Passing a list into a list .append() method

Peter Otten __peter__ at web.de
Tue Sep 9 09:13:16 CEST 2014

```JBB wrote:

> I have a list with a fixed number of elements which I need to grow; ie.
> add rows of a fixed number of elements, some of which will be blank.
>
> e.g. [['a','b','c','d'], ['A','B','C','D'], ['', 'aa', 'inky', ''], ['',
> 'bb', 'binky', ''], ... ]
>
> This is a reduced representation of a larger list-of-lists problem that
> had me running in circles today.
>
> I think I figured out _how_ to get what I want but I am looking to
> understand why one approach works and another doesn't.

The actual problem is that you are appending the same list multiple times.
In nuce:

>>> inner = [1, 2, 3]
>>> outer = [inner, inner, inner]

Remember that outer[0] is inner just like the two other items of the outer
list:

>>> outer[0] is inner
True
>>> outer[1] is inner
True
>>> outer[2] is inner
True

So

>>> outer[0][0] = 42

effectively changes inner

>>> inner
[42, 2, 3]

which is reflected in the following output:

>>> outer
[[42, 2, 3], [42, 2, 3], [42, 2, 3]]

With list(inner) you create a (shallow) copy of inner

>>> inner = [1, 2, 3]
>>> outer = [list(inner), list(inner), list(inner)]
>>> outer[0] is inner
False

and a subsequent assignment changes only that specific copy of inner:

>>> outer[0][0] = 42
>>> outer
[[42, 2, 3], [1, 2, 3], [1, 2, 3]]

So the key to the solution is that you create a new list on every iteration
of the loop. With that in mind I'd write

for a, b, in zip(qq, rr):
proc_file.append(["", a, b, ""])

or alternatively if the actual inner list is more complex

template = ["", "", "", ""]
for p in zip(qq, rr):
inner = list(template)
inner[1:3] = p
proc_file.append(inner)

> 1) What does NOT work as desired:
>     proc_file.append((blank_r))  # Add a row of blanks
>     proc_file[i+2][1] = j[0]
>     proc_file[i+2][2] = j[1]

> 2) What works as desired:

>     proc_file.append(list(blank_r))  # Change it to list(blank_r) and it
>     works
>     proc_file[i+2][1] = j[0]
>     proc_file[i+2][2] = j[1]
>     print len(proc_file), blank_r, proc_file

```