proper use of list comprehensions - python

Question

Normally, list comprehensions are used to derive a new list from an existing list. Eg:

>>> a = [1, 2, 3, 4, 5]
>>> [i for i in a if i > 2]
[3, 4, 5]

Should we use them to perform other procedures? Eg:

>>> a = [1, 2, 3, 4, 5]
>>> b = []
>>> [b.append(i) for i in a]
[None, None, None, None, None]
>>> print b
[1, 2, 3, 4, 5]

or should I avoid the above and use the following instead?:

for i in a:
    b.append(i)

Answer 1

You should indeed avoid using list comprehensions (along with dictionary comprehensions, set comprehensions and generator expressions) for side effects. Apart from the fact that they'd accumulate a bogus list and thus waste memory, it's also confusing. I expect a list comprehension to generate a (meaningful) value, and many would agree. Loops, on the other hand, are clearly a sequence of statements. They are expected to kick off side effects and generate no result value - no surprise.