python - list comprehension without assignment

Question

Today I was parsing a directory index with a list of paths to zip files using BeautifulSoup and came across an interesting thing. Let's assume I would like to take all the href properties of the tags I got and put them directly into a Queue:

q = Queue.Queue()
[q.put(tag['href']) for tag in soup.findAll('a')]

I've never run into a situation like this before where a comprehension could be used inline without assigning it to anything, just to generated another iterator through some routine call. Is this considered bad practice? Is it "pythonic", per se? Was there a better one-liner to put all the items into the queue?

Answer 1

There are many opinions on this thread, I can only speak from coding conventions at my organization.

there are many ways to affect a loop, but a key attribute of list comprehensions is that they create lists, with one item for each in the iterated over sequence.

>>> import Queue
>>> q = Queue.Queue()
>>> [q.put(item) for item in range(5)]
[None, None, None, None, None]
>>>

this unused list is obviously wasteful. As such, this construction, a list comprehension with unused return value; is forbidden from appearing in our code base. An explicit loop like above, or a generated combined with something that consumes it, for instance:

>>> any(q.put(item) for item in xrange(5))
False
>>>

or just:

>>> for item in xrange(5):
...     q.put(item)
...
>>>

is required to pass review.