Multicast delegates in Python. Uses operator overloading for subscription, C# style.

delegate.py

"""
A multicast delegate implementation. Subscribe or
unsubscribe callback functions using += or -=
operators, respectively.

Example:


```python
In [1]: from delegate import Multicast                                                                                                                             

In [2]: m = Multicast()                                                                                                                                            

In [3]: m += lambda: print("first")                                                                                                                                

In [4]: m += lambda: print("second")                                                                                                                               

In [5]: m()                                                                                                                                                        
first
second
```

This code is experimental. Its quality, and the design patterns
it encourages, are of questionable value in Python. I suspect
one should be wary of any class overriding `__call__` that isn't
a decorator, though I'd be happy to be proven wrong as they
are fun to write if nothing else.
"""


from typing import Any, Callable, List


class EmptyDelegateException(ValueError):
    pass


class Multicast:
    def __init__(self):
        self._invocation_list: List[Callable] = []

    @property
    def invocation_list(self):
        return self._invocation_list

    def __call__(self, *args, **kwargs) -> Any:
        # return only the last value
        if len(self._invocation_list) == 0:
            raise EmptyDelegateException()
        ret = None
        for method in self._invocation_list:
            ret = method(*args, **kwargs)
        return ret

    def __iadd__(self, method: Callable) -> "Multicast":
        """Multicast += some_function"""
        if method not in self._invocation_list:
            self._invocation_list.append(method)
        return self

    def __isub__(self, method: Callable):
        """Multicast -= some_function"""
        if method in self._invocation_list:
            self._invocation_list.remove(method)
        return self

    def __len__(self) -> int:
        return len(self._invocation_list)

    def __bool__(self) -> bool:
        return bool(len(self))

    def __str__(self) -> str:
        return f"Multicast[{len(self)}]"

    def __eq__(self, other) -> bool:
        return isinstance(other, self.__class__) and self._invocation_list == other._invocation_list

LICENSE

Copyright 2020 Christopher Lawlor

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

test_delegate.py

from unittest.mock import Mock
import pytest

from delegate import Multicast

@pytest.fixture
def delegate():
    return Multicast()

def test1():
    return "test1"


def test2():
    return "test2"


def test_no_methods_throws(delegate):
    """Invoking a delegate with no subscriptions throws"""
    with pytest.raises(ValueError):
        value = delegate()


def test_can_add_method(delegate):
    """Tests subscription via += operator"""
    delegate += test1

    value = delegate()

    assert value == "test1"


def test_len(delegate):
    """Delegate has a length"""
    assert len(delegate) == 0
    delegate += test1
    assert len(delegate) == 1


def test_can_remove_method(delegate):
    """Test unsubscribe by -= operator"""
    delegate += test1
    delegate -= test1

    assert len(delegate) == 0


def test_return_value_is_last_method(delegate):
    """Return the result of the last subscribed function when invoked"""
    delegate += test1
    delegate += test2

    value = delegate()

    assert value == "test2"


def test_eq():
    """Can compare for equality"""
    d1 = Multicast()
    d2 = Multicast()

    assert d1 == d2

    d1 += test1

    assert d1 != d2

    d2 += test1

    assert d1 == d2


def test_does_not_hide_exceptions(delegate):
    """Exceptions raised by subscribed functions are not swallowed"""
    method = Mock()
    method.side_effect = ValueError
    delegate += method

    with pytest.raises(ValueError):
        delegate()


def test_next_method_not_called_if_previous_throws(delegate):
    """Subsequent method is not called if the previous threw"""
    first = Mock()
    first.side_effect = ValueError
    second = Mock()
    delegate += first
    delegate += second

    with pytest.raises(ValueError):
        delegate()
        assert first.call_count == 1
        assert second.call_count == 0


if __name__ == "__main__":
    import sys
    sys.exit(pytest.main())