How to make a nested for loop using a generator in python?

I’m trying to convert this nested for loop:

        for k,v in all_R.iteritems():
            for pairs in v:
                print pairs[1]

to a one liner, something like this:

print ([pairs[1] for pairs in v for k,v in all_R.iteritems()])

But I’m getting this error:

    UnboundLocalError: local variable 'v' referenced before assignment

all_R is a defaultdict where every value has keys that are pairs, and I’m interested in just one value from that pair:

     {'1-0002': [('A-75G', 'dnaN'), ('I245T', 'recF'),... ], '1-0004': [('A-75G', 'dnaN'), ('L161', 'dnaN'),...]}

Answer

List comprehensions are written in the same order as for loops, so you are actually looking for (Note the order is reversed)

([pairs[1] for k,v in all_R.iteritems() for pairs in v ])

Here’s a break down that shows the structure a bit more clearly:

[
    pairs[1]
        for k,v in all_R.iteritems()
            for pairs in v
]

If you are looking to use generators as your title suggests, you can use parenthesis instead of brackets.

(pairs[1] for k,v in all_R.iteritems() for pairs in v)

That will create a generator with your desired properties.