Reduce function under functools

Take a look reduce Source code ：

################################################################################
### reduce() sequence to a single item
################################################################################
_initial_missing = object()

def reduce(function, sequence, initial=_initial_missing):
    """ reduce(function, sequence[, initial]) -> value Apply a function of two arguments cumulatively to the items of a sequence, from left to right, so as to reduce the sequence to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty. """

    it = iter(sequence)        # <-----------  Turn it into an iterator 

    if initial is _initial_missing:
        try:
            value = next(it)   # <-----------  obtain  it  The first element of 
        except StopIteration:
            raise TypeError("reduce() of empty sequence with no initial value") from None
    else:
        value = initial        # <-----------  This step is generally useless , Unless you manually specify the initial element 

    for element in it:
        value = function(value, element) #  If  value = next(it)  Then start with the second element 
		#  If  value = initial  be  for  from  it  The first element of begins 
		
    return value

Look at the notes above , It's even clearer ：

From left to right , Divide the sequence elements into pairs , Until it is an element
for instance ：

reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])

Finally, this result is returned ：

((((1+2)+3)+4)+5)

If initial Pass in the parameter , Then it will be placed before the items of the sequence in the calculation , And used as the default value when the sequence is empty

Let's take another example ：

stay PaddleSeg Transform Normalize Of __init__ in , There are three lines ：

from functools import reduce
if reduce(lambda x, y: x * y, self.std) == 0:
	raise ValueError('{}: std is invalid!'.format(self))

It's really just the self.std Multiply the three elements of , If one element is 0, be raise ValueError

（ because self.std Be the denominator ）

But it's actually from _functools The , I didn't use the above Python Code

try:
    from _functools import reduce
except ImportError:
    pass