logging
— Logging facility for Python¶
Source code: Lib/logging/__init__.py
This module defines functions and classes which implement a flexible event logging system for applications and libraries.
The key benefit of having the logging API provided by a standard library module is that all Python modules can participate in logging, so your application log can include your own messages integrated with messages from third-party modules.
The module provides a lot of functionality and flexibility. If you are unfamiliar with logging, the best way to get to grips with it is to see the tutorials (see the links on the right).
The basic classes defined by the module, together with their functions, are listed below.
Loggers expose the interface that application code directly uses.
Handlers send the log records (created by loggers) to the appropriate destination.
Filters provide a finer grained facility for determining which log records to output.
Formatters specify the layout of log records in the final output.
Logger Objects¶
Loggers have the following attributes and methods. Note that Loggers should
NEVER be instantiated directly, but always through the module-level function
logging.getLogger(name)
. Multiple calls to getLogger()
with the same
name will always return a reference to the same Logger object.
The name
is potentially a period-separated hierarchical value, like
foo.bar.baz
(though it could also be just plain foo
, for example).
Loggers that are further down in the hierarchical list are children of loggers
higher up in the list. For example, given a logger with a name of foo
,
loggers with names of foo.bar
, foo.bar.baz
, and foo.bam
are all
descendants of foo
. The logger name hierarchy is analogous to the Python
package hierarchy, and identical to it if you organise your loggers on a
per-module basis using the recommended construction
logging.getLogger(__name__)
. That’s because in a module, __name__
is the module’s name in the Python package namespace.
- class logging.Logger¶
- propagate¶
If this attribute evaluates to true, events logged to this logger will be passed to the handlers of higher level (ancestor) loggers, in addition to any handlers attached to this logger. Messages are passed directly to the ancestor loggers’ handlers - neither the level nor filters of the ancestor loggers in question are considered.
If this evaluates to false, logging messages are not passed to the handlers of ancestor loggers.
The constructor sets this attribute to
True
.Note
If you attach a handler to a logger and one or more of its ancestors, it may emit the same record multiple times. In general, you should not need to attach a handler to more than one logger - if you just attach it to the appropriate logger which is highest in the logger hierarchy, then it will see all events logged by all descendant loggers, provided that their propagate setting is left set to
True
. A common scenario is to attach handlers only to the root logger, and to let propagation take care of the rest.
- setLevel(level)¶
Sets the threshold for this logger to level. Logging messages which are less severe than level will be ignored; logging messages which have severity level or higher will be emitted by whichever handler or handlers service this logger, unless a handler’s level has been set to a higher severity level than level.
When a logger is created, the level is set to
NOTSET
(which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger). Note that the root logger is created with levelWARNING
.The term ‘delegation to the parent’ means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached.
If an ancestor is found with a level other than NOTSET, then that ancestor’s level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled.
If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root’s level will be used as the effective level.
See Logging Levels for a list of levels.
Changed in version 3.2: The level parameter now accepts a string representation of the level such as ‘INFO’ as an alternative to the integer constants such as
INFO
. Note, however, that levels are internally stored as integers, and methods such as e.g.getEffectiveLevel()
andisEnabledFor()
will return/expect to be passed integers.
- isEnabledFor(level)¶
Indicates if a message of severity level would be processed by this logger. This method checks first the module-level level set by
logging.disable(level)
and then the logger’s effective level as determined bygetEffectiveLevel()
.
- getEffectiveLevel()¶
Indicates the effective level for this logger. If a value other than
NOTSET
has been set usingsetLevel()
, it is returned. Otherwise, the hierarchy is traversed towards the root until a value other thanNOTSET
is found, and that value is returned. The value returned is an integer, typically one oflogging.DEBUG
,logging.INFO
etc.
- getChild(suffix)¶
Returns a logger which is a descendant to this logger, as determined by the suffix. Thus,
logging.getLogger('abc').getChild('def.ghi')
would return the same logger as would be returned bylogging.getLogger('abc.def.ghi')
. This is a convenience method, useful when the parent logger is named using e.g.__name__
rather than a literal string.New in version 3.2.
- debug(msg, *args, **kwargs)¶
Logs a message with level
DEBUG
on this logger. The msg is the message format string, and the args are the arguments which are merged into msg using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.) No % formatting operation is performed on msg when no args are supplied.There are four keyword arguments in kwargs which are inspected: exc_info, stack_info, stacklevel and extra.
