id: python-mutable-class-attr
name: Mutable Class Attribute
severity: warning
category: reliability
defect_class: correctness
inline_tier: blocking
language: python

message: "Class attribute '{{VARNAME}}' is mutable — shared across all instances"

description: |
  Mutable class-level attributes (list, dict, set) are shared across every instance.
  Modifying them on one instance modifies ALL instances.
  
  ❌ BUG:
    class Node:
        children = []  # shared! Node().children is Node().children
  
  ✅ FIX: Initialise in __init__:
    class Node:
        def __init__(self):
            self.children = []

query: |
  (class_definition
    body: (block
      (expression_statement
        (assignment
          left: (identifier) @VARNAME
          right: [
            (list) @VALUE
            (dictionary) @VALUE
            (set) @VALUE
          ]))))

metavars:
  - VARNAME
  - VALUE

post_filter: not_in_function

has_fix: false

tags:
  - reliability
  - oop
  - shared-state

examples:
  bad: |
    class Node:
        children = []   # all Node instances share one list
        metadata = {}   # all Node instances share one dict
  
  good: |
    class Node:
        def __init__(self):
            self.children = []
            self.metadata = {}