Python: Nested Loop

Question

Consider this:

>>> a = [("one","two"), ("bad","good")]

>>> for i in a:
...     for x in i:
...         print x
... 
one
two
bad
good

How can I write this code, but using a syntax like:

for i in a:
    print [x for x in i]

Obviously, This does not work, it prints:

['one', 'two']
['bad', 'good']

I want the same output. Can it be done?

Solution 1

>>> a = [("one","two"), ("bad","good")]
>>> print "\n".join(j for i in a for j in i)
one
two
bad
good



>>> for i in a:
...  print "\n".join(i)
... 
one
two
bad
good

Solution 2

List comprehensions and generators are only designed to be used as expressions, while printing is a statement. While you can effect what you're trying to do by doing

from __future__ import print_function
for x in a:
    [print(each) for each in x]

doing so is amazingly unpythonic, and results in the generation of a list that you don't actually need. The best thing you could do would simply be to write the nested for loops in your original example.

Solution 3

Given your example you could do something like this:

a = [("one","two"), ("bad","good")]

for x in sum(map(list, a), []):
    print x

This can, however, become quite slow once the list gets big.

The better way to do it would be like Tim Pietzcker suggested:

from itertools import chain

for x in chain(*a):
    print x

Using the star notation, *a, allows you to have n tuples in your list.

Solution 4

The print function really is superior, but here is a much more pythonic suggestion inspired by Benjamin Pollack's answer:

from __future__ import print_function
for x in a:
    print(*x, sep="\n")

Simply use * to unpack the list x as arguments to the function, and use newline separators.

Solution 5

import itertools
for item in itertools.chain(("one","two"), ("bad","good")):
    print item

will produce the desired output with just one for loop.

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Comments

[TM.]

This works but I'm guessing (although not positive) that in his real code he has n number of tuples, not a predefined set that he can hardcode.

[TM.]

To see what I'm getting at, read tgray's answer, with the *a notation.

[Jason Baker]

It should also be noted that this will only work in Python 2.6 or 3

[TM.]

much better off using the answer that tgray provided rather than this

[u0b34a0f6ae]

the print function really is awesome, however I think there is a much superior way to use it (hence my answer).

[stephan]

print '\n'.join(j for k in a for j in k)