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String format parameter checker

Build Status Coverage Status Pypi Entry

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
... P101 format string does contain unindexed parameters

$ flake8 --ignore P101


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

Error codes

This plugin is using the following error codes:

Presence of implicit parameters
P101 format string does contain unindexed parameters
P102 docstring does contain unindexed parameters
P103 other string does contain unindexed parameters
Missing values in the parameters
P201 format call uses too large index (INDEX)
P202 format call uses missing keyword (KEYWORD)
P203 format call uses keyword arguments but no named entries
P204 format call uses variable arguments but no numbered entries
P205 format call uses implicit and explicit indexes together
Unused values in the parameters
P301 format call provides unused index (INDEX)
P302 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.

The strings are basically sorted into three types corresponding to the P1XX range. Only the format string can cause all errors while any other string can only cause the corresponding P1XX error.

For this plugin all strings which are the first expression of the module or after a function or class definition are considered docstrings.

If the format method is used on a string or str.format with the string as the first parameter, it will consider this a format string and will analyze the parameters of that call. If that call uses variable arguments, it cannot issue P201 and P202 as missing entries might be hidden in those variable arguments. P301 and P302 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 P1XX errors. But it might still cause an error P301 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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