.getMessage(), computed just as the record is emitted

If a formatter sees that the format string includes “(asctime)s”, the creation time is formatted into the LogRecord’s asctime attribute. To allow flexibility in formatting dates, Formatters are initialized with a format string for the message as a whole, and a separate format string for date/time. The date/time format string should be in time.strftime format. The default value for the message format is “%(message)s”. The default date/time format is ISO8601.

The formatter uses a class attribute, “converter”, to indicate how to convert a time from seconds to a tuple. By default, the value of “converter” is “time.localtime”. If needed, a different converter (e.g. “time.gmtime”) can be set on an individual formatter instance, or the class attribute changed to affect all formatter instances.


When level-based filtering is insufficient, a Filter can be called by a Logger or Handler to decide if a LogRecord should be output. Loggers and Handlers can have multiple filters installed, and any one of them can veto a LogRecord being output.

class Filter:
    def filter(self, record):
        Return a value indicating true if the record is to be
        processed.  Possibly modify the record, if deemed
        appropriate by the filter.

The default behaviour allows a Filter to be initialized with a Logger name. This will only allow through events which are generated using the named logger or any of its children. For example, a filter initialized with “A.B” will allow events logged by loggers “A.B”, “A.B.C”, “A.B.C.D”, “A.B.D” etc. but not “A.BB”, “B.A.B” etc. If initialized with the empty string, all events are passed by the Filter. This filter behaviour is useful when it is desired to focus attention on one particular area of an application; the focus can be changed simply by changing a filter attached to the root logger.

There are many examples of Filters provided in [6].


The main benefit of a logging system like this is that one can control how much and what logging output one gets from an application without changing that application’s source code. Therefore, although configuration can be performed through the logging API, it must also be possible to change the logging configuration without changing an application at all. For long-running programs like Zope, it should be possible to change the logging configuration while the program is running.

Configuration includes the following:

  • What logging level a logger or handler should be interested in.
  • What handlers should be attached to which loggers.
  • What filters should be attached to which handlers and loggers.
  • Specifying attributes specific to certain handlers and filters.

In general each application will have its own requirements for how a user may configure logging output. However, each application will specify the required configuration to the logging system through a standard mechanism.

The most simple configuration is that of a single handler, writing to stderr, attached to the root logger. This configuration is set up by calling the basicConfig() function once the logging module has been imported.

def basicConfig(): ...

For more sophisticated configurations, this PEP makes no specific proposals, for the following reasons:

  • A specific proposal may be seen as prescriptive.
  • Without the benefit of wide practical experience in the Python community, there is no way to know whether any given configuration approach is a good one. That practice can’t really come until the logging module is used, and that means until after Python 2.3 has shipped.
  • There is a likelihood that different types of applications may require different configuration approaches, so that no “one size fits all”.

The reference implementation [6] has a working configuration file format, implemented for the purpose of proving the concept and suggesting one possible alternative. It may be that separate extension modules, not part of the core Python distribution, are created for logging configuration and log viewing, supplemental handlers and other features which are not of interest to the bulk of the community.

Thread Safety

The logging system should support thread-safe operation without any special action needing to be taken by its users.

Module-Level Functions

To support use of the logging mechanism in short scripts and small applications, module-level functions debug(), info(), warn(), error(), critical() and exception() are provided. These work in the same way as the correspondingly named methods of Logger - in fact they delegate to the corresponding methods on the root logger. A further convenience provided by these functions is that if no configuration has been done, basicConfig() is automatically called.

At application exit, all handlers can be flushed by calling the function:

def shutdown(): ...

This will flush and close all handlers.


The reference implementation is Vinay Sajip’s logging module [6].


The reference implementation is implemented as a single module. This offers the simplest interface - all users have to do is “import logging” and they are in a position to use all the functionality available.



Last modified: 2023-10-10 05:32:07 GMT