Joining strings. Generator or list comprehension?

Question

Consider the problem of extracting alphabets from a huge string.

One way to do is

\'\'.join([c for c in hugestring if c.isalpha()])

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Answer 1

join() does not need to be implemented as a sequential appending of elements of the sequence to a longer and longer accumulated string (which would indeed be very slow for long sequences); it just needs to produce the same result. So join() is probably just appending characters to some internal memory buffer, and creating a string from it at the end. The list comprehension construct, on the other hand, needs to first construct the list (by traversing hugestring's generator), and only then let join() begin its work.

Also, I doubt that join() looks at the list's length, since it can't know that each element is a single character (in most cases, it won't be) - it probably just obtains a generator from the list.