python reduce：查找元组列表中所有列表的总大小

Question

my data structure looks as below

itemsData = [('data1', [1, 2, 3, 4]), ('data2', [1, 2]), ('data3', [1, 2, 3])]

I want to find total number of items in the list of tuples above. For above example, len([1,2,3,4] + len([1,2]) + len([1,2,3]) = 9

reduce(lambda x,y: len(x[1]) + len(y[1]), itemsData )

error i get is

TypeError: 'int' object has no attribute '__getitem__'

Answer 1

You could simply try:

sum([len(elem[1]) for elem in itemsData])

Eg

>>> itemsData = [('data1', [1, 2, 3, 4]), ('data2', [1, 2]), ('data3', [1, 2, 3])]
>>> sum([len(elem[1]) for elem in itemsData])
9

Answer 2

I will explain why your code does not work

from https://docs.python.org/2/library/functions.html#reduce ,

The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable

So in the first iteration, your code len(x[1]) + len(y[1]) works since x=('data1', [1, 2, 3, 4]) , y=('data2', [1, 2]) , and the result is 6 ,

However in the second iteration, you get x=6 , y=('data3', [1, 2, 3])] , so len(x[1]) is invalid.

The correct code using reduce, is

reduce(lambda x,y: x+len(y[1]), itemsData, 0)

This works since

1st iteration ... x = 0, y = ('data1', [1, 2, 3, 4]), result = 4
2nd iteration ... x = 4, y = ('data2', [1, 2]), result = 6
3rd iteration ... x = 6, y = ('data3', [1, 2, 3]), result = 9