If exc_info does not evaluate as false, it causes exception information to be added to the logging message. If an exception tuple (in the format returned by
sys.exc_info()
) or an exception instance is provided, it is used; otherwise,sys.exc_info()
is called to get the exception information.The second optional keyword argument is stack_info, which defaults to
False
. If true, stack information is added to the logging message, including the actual logging call. Note that this is not the same stack information as that displayed through specifying exc_info: The former is stack frames from the bottom of the stack up to the logging call in the current thread, whereas the latter is information about stack frames which have been unwound, following an exception, while searching for exception handlers.You can specify stack_info independently of exc_info, e.g. to just show how you got to a certain point in your code, even when no exceptions were raised. The stack frames are printed following a header line which says:
Stack (most recent call last):
This mimics the
Traceback (most recent call last):
which is used when displaying exception frames.The third optional keyword argument is stacklevel, which defaults to
1
. If greater than 1, the corresponding number of stack frames are skipped when computing the line number and function name set in theLogRecord
created for the logging event. This can be used in logging helpers so that the function name, filename and line number recorded are not the information for the helper function/method, but rather its caller. The name of this parameter mirrors the equivalent one in thewarnings
module.The fourth keyword argument is extra which can be used to pass a dictionary which is used to populate the __dict__ of the
LogRecord
created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:FORMAT = '%(asctime)-15s %(clientip)s %(user)-8s %(message)s' logging.basicConfig(format=FORMAT) d = {'clientip': '192.168.0.1', 'user': 'fbloggs'} logger = logging.getLogger('tcpserver') logger.warning('Protocol problem: %s', 'connection reset', extra=d)
would print something like
2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset
The keys in the dictionary passed in extra should not clash with the keys used by the logging system. (See the
Formatter
documentation for more information on which keys are used by the logging system.)If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the
Formatter
has been set up with a format string which expects ‘clientip’ and ‘user’ in the attribute dictionary of theLogRecord
. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the extra dictionary with these keys.While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized
Formatter
s would be used with particularHandler
s.Changed in version 3.2: The stack_info parameter was added.
Changed in version 3.5: The exc_info parameter can now accept exception instances.
Changed in version 3.8: The stacklevel parameter was added.
- info(msg, *args, **kwargs)¶
Logs a message with level
INFO
on this logger. The arguments are interpreted as fordebug()
.
- warning(msg, *args, **kwargs)¶
Logs a message with level
WARNING
on this logger. The arguments are interpreted as fordebug()
.Note
There is an obsolete method
warn
which is functionally identical towarning
. Aswarn
is deprecated, please do not use it - usewarning
instead.
- error(msg, *args, **kwargs)¶
Logs a message with level
ERROR
on this logger. The arguments are interpreted as fordebug()
.
- critical(msg, *args, **kwargs)¶
Logs a message with level
CRITICAL
on this logger. The arguments are interpreted as fordebug()
.
- log(level, msg, *args, **kwargs)¶
Logs a message with integer level level on this logger. The other arguments are interpreted as for
debug()
.
- exception(msg, *args, **kwargs)¶
Logs a message with level
ERROR
on this logger. The arguments are interpreted as fordebug()
. Exception info is added to the logging message. This method should only be called from an exception handler.
- addFilter(filter)¶
Adds the specified filter filter to this logger.
- removeFilter(filter)¶
Removes the specified filter filter from this logger.
- filter(record)¶
Apply this logger’s filters to the record and return
True
if the record is to be processed. The filters are consulted in turn, until one of them returns a false value. If none of them return a false value, the record will be processed (passed to handlers). If one returns a false value, no further processing of the record occurs.
- addHandler(hdlr)¶
Adds the specified handler hdlr to this logger.
- removeHandler(hdlr)¶
Removes the specified handler hdlr from this logger.
- findCaller(stack_info=False, stacklevel=1)¶
Finds the caller’s source filename and line number. Returns the filename, line number, function name and stack information as a 4-element tuple. The stack information is returned as
None
unless stack_info isTrue
.The stacklevel parameter is passed from code calling the
debug()
and other APIs. If greater than 1, the excess is used to skip stack frames before determining the values to be returned. This will generally be useful when calling logging APIs from helper/wrapper code, so that the information in the event log refers not to the helper/wrapper code, but to the code that calls it.
- handle(record)¶
Handles a record by passing it to all handlers associated with this logger and its ancestors (until a false value of propagate is found). This method is used for unpickled records received from a socket, as well as those created locally. Logger-level filtering is applied using
filter()
.
- makeRecord(name, level, fn, lno, msg, args, exc_info, func=None, extra=None, sinfo=None)¶
This is a factory method which can be overridden in subclasses to create specialized
LogRecord
instances.
- hasHandlers()¶
Checks to see if this logger has any handlers configured. This is done by looking for handlers in this logger and its parents in the logger hierarchy. Returns
True
if a handler was found, elseFalse
. The method stops searching up the hierarchy whenever a logger with the ‘propagate’ attribute set to false is found - that will be the last logger which is checked for the existence of handlers.New in version 3.2.
