Helper functions

We feature several helper functions to make your developer experience better.


We also ship an utility function to compose two different functions together.

>>> from returns.functions import compose

>>> bool_after_int = compose(int, bool)
>>> bool_after_int('1')
>>> bool_after_int('0')

Composition is also type-safe. The only limitation is that we only support functions with one argument and one return to be composed.

Only works with regular functions (not async).


We also ship returns.functions.identity() function to help you with the composition.

Identity function is a simple concept: it just returns its argument. If you wonder why do we need this function, please read below:

tap and untap

We need tap() function to easily compose values with functions that does not return. For example you sometimes need to print() values inside your pipe:

>>> from returns.functions import tap

>>> result = tap(print)(1)  # will print and return 1
>>> assert result == 1

You can also use untap function to turn any function return type to None and still do its thing”

>>> from returns.functions import tap, untap

>>> result = untap(tap(print))(1)  # will print and return None
>>> assert result is None

This is also sometimes helpful for a typed function composition.


Sometimes you really want to reraise an exception from Failure[Exception] due to some existing API (or a dirty hack).

We allow you to do that with ease!

from returns.functions import raise_exception

def create_account_and_user(username: str) -> ...:
    Creates new Account-User pair.

    Imagine, that you need to reraise ValidationErrors due to existing API.
    return _validate_user(
        # What happens here is interesting, since you do not let your
        # unwrap to fail with UnwrapFailedError, but instead
        # allows you to reraise a wrapped exception.
        # In this case `ValidationError()` will be thrown
        # before `UnwrapFailedError`

def _validate_user(username: str) -> Result['User', ValidationError]:

Use this with caution. We try to remove exceptions from our code base. Original proposal is here.

API Reference


Function that returns its argument.

>>> identity(1)
>>> identity([1, 2, 3])
[1, 2, 3]

Why do we even need this? Identity functions help us with the composition.

Imagine, that you want to use returns.converters.coalesce_result() like so:

from returns.result import Result
from returns.converters import coalesce_result

numbers: Result[int, float]
# Now you want to coalesce `number` into `int` type:
number: int = coalesce_result(identity, int)(numbers)
# Done!
Return type


compose(first, second)[source]

Allows function composition.

Works as: second . first or first() |> second(). You can read it as “second after first”.

>>> compose(float, int)('123.5')

We can only compose functions with one argument and one return. Type checked.

Return type

Callable[[~_FirstType], ~_ThirdType]


Allows to apply some function and return an argument, instead of a result.

Is usefull for composing functions with side-effects like print(), logger.log(), etc.

>>> tap(print)(1)
>>> tap(lambda _: 1)(2)
Return type

Callable[[~_FirstType], ~_FirstType]


Allows to apply some function and always return None as a result.

Is usefull for composing functions that do some side effects and return some nosense.

Is the kind of a reverse of the tap function.

>>> def strange_log(arg: int) -> int:
...     print(arg)
...     return arg
>>> untap(strange_log)(2)
>>> untap(tap(lambda _: 1))(2)
Return type

Callable[[~_FirstType], None]


Helper function to raise exceptions as a function.

It might be required as a compatibility tool for existing APIs. That’s how it can be used:

>>> from returns.result import Failure, Result
>>> # Some operation result:
>>> user: Result[int, ValueError] = Failure(ValueError('boom'))
>>> # Here we unwrap internal exception and raise it:


>>> user.fix(raise_exception)
Traceback (most recent call last):
ValueError: boom
Return type