flake8-logging 1.2.0

A Flake8 plugin that checks for issues using the standard library logging module.

A Flake8 plugin that checks for issues using the standard library logging module.

Requirements

Python 3.8 to 3.12 supported.

Installation

First, install with pip:

python -m pip install flake8-logging

Second, if you define Flake8’s select setting, add the L prefix to it. Otherwise, the plugin should be active by default.

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Rules

LOG001 use logging.getLogger() to instantiate loggers

The Logger Objects documentation section starts:

Note that Loggers should NEVER be instantiated directly, but always through the module-level function logging.getLogger(name).

Directly instantiated loggers are not added into the logger tree, so their messages are discarded. This means you’ll never see any messages on such loggers. Use getLogger() to correctly instantiate loggers.

This rule detects any module-level calls to Logger().

Failing example:

import logging

logger = logging.Logger(__name__)

Corrected:

import logging

logger = logging.getLogger(__name__)

LOG002 use __name__ with getLogger()

The logging documentation recommends this pattern:

logging.getLogger(__name__)

__name__ is the fully qualified module name, such as camelot.spam, which is the intended format for logger names.

This rule detects probably-mistaken usage of similar module-level dunder constants:

• __cached__ - the pathname of the module’s compiled versio˜, such as camelot/__pycache__/spam.cpython-311.pyc.

• __file__ - the pathname of the module, such as camelot/spam.py.

Failing example:

import logging

logger = logging.getLogger(__file__)

Corrected:

import logging

logger = logging.getLogger(__name__)

LOG003 extra key '<key>' clashes with LogRecord attribute

The extra documentation states:

The keys in the dictionary passed in extra should not clash with the keys used by the logging system.

Such clashes crash at runtime with an error like:

KeyError: "Attempt to overwrite 'msg' in LogRecord"

Unfortunately, this error is only raised if the message is not filtered out by level. Tests may therefore not encounter the check, if they run with a limited logging configuration.

This rule detects such clashes by checking for keys matching the LogRecord attributes.

Failing example:

import logging

logger = logging.getLogger(__name__)

response = acme_api()
logger.info("ACME Response", extra={"msg": response.msg})

Corrected:

import logging

logger = logging.getLogger(__name__)

response = acme_api()
logger.info("ACME Response", extra={"response_msg": response.msg})

LOG004 avoid exception() outside of exception handlers

The exception() documentation states:

This function should only be called from an exception handler.

Calling exception() outside of an exception handler attaches None exception information, leading to confusing messages:

>>> logging.exception("example")
ERROR:root:example
NoneType: None

Use error() instead. To log a caught exception, pass it in the exc_info argument.

This rule detects exception() calls outside of exception handlers.

Failing example:

import logging

response = acme_api()
if response is None:
    logging.exception("ACME failed")

Corrected:

import logging

response = acme_api()
if response is None:
    logging.error("ACME failed")

LOG005 use exception() within an exception handler

Within an exception handler, the exception() method is preferable over logger.error(). The exception() method captures the exception automatically, whilst error() needs it to be passed explicitly in the exc_info argument. Both methods log with the level ERROR.

This rule detects error() calls within exception handlers, excluding those with a falsy exc_info argument.

Failing example:

try:
    acme_api()
except AcmeError as exc:
    logger.error("ACME API failed", exc_info=exc)

Corrected:

try:
    acme_api()
except AcmeError:
    logger.exception("ACME API failed")

Or alternatively, if the exception information is truly uninformative:

try:
    acme_api()
except DuplicateError:
    logger.error("ACME Duplicate Error", exc_info=False)

LOG006 redundant exc_info argument for exception()

The exception() method captures the exception automatically, making a truthy exc_info argument redundant.

This rule detects exception() calls within exception handlers with an exc_info argument that is truthy or the captured exception object.

Failing example:

try:
    acme_api()
except AcmeError:
    logger.exception("ACME API failed", exc_info=True)

Corrected:

try:
    acme_api()
except AcmeError:
    logger.exception("ACME API failed")

LOG007 use error() instead of exception() with exc_info=False

The exception() method captures the exception automatically. Disabling this by setting exc_info=False is the same as using error(), which is clearer and doesn’t need the exc_info argument.

This rule detects exception() calls with an exc_info argument that is falsy.

Failing example:

logger.exception("Left phalange missing", exc_info=False)

Corrected:

logger.error("Left phalange missing")

LOG008 warn() is deprecated, use warning() instead

The warn() method is a deprecated, undocumented alias for warning() warning() should always be used instead. The method was deprecated in Python 2.7, in commit 04d5bc00a2, and removed in Python 3.13, in commit dcc028d924.

This rule detects calls to warn().

Failing example:

logger.warn("Cheesy puns incoming")

Corrected:

logger.warning("Cheesy puns incoming")

LOG009 WARN is undocumented, use WARNING instead

The WARN constant is an undocumented alias for WARNING. Whilst it’s not deprecated, it’s not mentioned at all in the documentation, so the documented WARNING should always be used instead.

This rule detects any import or access of WARN.

Failing example:

import logging

logging.WARN

Corrected:

import logging

logging.WARNING

LOG010 exception() does not take an exception

Like other logger methods, the exception() method takes a string as its first argument. A common misunderstanding is to pass it an exception instead. Doing so is redundant, as exception() will already capture the exception object. It can also lead to unclear log messages, as the logger will call str() on the exception, which doesn’t always produce a sensible message.

This rule detects exception() calls with a first argument that is the current exception handler’s capture variable.

Failing example:

try:
    shuffle_deck()
except Exception as exc:
    logger.exception(exc)

Corrected:

try:
    shuffle_deck()
except Exception:
    logger.exception("Failed to shuffle deck")

LOG011 avoid pre-formatting log messages

Logger methods support string formatting for logging variable data, such as:

logger.info("Couldn’t chop %s", vegetable)

Formatting is skipped if the message isn’t be logged due to its level being lower than the configured one.

Passing a logger method a pre-formatted string, such as from an f-string, has no such optimization. Time is always spent on formatting when the message won’t be logged.

This rule detects logger method calls with a msg argument that is one of:

• an f-string

• a call to str.format()

• a string used with the modulus operator (%)

• a concatenation of strings with non-strings

Failing examples:

logging.error(f"Couldn’t chop {vegetable}")

logging.error("Couldn’t chop {}".format(vegetable))

logging.error("Couldn’t chop %s" % (vegetable,))

logging.error("Couldn’t chop " + vegetable)

Corrected:

logging.error("Couldn’t chop %s", vegetable)