MIGRATING.md

Migration guides

0.5 -> 0.6 migration

The 0.6 migration includes two breaking changes and some useful new functionality:

1. The Result.Ok() and Result.Err() class methods have been removed. These should be replaced by direct use of Ok() and Err(). As an example, the following code:

from result import Result
res1 = Result.Ok('yay')
res2 = Result.Err('nay')

should be replaced by:

from result import Ok, Err
res1 = Ok('yay')
res2 = Err('nay')

2. Result is now a Union type between Ok[T] and Err[E]. As such, you cannot use isinstance(res, Result) anymore. These should be replaced by isinstance(res, OkErr). As an example, the following code:

from result import Ok, Result
res = Ok('yay')
if isinstance(res, Result):
    print("Result type!")

should be replaced with:

from result import Ok, OkErr
res = Ok('yay')
if isinstance(res, OkErr):
    print("Result type!")

3. Because Result is now a union type MyPy can statically check the Result type. In previous versions MyPy saw the following types:

r: Result[int, str] = Ok(2)
if r.is_ok():
    reveal_type(r.value) # returns Union[int, str]

but now, by using isinstance:

r: Result[int, str] = Ok(2) # == Union[Ok[int], Err[str]]
if isinstance(r, Ok):
    reveal_type(r.value) # returns int

This allows for better type checking, but requires the use of isinstance instead of is_ok() or is_err().