Why Is Concatenating Strings With ''.join(list) So Popular?

I know that ''.join(list) is the preferred method to concatenate strings as opposed to say: for x in list: s += x My question is why is this much faster? Also, what if I need

Solution 1:

This is faster because the join method gets to dive "under the surface" and use lower-level optimizations not available from the Python layer. The loop has to plod through the sequence generator and deal with each object in turn. Also, your loop has to build a new string on each iteration, a slow process. join gets to use mutable strings on the C layer or below.

If the objects aren't already in a list ... it depends on the application. However, I suspect that almost any such application will have to go through that loop-ish overhead somewhere just to form the list, so you'd lose some of the advantage of join, although the mutable string would still save time.

Solution 2:

Yes, join is faster because it doesn't need to keep building new strings.

But you don't need a list to use join! You can give it any iterable, such as a generator expression:

''.join(x for x in lst if x != 'toss')

It appears that join is optimized when you use a list though. All of these are equivalent, but the one with a list comprehension is fastest.

>>>timeit("s=''.join('x' for i in range(200) if i!=47)")
15.870241802178043
>>>timeit("s=''.join(['x' for i in range(200) if i!=47])")
11.294011708363996
>>>timeit("s=''\nfor i in range(200):\n if i!=47:\n  s+='x'")
16.86279364279278