FCN001: False positive for positional-only parameters (Python 3.8+)

[rlobillo]

Description

FCN001 raises a false positive when calling functions with positional-only parameters, which were introduced in Python 3.8 (PEP 570).

Example

import socket
socket.setdefaulttimeout(30)  # FCN001 error: function should be called with keywords arguments

Problem

socket.setdefaulttimeout() has a positional-only parameter signature:

>>> import inspect
>>> inspect.signature(socket.setdefaulttimeout)
(object, /)

The / syntax means the parameter cannot accept keyword arguments. Attempting to use keywords results in:

>>> socket.setdefaulttimeout(timeout=30)
TypeError: _socket.setdefaulttimeout() takes no keyword arguments

Expected Behavior

FCN001 should detect positional-only parameters (where param.kind == inspect.Parameter.POSITIONAL_ONLY) and not require keyword arguments for those functions.

Actual Behavior

FCN001 raises an error even when using keyword arguments would cause a TypeError.

Environment

• Python: 3.14.0b2 (also affects 3.8+)
• flake8-plugins: v1.0.0

Suggested Fix

Check if parameters are positional-only before enforcing keyword arguments:

import inspect

sig = inspect.signature(func)
for param in sig.parameters.values():
    if param.kind == inspect.Parameter.POSITIONAL_ONLY:
        # Skip keyword argument check for positional-only params
        continue

Related

• PEP 570 – Python Positional-Only Parameters