Appending a list's elements to another list using a list comprehension

Hrvoje Niksic hniksic at
Thu Oct 18 11:21:48 CEST 2007

Paul Hankin <paul.hankin at> writes:

> On Oct 17, 10:03 pm, Debajit Adhikary <debaj... at> wrote:
>> How does "a.extend(b)" compare with "a += b" when it comes to
>> performance? Does a + b create a completely new list that it assigns
>> back to a? If so, a.extend(b) would seem to be faster. How could I
>> verify things like these?
> Use a += b rather than a.extend(b): I'm not sure what I was
> thinking.

Why?  a.extend(b) is at least as readable, and is guaranteed to extend
the same list.  In general, "a += b" can fall back to the slower "a =
a + b" for sequences that don't support +=.

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