Newbie Question, list = list + x VS append (x)

[Duncan Booth]

Chuck <cdreward at riaa.com> wrote in
news:mh4euu095fqqm9bmcgrpdrcvb5emgpkhlm at 4ax.com: 

> def add2(n):
>     return [n+2]
> 
> x = y = [1,2]
> 
> print x # [1,2] - Ok
> 
> x = x + add2(5)
> 
> print x # [1,2,7] - why not [1,2,[7]]? add2 returns answer inside 
"[ ]"
> 
> 
> #...vs...
> 
> 
> print y # [1,2] - Ok
> 
> y.append( add2(5) ) 
> 
> print y # [1,2,[7]] - Ok; What I expected it to do
> 
> 
> I'm not clear what's going on when I do "list = list + function 
that
> returns list", i.e. why it's taking the 7 out of a list, where 
.append
> keeps it in... 

Ignore the function for now, it is just confusing you.

>>> x = [1, 2] + [7]
>>> print x
[1, 2, 7]
>>> y = [1, 2]
>>> y.append([7])
>>> print y
[1, 2, [7]]
>>> 

The + operator concatenates lists. i.e. it takes two lists and 
sticks them together as a single list. In your case you had a single 
element, but if the second list had been longer it might be more 
obvious:

>>> print [1, 2] + [7, 8, 9]
[1, 2, 7, 8, 9]

The append method of a list takes one object of any type, and puts 
that object on the end of the list.

Note that in the first case, using + to concatenate lists, both 
arguments to + must be lists but the second case doesn't care.

The 'extend' method of lists is the one that corresponds more 
closely to '+' than the append method, (although of course it 
modifies in place rather than creating a new list):

>>> x = y = [1, 2]
>>> x.extend([7,8])
>>> print x
[1, 2, 7, 8]
>>> print y
[1, 2, 7, 8]
>>>