How to use values stored in variables as case patterns?
Feb 11
I'm trying to understand the new structural pattern matching syntax in Python 3.10. I understand that it is possible to match on literal values like this:
def handle(retcode):
match retcode:
case 200:
print('success')
case 404:
print('not found')
case _:
print('unknown')
handle(404)
# not found
However, if I refactor and move these values to module-level variables, it results in an error because the statements now represent structures or patterns rather than values:
SUCCESS = 200
NOT_FOUND = 404
def handle(retcode):
match retcode:
case SUCCESS:
print('success')
case NOT_FOUND:
print('not found')
case _:
print('unknown')
handle(404)
# File "<ipython-input-2-fa4ae710e263>", line 6
# case SUCCESS:
# ^
# SyntaxError: name capture 'SUCCESS' makes remaining patterns unreachable
Is there any way to use the match statement to match values that are stored within variables?
python1 answer
Accepted answer · original discussion
Feb 11
If the constant you're testing against is a dotted name, then it should be treated as a constant instead of as the name of the variable to put the capture in (see PEP 636 # Matching against constants and enums):
class Codes:
SUCCESS = 200
NOT_FOUND = 404
def handle(retcode):
match retcode:
case Codes.SUCCESS:
print('success')
case Codes.NOT_FOUND:
print('not found')
case _:
print('unknown')
Although, given how python is trying to implement pattern-matching, I think that for situations like this it's probably safer and clearer code to just use an if/elif/else tower when checking against constant values.
2 question comments
Use comments to ask for clarification. Post a solution as an answer.
Feb 11
SUCCESS should be interpreted as a capture pattern.Feb 11