Mapping Pattern Matching (Dictionaries)

1Concept

Mapping patterns match dictionary keys and bind values. Unlike sequence patterns, mapping patterns perform partial matching: the dict can contain extra keys not specified in the case pattern.

2Architecture Diagram

case {"type": "alert", "level": "CRITICAL", **rest}: ---> Matches even if payload has 10 other keys!

3Code Example

def inspect_event(event: dict):
    match event:
        case {"type": "LOGIN", "status": "FAILED", "ip": ip}:
            return f"SECURITY ALERT: Failed login from {ip}"
        case {"type": "LOGIN", "status": "SUCCESS", "user": user}:
            return f"User logged in: {user}"
        case _:
            return "Standard event"

print(inspect_event({"type": "LOGIN", "status": "FAILED", "ip": "192.168.1.55", "time": 1709420000}))
print(inspect_event({"type": "LOGIN", "status": "SUCCESS", "user": "alice"}))

4Expected Output

SECURITY ALERT: Failed login from 192.168.1.55
User logged in: alice

5Key Takeaways

• ✓Mapping patterns match if the specified keys exist, ignoring unmentioned keys.
• ✓Use `**rest` to capture unmentioned key-value pairs into a dictionary.
• ✓Keys must be literals or attribute lookups (`Enum.KEY`).