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A rust-like result type for Python

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A simple Result type for Python 3 inspired by Rust.

The idea is that a Result value can be either Ok(value) or Err(error), with a way to differentiate between the two. It will change code like this:

def get_user_by_email(email):
    """
    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

def get_user_by_email(email):
    """
    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 user_result.is_ok():
    do_something(user_result.value)
else:
    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. You still don’t get any type safety, but some easier handling of types 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')

Or through the class methods:

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

Checking whether a result is ok or not:

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

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 = Result.Ok()
>>> res1.value
True
>>> res2 = Ok()
>>> res2.value
True

The unwrap method returns the value if Ok, 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

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

>>> 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

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'

License

MIT License

Project details


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