如何在python中使用list comprehension从列表中获取元组

Question

I have two lists and want to merges them into one list of tuples . I want to do it with list comprehension , I can get it working using map . but would be nice to know how list comprehension here will work. code here

>>> lst =  [1,2,3,4,5]
>>> lst2 = [6,7,8,9,10]
>>> tup = map(None,lst,lst2) # works fine
>>> tup
[(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]
>>> l3 = [lst, lst2]
>>> l3
[[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]]
>>> zip(*l3) # works fine
[(1, 6), (2, 7), (3, 8), (4, 9), (5, 10)]

>>> [(i,j) for i in lst and for j in lst2] # does not work
  File "<stdin>", line 1
    [(i,j) for i in lst and for j in lst2]
                              ^
SyntaxError: invalid syntax
>>> 

I have written comments where it works and where it does not. How can a two for-loop be coupled in list comprehension

Answer 1

Think about list comprehensions as loops. How can you write 2 not nested loops?

You can do this with somewhat wierd list comprehension:

[(x, lst2[i]) for i, x in enumerate(lst)]

or

[(lst[i], lst2[i]) for i in xrange(len(lst))]

But actually, it's better to use zip .

Answer 2

The list comprehension way is silly, because it just wraps a do-nothing list comprehension around zip :

[(i,j) for i, j in zip(lst, lst2)]

Just use zip , this is what it's for. There's no sense in forcing yourself to use list comprehensions when they don't accomplish anything.

Edit: If your question is "how do I get two for loops in one list comprehension", you should ask that instead. The answer is "You can't get two PARALLEL for loops in one list comprehension". Any time you put two for clauses in a list comprehension, they will be nested. That, a list comprehension like this:

[... for a in list1 for b in list2]

Works like two nested for loops:

for a in list1:
    for b in list2:
        ...

You can't write a list comprehension that does this:

for a in list1:
    ...
for b in list2:
    ...

. . . and you don't need to, because you have the zip function to do that instead.

(You can sort of fake it using a solution like @Roman Pekar's, but that doesn't really do two for loops; it just does one and uses the values from that one to reach into the other list.)