slice-to-remove-prefix-or-suffix (FURB188)

Derived from the refurb linter.

Fix is always available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for the removal of a prefix or suffix from a string by assigning the string to a slice after checking .startswith() or .endswith(), respectively.

Why is this bad?

The methods [str.removeprefix][str.removeprefix] and [str.removesuffix][str.removesuffix], introduced in Python 3.9, have the same behavior and are more readable and efficient.

Example

filename[:-4] if filename.endswith(".txt") else filename

if text.startswith("pre"):
    text = text[3:]

Use instead:

filename = filename.removesuffix(".txt")

text = text.removeprefix("pre")

[str.removeprefix][str.removeprefix]: https://docs.python.org/3/library/stdtypes.html#str.removeprefix [str.removesuffix][str.removesuffix]: https://docs.python.org/3/library/stdtypes.html#str.removesuffix