Python version of Rust Result Sum Type

Jan 19, 2025 · 1 min read · python sum type rust ·

I try to implement a Python version of the Result<T, E> sum type in rust which can be used with match/case pattern matching.

The pattern matching with sub-patterns like case Err(e) could be implemented in Python with the __match_args__ dunder attribute. This is how Result[T, E], OK() and Err() defined:

 1import typing
 2
 3T = typing.TypeVar('T')
 4E = typing.TypeVar('E')
 5
 6class Result[T, E]:
 7    __match_args__ = ('_val',)
 8    def __init__(self, val):
 9        self._val = val
10
11class Ok[T](Result[T, ...]):
12    pass
13
14class Err[E](Result[..., E]):
15    pass

Below is the match/case block, note that if you have different types value to match(like different Error types), the guard(if statement behind the case pattern) must be used.

 1>>> def match(value):  
 2... match value:  
 3... case Ok(v): print(v)  
 4... case Err(e) if type(e) is str: print('str', e)  
 5... case Err(e) if type(e) is int: print('int', e)  
 6...  
 7>>> ok_result = Ok('Hello World')  
 8... e1 = Err('incorrect value')  
 9... e2 = Err(404)  
10...  
11>>> match(ok_result)  
12Hello World  
13>>> match(e1)  
14str incorrect value  
15>>> match(e2)  
16int 404