Get sum or any scalar from iterable

Built-In function reduce()

reduce(function, iterable, start_value)

reduce(lambda x, y: x+y, range(0,10))

The function here has two parameters. The calling flow is as below:

call: function(element[0], element[1]), save to temporary value tmp

call: function(tmp, element[2]), save to tmp

call: function(tmp, element[3]), save to tmp

Until loop completion.

The call-flow as described by official document is:

def reduce(function, iterable, initializer=None):
    it = iter(iterable)
    if initializer is None:
            initializer = next(it)
        except StopIteration:
            raise TypeError('reduce() of empty sequence with no initial value')
    accum_value = initializer
    for x in it:
        accum_value = function(accum_value, x)
    return accum_value

Therefore, the loop times is length of iterable minus 1.

However, if \_add\_(_, __) is visible to type, sum(list) is a quick way to return summary of all numbers.

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