New in version 2.3.
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 have the following attributes and methods. Note that Loggers are never instantiated directly, but always through the module-level function logging.getLogger(name).
If this evaluates to true, logging messages are passed by this logger and by its child loggers to the handlers of higher level (ancestor) loggers. 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 1.
Sets the threshold for this logger to lvl. Logging messages which are less severe than lvl will be ignored. 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 level WARNING.
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.
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 by logging.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 2.7.
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.)
There are two 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()) is provided, it is used; otherwise, sys.exc_info() is called to get the exception information.
The second 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 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 Formatters would be used with particular Handlers.
Finds the caller’s source filename and line number. Returns the filename, line number and function name as a 3-element tuple.
Changed in version 2.4: The function name was added. In earlier versions, the filename and line number were returned as a 2-element tuple.
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__().
For a list of handlers included as standard, see logging.handlers.
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.
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 String Formatting Operations for more information on string formatting.
The useful mapping keys in a LogRecord are given in the section on LogRecord attributes.
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, the ISO8601 date format is used.
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 with time.strftime() to format the creation time of the record. Otherwise, the ISO8601 format is used. 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 the converter attribute to a function with the same signature as time.localtime() or time.gmtime(). To change it for all formatters, for example if you want all logging times to be shown in GMT, set the converter attribute in the Formatter class.
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.
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.
Note that filters attached to handlers are consulted whenever an event is emitted by the handler, whereas filters attached to loggers are consulted whenever an event is logged to the handler (using debug(), info(), etc.) 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.
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 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).
Contains all the information pertinent to the event being logged.
The primary information is passed in msg and args, which are combined using msg % args to create the message field of the record.
| Parameters: |
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|---|
Changed in version 2.5: func was added.
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.
| Attribute name | Format | Description |
|---|---|---|
| args | You shouldn’t need to format this yourself. | The tuple of arguments merged into msg to produce message. |
| asctime | %(asctime)s | Human-readable time when the LogRecord was created. By default this is of the form ‘2003-07-08 16:49:45,896’ (the numbers after the comma are millisecond portion of the time). |
| created | %(created)f | Time when the LogRecord was created (as returned by time.time()). |
| exc_info | You shouldn’t need to format this yourself. | Exception tuple (à la sys.exc_info) or, if no exception has occurred, None. |
| filename | %(filename)s | Filename portion of pathname. |
| funcName | %(funcName)s | Name of function containing the logging call. |
| levelname | %(levelname)s | Text logging level for the message ('DEBUG', 'INFO', 'WARNING', 'ERROR', 'CRITICAL'). |
| levelno | %(levelno)s | Numeric logging level for the message (DEBUG, INFO, WARNING, ERROR, CRITICAL). |
| lineno | %(lineno)d | Source line number where the logging call was issued (if available). |
| module | %(module)s | Module (name portion of filename). |
| msecs | %(msecs)d | Millisecond portion of the time when the LogRecord was created. |
| message | %(message)s | The logged message, computed as msg % args. This is set when Formatter.format() is invoked. |
| msg | You shouldn’t need to format this yourself. | The format string passed in the original logging call. Merged with args to produce message, or an arbitrary object (see Using arbitrary objects as messages). |
| name | %(name)s | Name of the logger used to log the call. |
| pathname | %(pathname)s | Full pathname of the source file where the logging call was issued (if available). |
| process | %(process)d | Process ID (if available). |
| processName | %(processName)s | Process name (if available). |
| relativeCreated | %(relativeCreated)d | Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded. |
| thread | %(thread)d | Thread ID (if available). |
| threadName | %(threadName)s | Thread name (if available). |
Changed in version 2.5: funcName was added.
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.
New in version 2.6.
Returns an instance of LoggerAdapter initialized with an underlying Logger instance and a dict-like object.
In addition to the above, LoggerAdapter supports the following methods of Logger, i.e. debug(), info(), warning(), error(), exception(), critical(), log(), isEnabledFor(), getEffectiveLevel(), setLevel(), 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 2.7: The isEnabledFor() method was added to LoggerAdapter. This method delegates to the underlying logger.
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.
In addition to the classes described above, there are a number of module- level functions.
Return a logger with the specified name or, if no name is specified, 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.
Return either the standard Logger class, or the last class passed to setLoggerClass(). This function may be called from within a new class definition, to ensure that installing a customised Logger class will not undo customisations already applied by other code. For example:
class MyLogger(logging.getLoggerClass()):
# ... override behaviour here
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 two 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()) is provided, it is used; otherwise, sys.exc_info() is called to get the exception information.
The other 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 Formatters would be used with particular Handlers.
Changed in version 2.5: extra was added.
Logs a message with level level on the root logger. The other arguments are interpreted as for debug().
PLEASE NOTE: The above module-level functions which delegate to the root logger 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. These convenience functions call basicConfig() to ensure that at least one handler is available; in earlier versions of Python, 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.
Associates level lvl 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.
Does basic configuration for the logging system by creating a StreamHandler with a default Formatter and adding it to the root logger. The functions debug(), info(), warning(), error() and critical() will call basicConfig() automatically if no handlers are defined for the root logger.
This function does nothing if the root logger already has handlers configured for it.
Changed in version 2.4: Formerly, basicConfig() did not take any keyword arguments.
PLEASE 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 a StreamHandler. |
| filemode | Specifies the mode to open the file, if filename is specified (if filemode is unspecified, it defaults to ‘a’). |
| format | Use the specified format string for the handler. |
| datefmt | Use the specified date/time format. |
| 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, ‘stream’ is ignored. |
The captureWarnings() function can be used to integrate logging with the warnings module.
This function is used to turn the capture of warnings by logging on and off.
If capture is True, warnings issued by the warnings module will be redirected to the logging system. Specifically, a warning will be formatted using warnings.formatwarning() and the resulting string logged to a logger named ‘py.warnings’ with a severity of WARNING.
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 before captureWarnings(True) was called).
See also