本文详细介绍了Python内置函数reduce的工作原理及使用方法。reduce函数通过累积的方式将序列元素转化为单一值，支持传递初始值来处理空序列的情况。文章通过示例展示了如何使用lambda表达式与reduce结合实现序列求和。
>>> help(reduce)
Help on built-in function reduce in module __builtin__:
reduce(...)
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.
>>> l=[1,2,3,4,5,6,7]
>>> reduce(lambda x,y:x+y,l)
28
>>> reduce(lambda x,y:x+y,l,2)
30
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