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Python String Formatting Style (SFS) plugin for flake8

Project description

flake8-sfs - Python String Formatting Style Plugin

Released on the Python Package Index (PyPI) Released on Conda Testing with TravisCI PyPI downloads Code style: black


This is an MIT licensed flake8 plugin for enforcing a Python string formatting preference. It is available to install from the Python Package Index (PyPI).

For historical reasons, the Python programming language has accumlated multiple ways to do string formatting. The three main ones are:

  • Percent operator (violation codes SFS1##), as in this example:
>>> name = "Peter"
>>> price = 1.2
>>> print("Hello %s, do you have $%0.2f?" % (name, price))
Hello Peter, do you have $1.20?
  • Format method (violation codes SFS2##), available since Python 2.6,
>>> name = "Peter"
>>> price = 1.2
>>> print("Hello {}, do you have ${:0.2f}?".format(name, price))
Hello Peter, do you have $1.20?
  • f-strings (violation codes SFS3##), available since Python 3.6,
>>> name = "Peter"
>>> price = 1.2
>>> print(f"Hello {name}, do you have ${price:0.2f}?")

We are spoilt for choice, but quoting the Zen of Python, There should be one - and preferably only one - obvious way to do it. This flake8 plugin exists to let you define which of these styles your project allows.

By default this plugin complains about all three styles - we expect you to make an explicit choice and configure which codes to ignore. See uses cases below.

Flake8 Validation codes

Early versions of flake8 assumed a single character prefix for the validation codes, which became problematic with collisions in the plugin ecosystem. Since v3.0, flake8 has supported longer prefixes therefore this plugin uses SFS as its prefix (short for String Format Style).

Prefix SFS1 - percent operator:

Code Description
SFS101 String literal formatting using percent operator.
SFS102 Bytes literal formatting using percent operator.

Prefix SFS2 - format method:

Code Description
SFS201 String literal formatting using .format method.
SFS202 String formatting with str.format(‘…’, …) directly.

Prefix SFS3 - f-strings:

Code Description
SFS301 String literal formatting using f-string.

You can use a partial code like SFS1 in flake8 to ignore all the SFS1## percent formatting codes.

Use cases

If you accept that f-strings are best, you could run a tool like flynt to automatically convert all your code - and then use this flake8 plugin to enforce the style by configuring it to ignore the SFS301 violation.

You might be maintaining a project which still supports Python 2, where you have a mix of percent and format method string formatting. Here tell flake8 to ignore the SFS1 and SFS2 prefixes, and complain only about f-strings which would be a syntax error on Python 2 (i.e. enforce only prefix SFS3).

Alternatively, you might have a large legacy codebase with lots of the percent formatting - yet want to move any format methods to f-strings. Here you could ignore the SFS1 and SFS3 prefixes and enforce only the format method checks (SFS2 prefix).

Or you might say the old ways are the best, and configure flake8 to ignore the percent formatting but treat either the format method or f-strings as errors (by ignoring the SFS1 prefix).

Installation and usage

Python 3.6 or later is required (as we use need to parse the Python syntax which may include f-strings), but flake8 and this plugin can still be used on code intended support older versions of Python.

We recommend installing the plugin using pip, which handles the dependencies:

$ pip install flake8-sfs

Alternatively, if you are using the Anaconda packaging system, the following command will install the plugin with its dependencies:

$ conda install -c conda-forge flake8-sfs

The new validator should be automatically included when using flake8 which may now report additional validation codes starting with SFS (as defined above). For example:

$ flake8

You can request only the SFS codes be shown using:

$ flake8 --select SFS

You should add at least some SFS validation codes to your flake8 configuration file’s select or ignore list.


We assume you are familiar with flake8 configuration.

Unless your code performs no string formatting at all (which would be unusual), you should tell flake8 to ignore at least one of this plugin’s violation codes. For example:

extend-ignore =
    # Ignore f-strings, we like them:

Note that flake8 allows splitting comma separated lists over multiple lines, and allows including of hash comment lines.

Version History

Version Released Changes
v0.0.3 2020-01-22
  • Updates to documentation and PyPI metadata.
v0.0.2 2020-01-12
  • Codes now have a heirachy (so can ignore whole groups).
  • Added str.format("...", ...) check.
v0.0.1 2020-01-11
  • Initial public release (initial codes later reallocated).


This plugin is on GitHub at

To make a new release once tested locally and on TravisCI:

$ git tag vX.Y.Z
$ python sdist --formats=gztar
$ twine upload dist/flake8-sfs-X.Y.Z.tar.gz
$ git push origin master --tags

The PyPI upload should trigger an automated pull request updating the flake8-sfs conda-forge recipe.

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