Python 3.10+astterm

ast.MatchClass

pattern matching

A class pattern case Point(x, y=0): cls is the class name, patterns the positional patterns, kwd_attrs and kwd_patterns the keyword ones.

Syntax

ast.MatchClass(cls, patterns, kwd_attrs, kwd_patterns)

It looks like a call, but no constructor runs: isinstance is checked, then attributes. Positional patterns work only if the class has __match_args__ (a dataclass sets it for you), otherwise matching raises TypeError. kwd_attrs is a list of strings, not nodes. For built-in types 'case int(n)' captures the object itself.

Examples

import ast
p = ast.parse('match o:\n    case Point(x, y=0): pass').body[0].cases[0].pattern
print(type(p).__name__, p.cls.id, len(p.patterns), p.kwd_attrs)  # → MatchClass Point 1 ['y']

class P:
    def __init__(self, x):
        self.x = x
try:
    match P(1):
        case P(v):
            pass
except TypeError as e:
    print(type(e).__name__)  # → TypeError

See also

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