Changed in version 3.7: Loggers can now be pickled and unpickled.
Logging Levels¶
The numeric values of logging levels are given in the following table. These are primarily of interest if you want to define your own levels, and need them to have specific values relative to the predefined levels. If you define a level with the same numeric value, it overwrites the predefined value; the predefined name is lost.
Level |
Numeric value |
---|---|
|
50 |
|
40 |
|
30 |
|
20 |
|
10 |
|
0 |
Handler Objects¶
Handlers have the following attributes and methods. Note that Handler
is never instantiated directly; this class acts as a base for more useful
subclasses. However, the __init__()
method in subclasses needs to call
Handler.__init__()
.
- class logging.Handler¶
- __init__(level=NOTSET)¶
Initializes the
Handler
instance by setting its level, setting the list of filters to the empty list and creating a lock (usingcreateLock()
) for serializing access to an I/O mechanism.
- createLock()¶
Initializes a thread lock which can be used to serialize access to underlying I/O functionality which may not be threadsafe.
- acquire()¶
Acquires the thread lock created with
createLock()
.
- setLevel(level)¶
Sets the threshold for this handler to level. Logging messages which are less severe than level will be ignored. When a handler is created, the level is set to
NOTSET
(which causes all messages to be processed).See Logging Levels for a list of levels.
Changed in version 3.2: The level parameter now accepts a string representation of the level such as ‘INFO’ as an alternative to the integer constants such as
INFO
.
- addFilter(filter)¶
Adds the specified filter filter to this handler.
- removeFilter(filter)¶
Removes the specified filter filter from this handler.
- filter(record)¶
Apply this handler’s filters to the record and return
True
if the record is to be processed. The filters are consulted in turn, until one of them returns a false value. If none of them return a false value, the record will be emitted. If one returns a false value, the handler will not emit the record.
- flush()¶
Ensure all logging output has been flushed. This version does nothing and is intended to be implemented by subclasses.
- close()¶
Tidy up any resources used by the handler. This version does no output but removes the handler from an internal list of handlers which is closed when
shutdown()
is called. Subclasses should ensure that this gets called from overriddenclose()
methods.
- handle(record)¶
Conditionally emits the specified logging record, depending on filters which may have been added to the handler. Wraps the actual emission of the record with acquisition/release of the I/O thread lock.
- handleError(record)¶
This method should be called from handlers when an exception is encountered during an
emit()
call. If the module-level attributeraiseExceptions
isFalse
, exceptions get silently ignored. This is what is mostly wanted for a logging system - most users will not care about errors in the logging system, they are more interested in application errors. You could, however, replace this with a custom handler if you wish. The specified record is the one which was being processed when the exception occurred. (The default value ofraiseExceptions
isTrue
, as that is more useful during development).
- format(record)¶
Do formatting for a record - if a formatter is set, use it. Otherwise, use the default formatter for the module.
- emit(record)¶
Do whatever it takes to actually log the specified logging record. This version is intended to be implemented by subclasses and so raises a
NotImplementedError
.
For a list of handlers included as standard, see logging.handlers
.
Formatter Objects¶
Formatter
objects have the following attributes and methods. They are
responsible for converting a LogRecord
to (usually) a string which can
be interpreted by either a human or an external system. The base
Formatter
allows a formatting string to be specified. If none is
supplied, the default value of '%(message)s'
is used, which just includes
the message in the logging call. To have additional items of information in the
formatted output (such as a timestamp), keep reading.
A Formatter can be initialized with a format string which makes use of knowledge
of the LogRecord
attributes - such as the default value mentioned above
making use of the fact that the user’s message and arguments are pre-formatted
into a LogRecord
’s message attribute. This format string contains
standard Python %-style mapping keys. See section printf-style String Formatting
for more information on string formatting.
The useful mapping keys in a LogRecord
are given in the section on
LogRecord attributes.
- class logging.Formatter(fmt=None, datefmt=None, style='%', validate=True, *, defaults=None)¶
Returns a new instance of the
Formatter
class. The instance is initialized with a format string for the message as a whole, as well as a format string for the date/time portion of a message. If no fmt is specified,'%(message)s'
is used. If no datefmt is specified, a format is used which is described in theformatTime()
documentation.The style parameter can be one of ‘%’, ‘{’ or ‘$’ and determines how the format string will be merged with its data: using one of %-formatting,
str.format()
orstring.Template
. This only applies to the format string fmt (e.g.'%(message)s'
or{message}
), not to the actual log messages passed toLogger.debug
etc; see Using particular formatting styles throughout your application for more information on using {- and $-formatting for log messages.The defaults parameter can be a dictionary with default values to use in custom fields. For example:
logging.Formatter('%(ip)s %(message)s', defaults={"ip": None})
Changed in version 3.2: The style parameter was added.
