feature_06_pattern_matching.md

6. Structural Pattern Matching

Concept summary

Structural pattern matching (match / case) branches on data shape and values together. It can inspect dictionaries, sequences, class patterns, and guards in one construct.

What C# developers usually expect

C# switch and pattern matching are familiar, but Python emphasizes structural deconstruction of runtime data (especially dictionaries/lists) with concise syntax.

How this example works

The function matches an incoming dictionary. Case 1: {"kind": "http", "status": status} with guard status >= 500 returns server-error. Case 2: same shape without the guard returns http-{status}. Fallback case _ returns unknown. Input {"kind": "http", "status": 503} matches the guarded case first.

Why this matters in production

This style is useful for event routers, message handlers, protocol parsers, and API response classification. It reduces nested if/elif trees and keeps matching logic declarative.

Common mistakes

Assuming match is only value equality. Order matters: first matching case wins. Overusing very complex patterns can hurt readability; keep patterns focused.

Source

• feature_06_pattern_matching.py

Exact example from code file

def describe(value):
    match value:
        case {"kind": "http", "status": status} if status >= 500:
            return "server-error"
        case {"kind": "http", "status": status}:
            return f"http-{status}"
        case _:
            return "unknown"


def main() -> None:
    print(describe({"kind": "http", "status": 503}))


if __name__ == "__main__":
    main()

Run

uv run src/csharp_to_python_learning/python_only_features/feature_06_pattern_matching.py

Expected output

• server-error