Captures and returns an argument passed to a lambda expression.

capture.py

"""Captures and returns an argument passed to a lambda expression."""

class Capture:
    """Captures and returns an argument passed to a lambda expression.

    :param fn: The lambda expression.
    :type fn: function

    :Example:

    >>> from functools import reduce
    >>> f = Capture(lambda x, y: x.append(y))[0]
    >>> f([], 1)
    [1]
    >>> reduce(f, range(10), [])
    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

    """
    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args, **kwargs):
        return self.fn(*args, **kwargs)

    def __getitem__(self, key):
        if not isinstance(key, (int, str)):
            raise TypeError('index/key must be int or str, not ' + type(key).__name__)
        def inner_fn(*args, **kwargs):
            self(*args, **kwargs)
            return args[key] if isinstance(key, int) else kwargs[key]
        return inner_fn