Changed in version 3.8: The validate parameter was added. Incorrect or mismatched style and fmt will raise a
ValueError
. For example:logging.Formatter('%(asctime)s - %(message)s', style='{')
.Changed in version 3.10: The defaults parameter was added.
- format(record)¶
The record’s attribute dictionary is used as the operand to a string formatting operation. Returns the resulting string. Before formatting the dictionary, a couple of preparatory steps are carried out. The message attribute of the record is computed using msg % args. If the formatting string contains
'(asctime)'
,formatTime()
is called to format the event time. If there is exception information, it is formatted usingformatException()
and appended to the message. Note that the formatted exception information is cached in attribute exc_text. This is useful because the exception information can be pickled and sent across the wire, but you should be careful if you have more than oneFormatter
subclass which customizes the formatting of exception information. In this case, you will have to clear the cached value (by setting the exc_text attribute toNone
) after a formatter has done its formatting, so that the next formatter to handle the event doesn’t use the cached value, but recalculates it afresh.If stack information is available, it’s appended after the exception information, using
formatStack()
to transform it if necessary.
- formatTime(record, datefmt=None)¶
This method should be called from
format()
by a formatter which wants to make use of a formatted time. This method can be overridden in formatters to provide for any specific requirement, but the basic behavior is as follows: if datefmt (a string) is specified, it is used withtime.strftime()
to format the creation time of the record. Otherwise, the format ‘%Y-%m-%d %H:%M:%S,uuu’ is used, where the uuu part is a millisecond value and the other letters are as per thetime.strftime()
documentation. An example time in this format is2003-01-23 00:29:50,411
. The resulting string is returned.This function uses a user-configurable function to convert the creation time to a tuple. By default,
time.localtime()
is used; to change this for a particular formatter instance, set theconverter
attribute to a function with the same signature astime.localtime()
ortime.gmtime()
. To change it for all formatters, for example if you want all logging times to be shown in GMT, set theconverter
attribute in theFormatter
class.Changed in version 3.3: Previously, the default format was hard-coded as in this example:
2010-09-06 22:38:15,292
where the part before the comma is handled by a strptime format string ('%Y-%m-%d %H:%M:%S'
), and the part after the comma is a millisecond value. Because strptime does not have a format placeholder for milliseconds, the millisecond value is appended using another format string,'%s,%03d'
— and both of these format strings have been hardcoded into this method. With the change, these strings are defined as class-level attributes which can be overridden at the instance level when desired. The names of the attributes aredefault_time_format
(for the strptime format string) anddefault_msec_format
(for appending the millisecond value).Changed in version 3.9: The
default_msec_format
can beNone
.
- formatException(exc_info)¶
Formats the specified exception information (a standard exception tuple as returned by
sys.exc_info()
) as a string. This default implementation just usestraceback.print_exception()
. The resulting string is returned.
- formatStack(stack_info)¶
Formats the specified stack information (a string as returned by
traceback.print_stack()
, but with the last newline removed) as a string. This default implementation just returns the input value.
Filter Objects¶
Filters
can be used by Handlers
and Loggers
for more sophisticated
filtering than is provided by levels. The base filter class only allows events
which are below a certain point in the logger hierarchy. 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.
- class logging.Filter(name='')¶
Returns an instance of the
Filter
class. If name is specified, it names a logger which, together with its children, will have its events allowed through the filter. If name is the empty string, allows every event.- filter(record)¶
Is the specified record to be logged? Returns zero for no, nonzero for yes. If deemed appropriate, the record may be modified in-place by this method.
Note that filters attached to handlers are consulted before an event is
emitted by the handler, whereas filters attached to loggers are consulted
whenever an event is logged (using debug()
, info()
,
etc.), before sending an event to handlers. This means that events which have
been generated by descendant loggers will not be filtered by a logger’s filter
setting, unless the filter has also been applied to those descendant loggers.
You don’t actually need to subclass Filter
: you can pass any instance
which has a filter
method with the same semantics.
Changed in version 3.2: You don’t need to create specialized Filter
classes, or use other
classes with a filter
method: you can use a function (or other
callable) as a filter. The filtering logic will check to see if the filter
object has a filter
attribute: if it does, it’s assumed to be a
Filter
and its filter()
method is called. Otherwise, it’s
assumed to be a callable and called with the record as the single
parameter. The returned value should conform to that returned by
filter()
.
Although filters are used primarily to filter records based on more
sophisticated criteria than levels, they get to see every record which is
processed by the handler or logger they’re attached to: this can be useful if
you want to do things like counting how many records were processed by a
particular logger or handler, or adding, changing or removing attributes in
the LogRecord
being processed. Obviously changing the LogRecord needs
to be done with some care, but it does allow the injection of contextual
information into logs (see Using Filters to impart contextual information).
