flake8-sfs 0.0.2

Introduction

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.

For example, 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.

Alternatively, you might have a large legacy codebase with lots of the percent formatting - yet want to gradually move to f-strings. Here you could use this plugin to enforce a rule allowing those but rejecting the string format method.

Or you might the 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 default this plugin complains about all three styles - we expect you to make an explicit choice and configure which codes to ignore.

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:

Prefix SFS2 - format method:

Prefix SFS3 - f-strings:

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

Installation and usage

Python 3.6 or later is required (as we use need to parse the Python syntax which may include f-strings).

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 example.py

You can request only the SFS codes be shown using:

$ flake8 --select SFS example.py

Similarly you might add particular SFS validation codes to your flake8 configuration file’s select or ignore list.

Configuration

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:

[flake8]
extend-ignore =
    # Ignore f-strings, we like them:
    SFS103,

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

Version History

Developers

This plugin is on GitHub at https://github.com/peterjc/flake8-sfs

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

$ git tag vX.Y.Z
$ python setup.py 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.

TODO

• Can/should we spot byte formatting?

• Can we spot pointless f-strings? See flake8-pie’s PIE782 code.