Structural dispatch
Dispatch based on the shape or contents of dictionaries, sequences, or objects.
Python · Const
case
case introduces a pattern-matching block inside Python’s match statement. Cases are checked in order, can bind matched values, and may include guards or a wildcard fallback.
- Since
- 3.10
How it works
case introduces an individual pattern-and-action block inside a match statement. Python tests cases in source order, optionally evaluates a guard after a pattern succeeds, and runs the first successful case; a wildcard can provide a fallback.
Syntax
case patterns [guard] : suiteKey points
- Each case supplies the pattern against which the value provided to the enclosing match statement is tested.
- Patterns can be composed from smaller subpatterns, enabling nested structural checks.
- A successful pattern can bind portions of the subject to names for use in that case's suite.
- The case form consists of the keyword, a pattern, an optional guard, a colon, and an indented suite.
Common mistakes
caseis a soft keyword, so it remains usable as an identifier in contexts where the parser does not interpret it as part of pattern matching.- Without a matching case or wildcard fallback, the match statement performs no case action; provide
_when an unmatched input needs explicit handling. - A case block may include an optional guard expression after its pattern, allowing additional conditions beyond structural pattern matching.
Example: Matching mapping shapes
def describe(value):
match value:
case {"status": "ok", "data": data}:
return f"success: {data}"
case {"status": "error", "message": message}:
return f"failure: {message}"
case _:
return "unknown"
print(describe({"status": "ok", "data": 42}))This function matches mapping shapes, binds selected values, and uses a wildcard for all other inputs.
When to use it
Destructuring input
Extract fields from a matched value while selecting the handling branch.
Guarded cases
Add a boolean condition after a structural pattern succeeds.
Interview questions
QWhat is the role of case in Python pattern matching?
case defines a pattern block within match. The pattern is compared with the subject, and the block's suite runs when matching succeeds.
Follow-up: How are multiple cases selected? Python checks them from top to bottom and executes the first successful case.
QHow does a guard affect a case block?
A case can contain a pattern and an optional guard. The guard is evaluated after the pattern matches, and the suite runs only when that guard is true.
Follow-up: What happens when the guard is false? Python continues considering later case blocks.
QHow do you handle unmatched subjects?
case _: is the wildcard fallback. It matches when no earlier pattern succeeds, so it is useful for handling remaining input shapes.
Follow-up: Is the wildcard mandatory? No; without it, an unmatched subject simply leaves the match without selecting a case.
Related
match statementpatternsguardswildcard pattern `_`soft keywords