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logging-f-string (G004)

Derived from the flake8-logging-format linter.

What it does

Checks for uses of f-strings to format logging messages.

Why is this bad?

The logging module provides a mechanism for passing additional values to be logged using the extra keyword argument. This is more consistent, more efficient, and less error-prone than formatting the string directly.

Using f-strings to format a logging message requires that Python eagerly format the string, even if the logging statement is never executed (e.g., if the log level is above the level of the logging statement), whereas using the extra keyword argument defers formatting until required.

Additionally, the use of extra will ensure that the values are made available to all handlers, which can then be configured to log the values in a consistent manner.

As an alternative to extra, passing values as arguments to the logging method can also be used to defer string formatting until required.

Known problems

This rule detects uses of the logging module via a heuristic. Specifically, it matches against:

  • Uses of the logging module itself (e.g., import logging; logging.info(...)).
  • Uses of flask.current_app.logger (e.g., from flask import current_app; current_app.logger.info(...)).
  • Objects whose name starts with log or ends with logger or logging, when used in the same file in which they are defined (e.g., logger = logging.getLogger(); logger.info(...)).
  • Imported objects marked as loggers via the lint.logger-objects setting, which can be used to enforce these rules against shared logger objects (e.g., from module import logger; logger.info(...), when lint.logger-objects is set to ["module.logger"]).

Example

import logging

logging.basicConfig(format="%(message)s", level=logging.INFO)

user = "Maria"

logging.info(f"{user} - Something happened")

Use instead:

import logging

logging.basicConfig(format="%(user_id)s - %(message)s", level=logging.INFO)

user = "Maria"

logging.info("Something happened", extra=dict(user_id=user))

Or:

import logging

logging.basicConfig(format="%(message)s", level=logging.INFO)

user = "Maria"

logging.info("%s - Something happened", user)

Options

References