Function reduce

Lib/functools.py:237–265

reduce(function, iterable, /[, initial]) -> value Apply a function of two arguments cumulatively to the items of an iterable, from left to right. This effectively reduces the iterable to a single value. If initial is present, it is placed before the items of the iterable in the c

(function, sequence, initial=_initial_missing)

_initial_missing = object()

def reduce(function, sequence, initial=_initial_missing):
    """
    reduce(function, iterable, /[, initial]) -> value

    Apply a function of two arguments cumulatively to the items of an iterable, from left to right.

    This effectively reduces the iterable to a single value.  If initial is present,
    it is placed before the items of the iterable in the calculation, and serves as
    a default when the iterable is empty.

    For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
    calculates ((((1 + 2) + 3) + 4) + 5).
    """

    it = iter(sequence)

    if initial is _initial_missing:
        try:
            value = next(it)
        except StopIteration:
            raise TypeError(
                "reduce() of empty iterable with no initial value") from None
    else:
        value = initial

    for element in it:
        value = function(value, element)

    return value


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