LogRecord Objects¶
LogRecord
instances are created automatically by the Logger
every time something is logged, and can be created manually via
makeLogRecord()
(for example, from a pickled event received over the
wire).
- class logging.LogRecord(name, level, pathname, lineno, msg, args, exc_info, func=None, sinfo=None)¶
Contains all the information pertinent to the event being logged.
The primary information is passed in
msg
andargs
, which are combined usingmsg % args
to create themessage
field of the record.- Parameters
name – The name of the logger used to log the event represented by this LogRecord. Note that this name will always have this value, even though it may be emitted by a handler attached to a different (ancestor) logger.
level – The numeric level of the logging event (one of DEBUG, INFO etc.) Note that this is converted to two attributes of the LogRecord:
levelno
for the numeric value andlevelname
for the corresponding level name.pathname – The full pathname of the source file where the logging call was made.
lineno – The line number in the source file where the logging call was made.
msg – The event description message, possibly a format string with placeholders for variable data.
args – Variable data to merge into the msg argument to obtain the event description.
exc_info – An exception tuple with the current exception information, or
None
if no exception information is available.func – The name of the function or method from which the logging call was invoked.
sinfo – A text string representing stack information from the base of the stack in the current thread, up to the logging call.
- getMessage()¶
Returns the message for this
LogRecord
instance after merging any user-supplied arguments with the message. If the user-supplied message argument to the logging call is not a string,str()
is called on it to convert it to a string. This allows use of user-defined classes as messages, whose__str__
method can return the actual format string to be used.
Changed in version 3.2: The creation of a
LogRecord
has been made more configurable by providing a factory which is used to create the record. The factory can be set usinggetLogRecordFactory()
andsetLogRecordFactory()
(see this for the factory’s signature).This functionality can be used to inject your own values into a
LogRecord
at creation time. You can use the following pattern:old_factory = logging.getLogRecordFactory() def record_factory(*args, **kwargs): record = old_factory(*args, **kwargs) record.custom_attribute = 0xdecafbad return record logging.setLogRecordFactory(record_factory)
With this pattern, multiple factories could be chained, and as long as they don’t overwrite each other’s attributes or unintentionally overwrite the standard attributes listed above, there should be no surprises.
LogRecord attributes¶
The LogRecord has a number of attributes, most of which are derived from the parameters to the constructor. (Note that the names do not always correspond exactly between the LogRecord constructor parameters and the LogRecord attributes.) These attributes can be used to merge data from the record into the format string. The following table lists (in alphabetical order) the attribute names, their meanings and the corresponding placeholder in a %-style format string.
If you are using {}-formatting (str.format()
), you can use
{attrname}
as the placeholder in the format string. If you are using
$-formatting (string.Template
), use the form ${attrname}
. In
both cases, of course, replace attrname
with the actual attribute name
you want to use.
In the case of {}-formatting, you can specify formatting flags by placing them
after the attribute name, separated from it with a colon. For example: a
placeholder of {msecs:03d}
would format a millisecond value of 4
as
004
. Refer to the str.format()
documentation for full details on
the options available to you.
Attribute name |
Format |
Description |
---|---|---|
args |
You shouldn’t need to format this yourself. |
The tuple of arguments merged into |
asctime |
|
Human-readable time when the
|
created |
|
Time when the |
exc_info |
You shouldn’t need to format this yourself. |
Exception tuple (à la |
filename |
|
Filename portion of |
funcName |
|
Name of function containing the logging call. |
levelname |
|
Text logging level for the message
( |
levelno |
|
Numeric logging level for the message
( |
lineno |
|
Source line number where the logging call was issued (if available). |
message |
|
The logged message, computed as |
module |
|
Module (name portion of |
msecs |
|
Millisecond portion of the time when the
|
msg |
You shouldn’t need to format this yourself. |
The format string passed in the original
logging call. Merged with |
name |
|
Name of the logger used to log the call. |
pathname |
|
Full pathname of the source file where the logging call was issued (if available). |
process |
|
Process ID (if available). |
processName |
|
Process name (if available). |
relativeCreated |
|
Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded. |
stack_info |
You shouldn’t need to format this yourself. |
Stack frame information (where available) from the bottom of the stack in the current thread, up to and including the stack frame of the logging call which resulted in the creation of this record. |
thread |
|
Thread ID (if available). |
threadName |
|
Thread name (if available). |
Changed in version 3.1: processName was added.
LoggerAdapter Objects¶
LoggerAdapter
instances are used to conveniently pass contextual
information into logging calls. For a usage example, see the section on
adding contextual information to your logging output.
