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from collections.abc import Callable
from functools import wraps
from typing import Any, TypeVar
from typing_extensions import Never, ParamSpec
_FirstType = TypeVar('_FirstType')
_SecondType = TypeVar('_SecondType')
_ThirdType = TypeVar('_ThirdType')
_FuncParams = ParamSpec('_FuncParams')
def identity(instance: _FirstType) -> _FirstType:
"""
Function that returns its argument.
.. code:: python
>>> assert identity(1) == 1
>>> assert identity([1, 2, 3]) == [1, 2, 3]
This function is really helpful for some composition.
It is also useful for "do nothing" use-case.
See also:
- https://en.wikipedia.org/wiki/Identity_function
- https://stackoverflow.com/a/21506571/4842742
"""
return instance
def compose(
first: Callable[[_FirstType], _SecondType],
second: Callable[[_SecondType], _ThirdType],
) -> Callable[[_FirstType], _ThirdType]:
"""
Allows function composition.
Works as: ``second . first`` or ``first() |> second()``.
You can read it as "second after first".
.. code:: python
>>> assert compose(float, int)('123.5') == 123
We can only compose functions with one argument and one return.
Type checked.
"""
return lambda argument: second(first(argument))
def tap(
function: Callable[[_FirstType], Any],
) -> Callable[[_FirstType], _FirstType]:
"""
Allows to apply some function and return an argument, instead of a result.
Is useful for composing functions with
side-effects like ``print()``, ``logger.log()``, etc.
.. code:: python
>>> assert tap(print)(1) == 1
1
>>> assert tap(lambda _: 1)(2) == 2
See also:
- https://github.com/dry-python/returns/issues/145
"""
def decorator(argument_to_return: _FirstType) -> _FirstType:
function(argument_to_return)
return argument_to_return
return decorator
def untap(
function: Callable[[_FirstType], Any],
) -> Callable[[_FirstType], None]:
"""
Allows to apply some function and always return ``None`` as a result.
Is useful for composing functions that do some side effects
and return some nosense.
Is the kind of a reverse of the ``tap`` function.
.. code:: python
>>> def strange_log(arg: int) -> int:
... print(arg)
... return arg
>>> assert untap(strange_log)(2) is None
2
>>> assert untap(tap(lambda _: 1))(2) is None
See also:
- https://github.com/dry-python/returns/issues/145
"""
def decorator(argument_to_return: _FirstType) -> None:
function(argument_to_return)
return decorator
def raise_exception(exception: Exception) -> Never:
"""
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:
.. code:: pycon
>>> 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.alt(raise_exception)
Traceback (most recent call last):
...
ValueError: boom
See also:
- https://github.com/dry-python/returns/issues/56
"""
raise exception
def not_(function: Callable[_FuncParams, bool]) -> Callable[_FuncParams, bool]:
"""
Denies the function returns.
.. code:: python
>>> from returns.result import Result, Success, Failure
>>> def is_successful(result_container: Result[float, int]) -> bool:
... return isinstance(result_container, Success)
>>> assert not_(is_successful)(Success(1.0)) is False
>>> assert not_(is_successful)(Failure(1)) is True
"""
@wraps(function)
def decorator(
*args: _FuncParams.args,
**kwargs: _FuncParams.kwargs,
) -> bool:
return not function(*args, **kwargs)
return decorator
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