pi-lens/rules/tree-sitter-queries/python/slots-assignment-mismatch.yml

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# Slotted Class Attribute Assignment
# Detects `self.X = ...` inside a class with `__slots__` when X is NOT in the slots list.
id: slots-assignment-mismatch
name: Slotted Class Attribute Assignment
severity: warning
category: reliability
defect_class: convention
inline_tier: blocking
language: python

message: "Attribute '{{name}}' assigned in class with __slots__ but not declared"

description: |
  Python `__slots__` declares a fixed set of allowed attributes for
  instances of a class. Assigning to an attribute NOT in the slots
  list will raise `AttributeError` at runtime (unless the parent
  class has `__dict__`, in which case it's allowed but defeats the
  purpose of __slots__).

  Matches: a `self.X = ...` assignment inside any method of a class
  whose body contains `__slots__ = ...`. Post-filter:
  1. Parse the `__slots__` assignment (single string, tuple, or list)
  2. If the assigned attribute X is in the slots list → suppress
  3. Otherwise → fire

  Caveat: If the parent class has its own `__dict__` (e.g. generated
  ANTLR `ParserRuleContext` classes), the child's `__slots__` is
  effectively a no-op for the purpose of preventing attribute
  creation. SonarCloud S8494 still flags these as a code-smell
  (misleading intent), but no runtime error will occur.

  ✅ FIX: Add the attribute to `__slots__`, or remove `__slots__`.

query: |
  (function_definition
    body: (block
      (expression_statement
        (assignment
          left: (attribute
            object: (identifier) @SELF
            attribute: (identifier) @ATTR)
          right: (_) @VAL)))) @METHOD

metavars:
  - SELF
  - ATTR
  - VAL
  - METHOD

post_filter: slots_attribute_mismatch

has_fix: false

tags:
  - reliability
  - python-specific
  - slots
  - attribute-assignment

examples:
  bad: |
    class Foo:
        __slots__ = ("x",)
        def __init__(self):
            self.x = 1       # OK - in slots
            self.y = 2       # BAD - raises AttributeError
  
  good: |
    class Foo:
        __slots__ = ("x", "y")
        def __init__(self):
            self.x = 1
            self.y = 2