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

I have two lists:

a = [1, 2, 3]
b = [4, 5, 6]

What I'd like to do is append all of the elements of b at the end of
a, so that a looks like:

a = [1, 2, 3, 4, 5, 6]

I can do this using

map(a.append, b)

How do I do this using a list comprehension?

(In general, is using a list comprehension preferable (or more
"pythonic") as opposed to using map / filter etc.?)

Oct 17 '07
10 15043
On Thu, Oct 18, 2007 at 11:57:10AM -0000, Paul Hankin wrote regarding Re: Appending a list's elements to another list using a list comprehension:
>
Not to me: I can never remember which of a.append and a.extend is
which. Falling back to a = a + b is exactly what you want. For
instance:

a = (1, 2, 3)
a += (4, 5, 6)

works, whereas:

a = (1, 2, 3)
a.extend((4, 5, 6))

doesn't. So using += makes your code more general. There's no standard
sequence type that has extend and not +=, so worrying that += is
slower isn't a concern unless you're using a user-defined class. Even
then, it's probably a mistake that should be fixed in the class rather
than requiring .extend() to be used instead of +=.
I was going to argue that in fact += is not more general, it just covers a different set of use cases, but then I tested my hypothesis...
>>a = [1,2,3]
b = a
c = [4,5,6]
d = c
e = [7,8,9]
a.extend(e)
b
[1, 2, 3, 7, 8, 9]
>>c += e
d # I expected [4, 5, 6]
[4, 5, 6, 7, 8, 9]
>>c = c + e # But += doesn't do the same as this
c
[4, 5, 6, 7, 8, 9, 7, 8, 9]
>>d
[4, 5, 6, 7, 8, 9]

So I learned something new.

Cheers,
Cliff
Oct 18 '07 #11

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