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Completes #41 

* Add pytest-mypy-plugins for type tests

* Remove typetests.py

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A simple Result type for Python 3 inspired by Rust, fully type annotated.

The idea is that a result value can be either Ok(value) or Err(error), with a way to differentiate between the two. Ok and Err are both classes encapsulating an arbitrary value. Result[T, E] is a generic type alias for typing.Union[Ok[T], Err[E]]. It will change code like this:

def get_user_by_email(email: str) -> Tuple[Optional[User], Optional[str]]:
    """
    Return the user instance or an error message.
    """
    if not user_exists(email):
        return None, 'User does not exist'
    if not user_active(email):
        return None, 'User is inactive'
    user = get_user(email)
    return user, None

user, reason = get_user_by_email('ueli@example.com')
if user is None:
    raise RuntimeError('Could not fetch user: %s' % reason)
else:
    do_something(user)

To something like this:

from result import Ok, Err, Result

def get_user_by_email(email: str) -> Result[User, str]:
    """
    Return the user instance or an error message.
    """
    if not user_exists(email):
        return Err('User does not exist')
    if not user_active(email):
        return Err('User is inactive')
    user = get_user(email)
    return Ok(user)

user_result = get_user_by_email(email)
if isinstance(user_result, Ok):
    # type(user_result.value) == User
    do_something(user_result.value)
else:
    # type(user_result.value) == str
    raise RuntimeError('Could not fetch user: %s' % user_result.value)

As this is Python and not Rust, you will lose some of the advantages that it brings, like elegant combinations with the match statement. On the other side, you don't have to return semantically unclear tuples anymore.

Not all methods (https://doc.rust-lang.org/std/result/enum.Result.html) have been implemented, only the ones that make sense in the Python context. By using isinstance to check for Ok or Err you get type safe access to the contained value when using MyPy to typecheck your code. All of this in a package allowing easier handling of values that can be OK or not, without resorting to custom exceptions.

API

Creating an instance:

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

Checking whether a result is Ok or Err. With isinstance you get type safe access that can be checked with MyPy. The is_ok() or is_err() methods can be used if you don't need the type safety with MyPy:

>>> res = Ok('yay')
>>> isinstance(res, Ok)
True
>>> isinstance(res, Err)
False
>>> res.is_ok()
True
>>> res.is_err()
False

You can also check if an object is Ok or Err by using the OkErr type. Please note that this type is designed purely for convenience, and should not be used for anything else. Using (Ok, Err) also works fine:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> isinstance(res1, OkErr)
True
>>> isinstance(res2, OkErr)
True
>>> isinstance(1, OkErr)
False
>>> isinstance(res1, (Ok, Err))
True

Convert a Result to the value or None:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.ok()
'yay'
>>> res2.ok()
None

Convert a Result to the error or None:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.err()
None
>>> res2.err()
'nay'

Access the value directly, without any other checks:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.value
'yay'
>>> res2.value
'nay'

Note that this is a property, you cannot assign to it. Results are immutable.

For your convenience, simply creating an Ok result without value is the same as using True:

>>> res1 = Ok()
>>> res1.value
True

The unwrap method returns the value if Ok and unwrap_err method returns the error value if Err, otherwise it raises an UnwrapError:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap()
'yay'
>>> res2.unwrap()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\project\result\result.py", line 107, in unwrap
    return self.expect("Called `Result.unwrap()` on an `Err` value")
File "C:\project\result\result.py", line 101, in expect
    raise UnwrapError(message)
result.result.UnwrapError: Called `Result.unwrap()` on an `Err` value
>>> res1.unwrap_err()
Traceback (most recent call last):
...
>>>res2.unwrap_err()
'nay'

A custom error message can be displayed instead by using expect and expect_err:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.expect('not ok')
'yay'
>>> res2.expect('not ok')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\project\result\result.py", line 101, in expect
    raise UnwrapError(message)
result.result.UnwrapError: not ok
>>> res1.expect_err('not err')
Traceback (most recent call last):
...
>>> res2.expect_err('not err')
'nay'

A default value can be returned instead by using unwrap_or:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap_or('default')
'yay'
>>> res2.unwrap_or('default')
'default'

Values and errors can be mapped using map, map_or, map_or_else and map_err:

>>> Ok(1).map(lambda x: x + 1)
Ok(2)
>>> Err('nay').map(lambda x: x + 1)
Err('nay')
>>> Ok(1).map_or(-1, lambda x: x + 1)
2
>>> Err(1).map_or(-1, lambda x: x + 1)
-1
>>> Ok(1).map_or_else(lambda: 3, lambda x: x + 1)
2
>>> Err('nay').map_or_else(lambda: 3, lambda x: x + 1)
3
>>> Ok(1).map_err(lambda x: x + 1)
Ok(1)
>>> Err(1).map_err(lambda x: x + 1)
Err(2)

FAQ

  • Why do I get the "Cannot infer type argument" error with MyPy?

There is a bug in MyPy which can be triggered in some scenarios. Using if isinstance(res, Ok) instead of if res.is_ok() will help in some cases. Otherwise using one of these workarounds can help.

License

MIT License

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A simple Rust like Result type for Python 3. Fully type annotated.

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