# Mutable Default Argument
# Detects mutable defaults in function arguments (common Python pitfall)
id: mutable-default-arg
name: Mutable Default Argument
severity: warning
category: reliability
defect_class: correctness
inline_tier: blocking
language: python

message: "Mutable default argument — list/dict/set as default value"

description: |
  Mutable default arguments are evaluated once at function definition time,
  not each call. This causes unexpected shared state between calls.
  
  ✅ FIX: Use None as default, create mutable inside function
  
  ```python
  def process(items=None):  # GOOD
      if items is None:
          items = []
      items.append(item)
  ```

query: |
  (function_definition
    (parameters
      (default_parameter
        (identifier) @PARAM
        [(list) (dictionary) (set)] @MUTABLE)))

metavars:
  - PARAM
  - MUTABLE

tags:
  - reliability
  - common-mistake
  - python-specific

examples:
  bad: |
    def add_item(item, items=[]):  # BAD - shared list!
        items.append(item)
        return items
    
    print(add_item(1))  # [1]
    print(add_item(2))  # [1, 2] - unexpected!
  
  good: |
    def add_item(item, items=None):  # GOOD
        if items is None:
            items = []
        items.append(item)
        return items

has_fix: true
fix_action: convert_to_none_default