- class logging.LoggerAdapter(logger, extra)¶
Returns an instance of
LoggerAdapter
initialized with an underlyingLogger
instance and a dict-like object.- process(msg, kwargs)¶
Modifies the message and/or keyword arguments passed to a logging call in order to insert contextual information. This implementation takes the object passed as extra to the constructor and adds it to kwargs using key ‘extra’. The return value is a (msg, kwargs) tuple which has the (possibly modified) versions of the arguments passed in.
In addition to the above, LoggerAdapter
supports the following
methods of Logger
: debug()
, info()
,
warning()
, error()
, exception()
,
critical()
, log()
, isEnabledFor()
,
getEffectiveLevel()
, setLevel()
and
hasHandlers()
. These methods have the same signatures as their
counterparts in Logger
, so you can use the two types of instances
interchangeably.
Changed in version 3.2: The isEnabledFor()
, getEffectiveLevel()
,
setLevel()
and hasHandlers()
methods were added
to LoggerAdapter
. These methods delegate to the underlying logger.
Changed in version 3.6: Attribute manager
and method _log()
were added, which
delegate to the underlying logger and allow adapters to be nested.
Thread Safety¶
The logging module is intended to be thread-safe without any special work needing to be done by its clients. It achieves this though using threading locks; there is one lock to serialize access to the module’s shared data, and each handler also creates a lock to serialize access to its underlying I/O.
If you are implementing asynchronous signal handlers using the signal
module, you may not be able to use logging from within such handlers. This is
because lock implementations in the threading
module are not always
re-entrant, and so cannot be invoked from such signal handlers.
Module-Level Functions¶
In addition to the classes described above, there are a number of module-level functions.
- logging.getLogger(name=None)¶
Return a logger with the specified name or, if name is
None
, return a logger which is the root logger of the hierarchy. If specified, the name is typically a dot-separated hierarchical name like ‘a’, ‘a.b’ or ‘a.b.c.d’. Choice of these names is entirely up to the developer who is using logging.All calls to this function with a given name return the same logger instance. This means that logger instances never need to be passed between different parts of an application.
- logging.getLoggerClass()¶
Return either the standard
Logger
class, or the last class passed tosetLoggerClass()
. This function may be called from within a new class definition, to ensure that installing a customizedLogger
class will not undo customizations already applied by other code. For example:class MyLogger(logging.getLoggerClass()): # ... override behaviour here
- logging.getLogRecordFactory()¶
Return a callable which is used to create a
LogRecord
.New in version 3.2: This function has been provided, along with
setLogRecordFactory()
, to allow developers more control over how theLogRecord
representing a logging event is constructed.See
setLogRecordFactory()
for more information about the how the factory is called.
- logging.debug(msg, *args, **kwargs)¶
Logs a message with level
DEBUG
on the root logger. The msg is the message format string, and the args are the arguments which are merged into msg using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.)There are three keyword arguments in kwargs which are inspected: exc_info which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by
sys.exc_info()
) or an exception instance is provided, it is used; otherwise,sys.exc_info()
is called to get the exception information.The second optional keyword argument is stack_info, which defaults to
False
. If true, stack information is added to the logging message, including the actual logging call. Note that this is not the same stack information as that displayed through specifying exc_info: The former is stack frames from the bottom of the stack up to the logging call in the current thread, whereas the latter is information about stack frames which have been unwound, following an exception, while searching for exception handlers.You can specify stack_info independently of exc_info, e.g. to just show how you got to a certain point in your code, even when no exceptions were raised. The stack frames are printed following a header line which says:
Stack (most recent call last):
This mimics the
Traceback (most recent call last):
which is used when displaying exception frames.The third optional keyword argument is extra which can be used to pass a dictionary which is used to populate the __dict__ of the LogRecord created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:
FORMAT = '%(asctime)-15s %(clientip)s %(user)-8s %(message)s' logging.basicConfig(format=FORMAT) d = {'clientip': '192.168.0.1', 'user': 'fbloggs'} logging.warning('Protocol problem: %s', 'connection reset', extra=d)
would print something like:
2006-02-08 22:20:02,165 192.168.0.1 fbloggs Protocol problem: connection reset
The keys in the dictionary passed in extra should not clash with the keys used by the logging system. (See the
Formatter
documentation for more information on which keys are used by the logging system.)If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the
Formatter
has been set up with a format string which expects ‘clientip’ and ‘user’ in the attribute dictionary of the LogRecord. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the extra dictionary with these keys.While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized
Formatter
s would be used with particularHandler
s.Changed in version 3.2: The stack_info parameter was added.
- logging.info(msg, *args, **kwargs)¶
Logs a message with level
INFO
on the root logger. The arguments are interpreted as fordebug()
.
- logging.warning(msg, *args, **kwargs)¶
Logs a message with level
WARNING
on the root logger. The arguments are interpreted as fordebug()
.Note
There is an obsolete function
warn
which is functionally identical towarning
. Aswarn
is deprecated, please do not use it - usewarning
instead.
