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String format parameter checker

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An extension for Flake8 to check the strings and parameters using str.format. It checks all strings whether they use numbered parameters with an implicit index which isn't support in Python 2.6.

In all instances of '…'.format(…) it will also check whether there are enough parameters given. If the format call uses variable arguments, it'll just check whether the right types of arguments are present.

Plugin for Flake8

When both Flake8 and flake8-string-format are installed, the plugin is available in flake8:

$ flake8 --version
3.0.2 (flake8-string-format: 0.2.3, […]

This plugin supports Flake8 2.6 as well as Flake8 3.0. Older or newer versions may be supported too but they weren't tested.

Via --ignore it's possible to ignore unindexed parameters:

$ flake8
... FMT101 format string does contain unindexed parameters

$ flake8 --ignore FMT101


This module doesn't add any additional parameters to Flake8.

Error codes

This plugin is using the following error codes:

Presence of implicit parameters
FMT101 format string does contain unindexed parameters
FMT102 docstring does contain unindexed parameters
FMT103 other string does contain unindexed parameters
Missing values in the parameters
FMT201 format call uses too large index (INDEX)
FMT202 format call uses missing keyword (KEYWORD)
FMT203 format call uses keyword arguments but no named entries
FMT204 format call uses variable arguments but no numbered entries
FMT205 format call uses implicit and explicit indexes together
Unused values in the parameters
FMT301 format call provides unused index (INDEX)
FMT302 format call provides unused keyword (KEYWORD)


The plugin will go through all bytes, str and unicode instances. If it encounters bytes instances on Python 3, it'll decode them using ASCII and if that fails it'll skip that entry.

Depending on the usage the string is handled differently. When it is not being formatted, it can only cause FMT102 and FMT103. For this plugin all strings which are the first expression of the module or after a function or class definition are considered docstrings.

Both FMT102 and FMT103 issue many false positives and should only be used with Python 2.6 which does not support unindexed parameters.

Format strings

Every string where either the format method is called or where it is the first parameter of str.format, is considered a format string and cause the higher numbers.

If that call to format uses variable arguments, it cannot issue FMT201 and FMT202 as missing entries might be hidden in those variable arguments. FMT301 and FMT302 can still be checked for any argument which is defined statically.

Python 2.6 support

Python 2.6 is only partially supported as it's using Python's capability to format a string. So if a string contains implicit parameters, it won't be detected as a parameter on Python 2.6 and thus it won't cause any FMT1XX errors. But it might still cause an error FMT301 when variable arguments aren't used.

So if Python 2.6 compatibility is wished and thus implicit parameters aren't allowed, this plugin won't cause false positives.


0.3.0 - 2020-02-16

  • Removed support for standalone version.
  • Support multiple starargs and at any location.

0.2.3 - 2016-07-27

  • Properly register with Flake8 so it will be selected on Flake8 3.x by default and it can be selected on Flake8 2.x.

0.2.2 - 2016-05-29

  • Do not check simple expressions, except for docstrings, because they cannot be accessed anyway.
  • Properly assert starred arguments in Python 3.5. Only the last element must be a vararg if varargs are present and not the complete list.
  • Output correct column offset on Python 3.4.2, as that used the wrong offset inside calls.

0.2.1 - 2015-09-20

  • Support str.format("…", …) calls and handle them like "…".format(…)

0.2.0 - 2015-09-12

  • Instead of using a regex it's trying to parse it using Python's parser
  • This result can also be used now to verify that enough parameters are given
  • Limited Python 2.6 support

0.1.0 - 2015-09-10

  • Detect unindexed parameters in all strings
  • Separate error code for docstrings


Check that indexed parameters are used in strings







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