Question

I’m writing a generator function. I want to know if there’s a better (read: more pythonic, ideally with a list comprehension) way to implement something like this:

generator = gen()
captures = []
for _ in xrange(x):
    foo = next(generator)
    directories.append(foo['name'])
    yield foo

The key here is that I don’t want to capture the WHOLE yield- the dictionary returned by gen() is large, which is why I’m using a generator. I do need to capture all of the ‘name’s, though. I feel like there’s a way to do this with a list comprehension, but I’m just not seeing it. Thoughts?

Answer 1

There is another / shorter way to do this, but I wouldn’t call it more Pythonic:

generator = gen()
directories = []
generator_wrapper = (directories.append(foo['name']) or foo 
                         for foo in generator)

This takes advantage of the fact that append, like all mutating methods in Python, always returns None so .append(...) or foo will always evaluate to foo.

That way the whole dictionary is still the result of the generator expression, and you still get lazy evaluation, but the name still gets saved to the directories list.

You could also use this method in an explicit for loop:

for foo in generator:
    yield directories.append(foo['name']) or foo

or even just simplify your loop a bit:

for foo in generator:
    directories.append(foo['name'])
    yield foo

as there is no reason to use an xrange just to iterate over the generator (unless you actually want to only iterate some known number of steps in).