pi-lens/rules/tree-sitter-queries/python/python-special-method-arity.yml

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# Special Method Arity Violation
# Detects __init__, __eq__, __hash__, etc. with wrong parameter counts
id: python-special-method-arity
name: Special Method Wrong Arity
severity: error
category: reliability
defect_class: correctness
inline_tier: blocking
language: python

message: "Special method {{NAME}} has wrong arity — Python protocol expects {{EXPECTED}}"

description: |
  Python's data model expects specific arities for dunder methods:
  - `__init__(self, ...)` — variable args (after self)
  - `__new__(cls, ...)` — variable args (after cls)
  - `__del__(self)` — exactly self
  - `__repr__(self)` — exactly self
  - `__str__(self)` — exactly self
  - `__hash__(self)` — exactly self
  - `__bool__(self)` — exactly self
  - `__len__(self)` — exactly self
  - `__eq__(self, other)` — exactly self + other
  - `__lt__`, `__le__`, `__gt__`, `__ge__`, `__ne__` — self + other

  The rule uses `count_params` post_filter with min_params to
  flag missing parameters. The post_filter is run only against
  function names matching the special-method pattern.

  Note: SonarCloud S5722 in full also checks `__add__`,
  `__mul__`, etc. — full list is 30+ methods. This rule covers
  the most common ones.

  ✅ FIX: Add the required parameters

query: |
  (function_definition
    name: (identifier) @NAME
    parameters: (parameters) @PARAMS)
  (#match? @NAME "^__(init|new|del|repr|str|hash|bool|len|eq|lt|le|gt|ge|ne)__$")

metavars:
  - NAME
  - PARAMS

post_filter: special_method_arity

has_fix: false

tags:
  - reliability
  - python-specific
  - data-model
  - dunder

examples:
  bad: |
    class Foo:
        def __eq__(self): pass  # BAD - missing 'other'
        def __hash__(self, x): pass  # BAD - extra arg
        def __repr__(self, x): pass  # BAD - extra arg
  
  good: |
    class Foo:
        def __eq__(self, other): return self.x == other.x
        def __hash__(self): return hash(self.x)
        def __repr__(self): return f"Foo({self.x})"