Python中reduce与lambda的结合使用

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.

function不能为None。 考虑没有initial，且function为lambda表达式的情形。此时，lambda表达式的:左边有且须有2个参数，:右边是关于这2个参数的表达式。

reduce(lambda x,y:x+y, [1,2,3]) #6

reduce(lambda x,y:x * y, [1,2,4]) #8

reduce(lambda x,y: x and y, [True,False,True]) #False

def f(x,y):
    return x+y
reduce(lambda x,y:f(x,y), [1,2,3]) #6

考虑没有initial，且function不为lambda表达式的情形。

def f(x,y):
    return x+y
reduce(f, [1,2,3]) #6

考虑有initial，且function为lambda表达式的情形。同样，lambda表达式的:左边有且须有2个参数，:右边是关于这2个参数的表达式。

reduce(lambda x,y:x+y, [1,2,3], 10) #16

reduce(lambda x,y:x+y, [], 10) #10

另外，考虑一种特殊情况，即sequence为函数列表(即列表中的每个元素均是某一个函数)的情况。

def f1(x):
    return x + [1]
def f2(x):
    return x + [2]
reduce(lambda x, y: y(x), [f1, f2], [0]) #[0, 1, 2]