Is there a simple way to flatten a list of iterables with a list comprehension, or failing that, what would you all consider to be the best way to flatten a shallow list like this, balancing performance and readability?

I tried to flatten such a list with a nested list comprehension, like this:

[image for image in menuitem for menuitem in list_of_menuitems]

But I get in trouble of the NameError variety there, because the name "menuitem" is not defined. After googling and looking around on Stack Overflow, I got the desired results with a reduce statement:

reduce(list.__add__, map(lambda x: list(x), list_of_menuitems))

But this method is fairly unreadable because I need that list(x) call there because x is a Django QuerySet object.

Conclusion:

Thanks to everyone who contributed to this question. Here is a summary of what I learned. I"m also making this a community wiki in case others want to add to or correct these observations.

My original reduce statement is redundant and is better written this way:

>>> reduce(list.__add__, (list(mi) for mi in list_of_menuitems))

This is the correct syntax for a nested list comprehension (Brilliant summary dF!):

>>> [image for mi in list_of_menuitems for image in mi]

But neither of these methods are as efficient as using itertools.chain:

>>> from itertools import chain
>>> list(chain(*list_of_menuitems))

And as @cdleary notes, it"s probably better style to avoid * operator magic by using chain.from_iterable like so:

>>> chain = itertools.chain.from_iterable([[1,2],[3],[5,89],[],[6]])
>>> print(list(chain))
>>> [1, 2, 3, 5, 89, 6]

>>> list_of_menuitems = [["image00", "image01"], ["image10"], []]
>>> import itertools
>>> chain = itertools.chain(*list_of_menuitems)
>>> print(list(chain))
["image00", "image01", "image10"]

for inner_list in outer_list:
    for item in inner_list:
        ...

[... for inner_list in outer_list for item in inner_list]

[image for menuitem in list_of_menuitems for image in menuitem]