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match statement (structural pattern matching)

Monty implements PEP 634 match statements: value, singleton, capture, wildcard, sequence (with one *rest), mapping (with **rest), class, as and | patterns, with guards. Names are bound only after the whole pattern matches, before the guard runs, as in CPython, so a case whose pattern fails part-way leaves no bindings behind and a case whose guard fails leaves them all. The compile-time SyntaxErrors match CPython’s wording and location: duplicate captures, alternatives binding different names, an unguarded irrefutable pattern before the last case, duplicate constant mapping keys, repeated class-pattern attributes and two starred names. @dataclass classes get __match_args__ (the field names, in order) and collections.namedtuple classes expose it too, so positional class patterns work on both.

Sequence and mapping patterns

  • Only builtin containers match. Sequence patterns accept list, tuple, namedtuples, range and deque; mapping patterns accept dict, defaultdict and Counter. Monty has no inheritance and no collections.abc, so no user or host class can be a Sequence or Mapping, and its instances never match these patterns.

Class patterns

  • isinstance has no inheritance. A class pattern on a user-defined or host class matches instances of exactly that class; see classes.md.
  • Host classes only have the __match_args__ the host sent as a class attribute. Without one, a positional sub-pattern raises TypeError: Cls() accepts 0 positional sub-patterns (1 given) even when the host-side class is a dataclass; keyword sub-patterns (case Cls(x=0)) work.

Parse-time checks

  • A pattern after **rest reports SyntaxError: Pattern cannot follow a double star pattern (Ruff’s wording) where CPython says invalid syntax.
  • Bytecode operand limits. A sequence, mapping or class pattern with more than 255 sub-patterns, or a pattern that captures more than 256 names, raises SyntaxError: too many sub-patterns in ... pattern (or too many names in pattern) at compile time; CPython has no such limit.