pi-lens/rules/tree-sitter-queries/python/string-format-arity-mismatch.yml

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# String Formatting Argument Count Mismatch
# Detects %-formatting calls with wrong argument counts
id: string-format-arity-mismatch
name: String Format Argument Count Mismatch
severity: error
category: reliability
defect_class: correctness
inline_tier: blocking
language: python

message: "String format has wrong number of arguments — RuntimeError at format time"

description: |
  `%`-style formatting throws TypeError if the number of arguments
  doesn't match the format spec. Static detection catches most cases:
  - `"%s" % (1, 2)` — too few / too many args
  - `"%s %s" % (1,)` — too few
  - `"%(a)s" % {"b": 1}` — missing key

  ✅ FIX: Match format string placeholders to argument count,
     or use f-strings.

query: |
  (binary_operator
    left: (string) @FORMAT
    operator: "%"
    right: [(tuple) (dictionary)] @ARGS)

metavars:
  - FORMAT
  - ARGS

post_filter: format_arity_mismatch

has_fix: false

tags:
  - reliability
  - python-specific
  - format-string
  - runtime-error

examples:
  bad: |
    "%s" % (1, 2)         # TypeError: not all arguments converted
    "%s %s" % (1,)        # TypeError: not enough arguments
    "%(a)s" % {"b": 1}    # KeyError: 'a'
  
  good: |
    "%s" % 1              # OK
    "%s %s" % (1, 2)      # OK
    "%(a)s" % {"a": 1}    # OK