如何在一行中从python列表中收集许多元组？

Question

for example:

If I want to make a array b from a like this:

a = [1, 2, 3]
b = [i for i in a]

then b will be [1, 2, 3]

now, I want to make a tuple list like this: [(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)] from a

how to write this in one line?

I know how to write this function like this:

b = []
for i in a:
    for j in a:
        if i == j: continue
        b.append((i, j))

but I wonder how to write this function in one line?

Answer 1

Simply use itertools.permutations :

>>> from itertools import permutations
>>> a = [1, 2, 3]
>>> list(permutations(a, 2))
[(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]

Answer 2

>>>a = [1, 2, 3]
>>>b = [(n,m) for n in a for m in a if n!=m]
>>>
>>>print b
[(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]