- logging.error(msg, *args, **kwargs)¶
Logs a message with level
ERROR
on the root logger. The arguments are interpreted as fordebug()
.
- logging.critical(msg, *args, **kwargs)¶
Logs a message with level
CRITICAL
on the root logger. The arguments are interpreted as fordebug()
.
- logging.exception(msg, *args, **kwargs)¶
Logs a message with level
ERROR
on the root logger. The arguments are interpreted as fordebug()
. Exception info is added to the logging message. This function should only be called from an exception handler.
- logging.log(level, msg, *args, **kwargs)¶
Logs a message with level level on the root logger. The other arguments are interpreted as for
debug()
.Note
The above module-level convenience functions, which delegate to the root logger, call
basicConfig()
to ensure that at least one handler is available. Because of this, they should not be used in threads, in versions of Python earlier than 2.7.1 and 3.2, unless at least one handler has been added to the root logger before the threads are started. In earlier versions of Python, due to a thread safety shortcoming inbasicConfig()
, this can (under rare circumstances) lead to handlers being added multiple times to the root logger, which can in turn lead to multiple messages for the same event.
- logging.disable(level=CRITICAL)¶
Provides an overriding level level for all loggers which takes precedence over the logger’s own level. When the need arises to temporarily throttle logging output down across the whole application, this function can be useful. Its effect is to disable all logging calls of severity level and below, so that if you call it with a value of INFO, then all INFO and DEBUG events would be discarded, whereas those of severity WARNING and above would be processed according to the logger’s effective level. If
logging.disable(logging.NOTSET)
is called, it effectively removes this overriding level, so that logging output again depends on the effective levels of individual loggers.Note that if you have defined any custom logging level higher than
CRITICAL
(this is not recommended), you won’t be able to rely on the default value for the level parameter, but will have to explicitly supply a suitable value.Changed in version 3.7: The level parameter was defaulted to level
CRITICAL
. See bpo-28524 for more information about this change.
- logging.addLevelName(level, levelName)¶
Associates level level with text levelName in an internal dictionary, which is used to map numeric levels to a textual representation, for example when a
Formatter
formats a message. This function can also be used to define your own levels. The only constraints are that all levels used must be registered using this function, levels should be positive integers and they should increase in increasing order of severity.Note
If you are thinking of defining your own levels, please see the section on Custom Levels.
- logging.getLevelNamesMapping()¶
Returns a mapping from level names to their corresponding logging levels. For example, the string “CRITICAL” maps to
CRITICAL
. The returned mapping is copied from an internal mapping on each call to this function.New in version 3.11.
- logging.getLevelName(level)¶
Returns the textual or numeric representation of logging level level.
If level is one of the predefined levels
CRITICAL
,ERROR
,WARNING
,INFO
orDEBUG
then you get the corresponding string. If you have associated levels with names usingaddLevelName()
then the name you have associated with level is returned. If a numeric value corresponding to one of the defined levels is passed in, the corresponding string representation is returned.The level parameter also accepts a string representation of the level such as ‘INFO’. In such cases, this functions returns the corresponding numeric value of the level.
If no matching numeric or string value is passed in, the string ‘Level %s’ % level is returned.
Note
Levels are internally integers (as they need to be compared in the logging logic). This function is used to convert between an integer level and the level name displayed in the formatted log output by means of the
%(levelname)s
format specifier (see LogRecord attributes), and vice versa.Changed in version 3.4: In Python versions earlier than 3.4, this function could also be passed a text level, and would return the corresponding numeric value of the level. This undocumented behaviour was considered a mistake, and was removed in Python 3.4, but reinstated in 3.4.2 due to retain backward compatibility.
- logging.makeLogRecord(attrdict)¶
Creates and returns a new
LogRecord
instance whose attributes are defined by attrdict. This function is useful for taking a pickledLogRecord
attribute dictionary, sent over a socket, and reconstituting it as aLogRecord
instance at the receiving end.
- logging.basicConfig(**kwargs)¶
Does basic configuration for the logging system by creating a
StreamHandler
with a defaultFormatter
and adding it to the root logger. The functionsdebug()
,info()
,warning()
,error()
andcritical()
will callbasicConfig()
automatically if no handlers are defined for the root logger.This function does nothing if the root logger already has handlers configured, unless the keyword argument force is set to
True
.Note
This function should be called from the main thread before other threads are started. In versions of Python prior to 2.7.1 and 3.2, if this function is called from multiple threads, it is possible (in rare circumstances) that a handler will be added to the root logger more than once, leading to unexpected results such as messages being duplicated in the log.
The following keyword arguments are supported.
