I’m trying to undertand 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 on values that are stored within variables?
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.