I would like to understand how pattern matching works in Python.
I know that I can match a value like so:
>>> t = 12.0
>>> match t:
... case 13.0:
... print("13")
... case 12.0:
... print("12")
...
12
But I notice that when I use matching with a type like float(), it matches 12.0:
>>> t = 12.0
>>> match t:
... case float():
... print("13")
... case 12.0:
... print("12")
...
13
This seems strange, because float() evaluates to 0.0, but the results are different if that is substituted in:
>>> t = 12.0
>>> match t:
... case 0.0:
... print("13")
... case 12.0:
... print("12")
...
12
I would expect that if 12.0 matches float(), it would also match 0.0.
There are cases where I would like to match against types, so this result seems useful. But why does it happen? How does it work?
The thing that follows the case keyword is not an expression, but special syntax called a pattern.
0.0 is a literal pattern. It checks equality with 0.0.
float() is a class pattern. It checks that the type is float. Since it is not an expression, it isn't evaluated and therefore is different from 0.0.