python中两个for循环之间的交替执行

Question

I have two strings that contains values that should be joined together, that means value one in string1 with values two in string2. Here is the code that I have tried it:

str1 = "John,Gabriel,Isaac,Albert"
str2 = "Cena,Heins,Newton,Einstein"
for x,y in str1.split(','),str2.split(','):
    print (x+y) 

I have got this error:

Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: too many values to unpack

Answer 1

You could use zip , it returns an iterator (in Python 2 just a list) which spits out tuples with values combined from specified sources:

str1 = 'John,Gabriel,Isaac,Albert'
str2 = 'Cena,Heins,Newton,Einstein'

for x, y in zip(str1.split(','), str2.split(',')):
    print(x + y)

Answer 2

To address what the actual problem is, this line:

for x,y in str1.split(','),str2.split(','):

Does not mean what you intend.

There are two things going on.

for i in s:

Iterates over elements of s , assigning them to i . s in your case is:

>>> str1.split(','),str2.split(',')
(['John', 'Gabriel', 'Isaac', 'Albert'], ['Cena', 'Heins', 'Newton', 'Einstein'])

That is, a sequence of 2 elements, each being a list. So your loop would iterate twice with i being set to one of those lists each time.

On top of that, you're also trying to unpack i , effectively, trying to assign x, y with one of those lists, the same as this:

>>> x, y = ['John', 'Gabriel', 'Isaac', 'Albert']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack

This doesn't work because there are more values on the right, than names on the left.

The solution is to construct a new sequence that has pairs of values from each list, and iterate over that instead, as other have said, using zip .

Answer 3

In case the number of fields in each string might be different you can use itertools.zip_longest() for Python 3, or itertools.izip_longest() in Python 2:

from itertools import zip_longest

str1 = "John,Gabriel,Isaac,Albert,Aristotle"
str2 = "Cena,Heins,Newton,Einstein"

for firstname, surname in zip_longest(str1.split(','), str2.split(','), fillvalue=''):
    print('{}{}{}'.format(firstname, ' ' if firstname and surname else '', surname))

Output:

John Cena
Gabriel Heins
Isaac Newton
Albert Einstein
Aristotle

Otherwise, if you know for certain that the names are always paired, you can just use zip() .