Mapping a nested list with List Comprehension in Python?

Question

I have the following code which I use to map a nested list in Python to produce a list with the same structure.

>>> nested_list = [['Hello', 'World'], ['Goodbye', 'World']]
>>> [map(str.upper, x) for x in nested_list]
[['HELLO', 'WORLD'], ['GOODBYE', 'WORLD']]

Can this be done with list comprehension alone (without using the map function)?

Answer 1

For nested lists you can use nested list comprehensions:

nested_list = [[s.upper() for s in xs] for xs in nested_list]

Personally I find map to be cleaner in this situation, even though I almost always prefer list comprehensions. So it's really your call, since either will work.

Answer 2

Map is certainly a much cleaner way of doing what you want. You can nest the list comprehensions though, maybe that's what you're after?

[[ix.upper() for ix in x] for x in nested_list]

Answer 3

Other posters have given the answer, but whenever I'm having trouble wrapping my head around a functional construct, I swallow my pride and spell it out longhand with explicitly non-optimal methods and/or objects. You said you wanted to end up with a generator, so:

for xs in n_l:
    def doUpper(l):
        for x in l:
            yield x.upper()
    yield doUpper(xs)

for xs in n_l:
    yield (x.upper() for x in xs)

((x.upper() for x in xs) for xs in n_l)

Sometimes it's cleaner to keep one of the longhand versions. For me, map and reduce sometimes make it more obvious, but Python idioms might be more obvious for others.