flake8-spm

Flake8 plugin for finding issues with structural pattern matching

NOTE: this plugin is under development. Coming up soon!

Motivation

Structural pattern matching has been introduced in PEP 634 with the tutorial added in PEP 636.

The present plugin intends to catch default cases, which do not match any pattern, but do not raise an exception.

In the problematic example below we handle unexpected pattern as Default.

def func(value):
    match value:
        case 1:
            return 'One'
        case _:
            return 'Default'  # <-- ignoring exceptional case

However, for the non-matching cases we would better raise an error, to follow fail-fast methodology. Indeed, if we could not find a mathing pattern, then it is an unexpected event, and we need to raise an error, as shown below.

def func(value):
    match value:
        case 1:
            return 'One'
        case _:
            raise ValueError(f'Unexpected case: {value}')

Then in the client code we would handle this error appropriately, instead of sweeping the issue under the rug.

List of warnings

SPM001: not raising when matching default value.

Installation (IN PROGRESS)

Install via pip with:

$ pip install flake8-spm

It will then automatically be run as part of flake8. You can check is has been picked up via:

$ flake8 --version