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`__y` is not considered positional-only

crates/ty_python_semantic/resources/lint_docs/invalid-legacy-positional-parameter.md

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What it does

Checks for parameters that appear to be attempting to use the legacy convention to specify that a parameter is positional-only, but do so incorrectly.

The "legacy convention" for specifying positional-only parameters was specified in PEP 484. It states that parameters with names starting with __ should be considered positional-only by type checkers. PEP 570, introduced in Python 3.8, added dedicated syntax for specifying positional-only parameters, rendering the legacy convention obsolete. However, some codebases may still use the legacy convention for compatibility with older Python versions.

Why is this bad?

In most cases, a type checker will not consider a parameter to be positional-only if it comes after a positional-or-keyword parameter, even if its name starts with __. This may be unexpected to the author of the code.

Example

python
# `__y` is not considered positional-only
def f(x, __y):  # error
    pass

Use instead:

python
def f(__x, __y):  # If you need compatibility with Python <=3.7
    pass

or:

python
def f(x, y, /):  # Python 3.8+ syntax
    pass

References