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Flake8 extension to validate (lack of) logging format strings


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Flake8 extension to validate (lack of) logging format strings

What's This?

Python logging supports a special extra keyword for passing a dictionary of user-defined attributes to include in a logging event. One way to ensure consistency and rigor in logging is to always use extra to pass non-constant data and, therefore, to never use format strings, concatenation, or other similar techniques to construct a log string.

In other words, do this:
    "Hello {world}",

Instead of:
    "Hello {world}".format(world=Earth)

Extra Whitelist

As a further level of rigor, we can enforce that extra dictionaries only use keys from a well-known whitelist.


flake8 --enable-extra-whitelist

The built-in Whitelist supports plugins using entry_points with a key of "logging.extra.whitelist". Each registered entry point must be a callable that returns an iterable of string.

In some cases you may want to log sensitive data only in debugging scenarios. This is supported in 2 ways:

  1. We do not check the logging.extra.whitelist for lines logged at the debug level
  2. You may also prefix a keyword with 'debug_' and log it at another level. You can safely assume these will be filtered out of shipped logs.

Violations Detected

  • G001 Logging statements should not use string.format() for their first argument
  • G002 Logging statements should not use % formatting for their first argument
  • G003 Logging statements should not use + concatenation for their first argument
  • G004 Logging statements should not use f"..." for their first argument (only in Python 3.6+)
  • G010 Logging statements should not use warn (use warning instead)
  • G100 Logging statements should not use extra arguments unless whitelisted
  • G101 Logging statement should not use extra arguments that clash with LogRecord fields
  • G200 Logging statements should not include the exception in logged string (use exception or exc_info=True)
  • G201 Logging statements should not use error(..., exc_info=True) (use exception(...) instead)
  • G202 Logging statements should not use redundant exc_info=True in exception

These violations are disabled by default. To enable them for your project, specify the code(s) in your setup.cfg:



Our motivation has to do with balancing the needs of our team and those of our customers. On the one hand, developers and front-line support should be able to look at application logs. On the other hand, our customers don't want their data shared with anyone, including internal employees.

The implementation approaches this in two ways:

  1. By trying to prevent the use of string concatenation in logs (vs explicit variable passing in the standard logging extra dictionary)

  2. By providing an (optional) mechanism for whitelisting which field names may appear in the extra dictionary

Naturally, this does not prevent developers from doing something like:


but then avoiding a case like this falls back to other processes around pull-requests, code review and internal policy.