result 0.8.0b1

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 or unwrap_or_else:

>>> res1 = Ok('yay')
>>> res2 = Err('nay')
>>> res1.unwrap_or('default')
'yay'
>>> res2.unwrap_or('default')
'default'
>>> res1.unwrap_or_else(str.upper)
'yay'
>>> res2.unwrap_or_else(str.upper)
'NAY'

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)

To save memory, both the Ok and Err classes are ‘slotted’, i.e. they define __slots__. This means assigning arbitrary attributes to instances will raise AttributeError.

The as_result() decorator can be used to quickly turn ‘normal’ functions into Result returning ones by specifying one or more exception types:

@as_result(ValueError, IndexError)
def f(value: int) -> int:
    if value == 0:
        raise ValueError  # becomes Err
    elif value == 1:
        raise IndexError  # becomes Err
    elif value == 2:
        raise KeyError  # raises Exception
    else:
        return value  # becomes Ok

res = f(0)  # Err[ValueError()]
res = f(1)  # Err[IndexError()]
res = f(2)  # raises KeyError
res = f(3)  # Ok[3]

Exception (or even BaseException) can be specified to create a ‘catch all’ Result return type. This is effectively the same as try followed by except Exception, which is not considered good practice in most scenarios, and hence this requires explicit opt-in.

Since as_result is a regular decorator, it can be used to wrap existing functions (also from other libraries), albeit with a slightly unconventional syntax (without the usual @):

import third_party

x = third_party.do_something(...)  # could raise; who knows?

safe_do_something = as_result(Exception)(third_party.do_something)

res = safe_do_something(...)  # Ok(...) or Err(...)
if isinstance(res, Ok):
    print(res.value)

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