Format
Description
filename
Specifies that a
FileHandler
be created, using the specified filename, rather than aStreamHandler
.filemode
If filename is specified, open the file in this mode. Defaults to
'a'
.format
Use the specified format string for the handler. Defaults to attributes
levelname
,name
andmessage
separated by colons.datefmt
Use the specified date/time format, as accepted by
time.strftime()
.style
If format is specified, use this style for the format string. One of
'%'
,'{'
or'$'
for printf-style,str.format()
orstring.Template
respectively. Defaults to'%'
.level
Set the root logger level to the specified level.
stream
Use the specified stream to initialize the
StreamHandler
. Note that this argument is incompatible with filename - if both are present, aValueError
is raised.handlers
If specified, this should be an iterable of already created handlers to add to the root logger. Any handlers which don’t already have a formatter set will be assigned the default formatter created in this function. Note that this argument is incompatible with filename or stream - if both are present, a
ValueError
is raised.force
If this keyword argument is specified as true, any existing handlers attached to the root logger are removed and closed, before carrying out the configuration as specified by the other arguments.
encoding
If this keyword argument is specified along with filename, its value is used when the
FileHandler
is created, and thus used when opening the output file.errors
If this keyword argument is specified along with filename, its value is used when the
FileHandler
is created, and thus used when opening the output file. If not specified, the value ‘backslashreplace’ is used. Note that ifNone
is specified, it will be passed as such toopen()
, which means that it will be treated the same as passing ‘errors’.Changed in version 3.2: The style argument was added.
Changed in version 3.3: The handlers argument was added. Additional checks were added to catch situations where incompatible arguments are specified (e.g. handlers together with stream or filename, or stream together with filename).
Changed in version 3.8: The force argument was added.
Changed in version 3.9: The encoding and errors arguments were added.
- logging.shutdown()¶
Informs the logging system to perform an orderly shutdown by flushing and closing all handlers. This should be called at application exit and no further use of the logging system should be made after this call.
When the logging module is imported, it registers this function as an exit handler (see
atexit
), so normally there’s no need to do that manually.
- logging.setLoggerClass(klass)¶
Tells the logging system to use the class klass when instantiating a logger. The class should define
__init__()
such that only a name argument is required, and the__init__()
should callLogger.__init__()
. This function is typically called before any loggers are instantiated by applications which need to use custom logger behavior. After this call, as at any other time, do not instantiate loggers directly using the subclass: continue to use thelogging.getLogger()
API to get your loggers.
- logging.setLogRecordFactory(factory)¶
Set a callable which is used to create a
LogRecord
.- Parameters
factory – The factory callable to be used to instantiate a log record.
New in version 3.2: This function has been provided, along with
getLogRecordFactory()
, to allow developers more control over how theLogRecord
representing a logging event is constructed.The factory has the following signature:
factory(name, level, fn, lno, msg, args, exc_info, func=None, sinfo=None, **kwargs)
- name
The logger name.
- level
The logging level (numeric).
- fn
The full pathname of the file where the logging call was made.
- lno
The line number in the file where the logging call was made.
- msg
The logging message.
- args
The arguments for the logging message.
- exc_info
An exception tuple, or
None
.- func
The name of the function or method which invoked the logging call.
- sinfo
A stack traceback such as is provided by
traceback.print_stack()
, showing the call hierarchy.- kwargs
Additional keyword arguments.
Module-Level Attributes¶
- logging.lastResort¶
A “handler of last resort” is available through this attribute. This is a
StreamHandler
writing tosys.stderr
with a level ofWARNING
, and is used to handle logging events in the absence of any logging configuration. The end result is to just print the message tosys.stderr
. This replaces the earlier error message saying that “no handlers could be found for logger XYZ”. If you need the earlier behaviour for some reason,lastResort
can be set toNone
.New in version 3.2.
Integration with the warnings module¶
The captureWarnings()
function can be used to integrate logging
with the warnings
module.
- logging.captureWarnings(capture)¶
This function is used to turn the capture of warnings by logging on and off.
If capture is
True
, warnings issued by thewarnings
module will be redirected to the logging system. Specifically, a warning will be formatted usingwarnings.formatwarning()
and the resulting string logged to a logger named'py.warnings'
with a severity ofWARNING
.If capture is
False
, the redirection of warnings to the logging system will stop, and warnings will be redirected to their original destinations (i.e. those in effect beforecaptureWarnings(True)
was called).
See also
- Module
logging.config
Configuration API for the logging module.
- Module
logging.handlers
Useful handlers included with the logging module.
- PEP 282 - A Logging System
The proposal which described this feature for inclusion in the Python standard library.
- Original Python logging package
This is the original source for the
logging
package. The version of the package available from this site is suitable for use with Python 1.5.2, 2.1.x and 2.2.x, which do not include thelogging
package in the standard library.