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

In the example you give it would make the most sense to do:

b = [i for i in a]

if for some reason you wanted to create b. In general, there is some common sense that must be employed. If using a comprehension makes your code unreadable, don't use it. Otherwise go for it.

Answer 2

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.

Answer 3

b = []
a = [1, 2, 3, 4, 5]
b.extend (a)

Answer 4

Only use list comprehensions if you plan to use the created list. Otherwise you create it just for the GC to throw it again without ever being used.

So instead of [b.append(i) for i in a] you should use a proper for loop:

for i in a:
    b.append(i)

Another solution would be through a generator expression:

b += (i for i in a)

---

However, if you want to append the whole list, you can simply do

b += a

And if you just need to apply a function to the elements before adding them to the list, you can always use map:

b += map(somefunc, a)

Answer 5

From python documentation:

List comprehensions provide a concise way to create lists. Common applications are to make new lists

Perhaps you want to learn more about reduce(), filter() and map() functions.