Passing a list into a list .append() method

Rustom Mody rustompmody at gmail.com
Tue Sep 9 14:07:54 CEST 2014


On Tuesday, September 9, 2014 11:25:29 AM UTC+5:30, 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.

> 1) What does NOT work as desired:

> proc_file = []
> proc_file = [['a','b','c','d']]
> proc_file.append(['A','B','C','D'])
> blank_r = ['','','','']

> qq = ['aa','bb','cc','dd']
> rr = ['inky', 'binky', 'pinky', 'clyde']

> for i,j in enumerate(zip(qq,rr)):
>     proc_file.append((blank_r))  # Add a row of blanks
>     proc_file[i+2][1] = j[0]
>     proc_file[i+2][2] = j[1]
>     print len(proc_file), blank_r, proc_file

> print
> print
> proc_file

> 3 ['', 'aa', 'inky', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['',
> 'aa', 'inky', '']]
> 4 ['', 'bb', 'binky', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['',
> 'bb', 'binky', ''], ['', 'bb', 'binky', '']]
> 5 ['', 'cc', 'pinky', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['',
> 'cc', 'pinky', ''], ['', 'cc', 'pinky', ''], ['', 'cc', 'pinky', '']]
> 6 ['', 'dd', 'clyde', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['',
> 'dd', 'clyde', ''], ['', 'dd', 'clyde', ''], ['', 'dd', 'clyde', ''], ['',
> 'dd', 'clyde', '']]

> Out[82]:

> [['a', 'b', 'c', 'd'],
>  ['A', 'B', 'C', 'D'],
>  ['', 'dd', 'clyde', ''],
>  ['', 'dd', 'clyde', ''],
>  ['', 'dd', 'clyde', ''],
>  ['', 'dd', 'clyde', '']]

> 2) What works as desired:

> proc_file = []
> proc_file = [['a','b','c','d']]
> proc_file.append(['A','B','C','D'])
> blank_r = ['','','','']

> qq = ['aa','bb','cc','dd']
> rr = ['inky', 'binky', 'pinky', 'clyde']

> for i,j in enumerate(zip(qq,rr)):
>     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

> print
> print
> proc_file

> 3 ['', '', '', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['', 'aa',
> 'inky', '']]
> 4 ['', '', '', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['', 'aa',
> 'inky', ''], ['', 'bb', 'binky', '']]
> 5 ['', '', '', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['', 'aa',
> 'inky', ''], ['', 'bb', 'binky', ''], ['', 'cc', 'pinky', '']]
> 6 ['', '', '', ''] [['a', 'b', 'c', 'd'], ['A', 'B', 'C', 'D'], ['', 'aa',
> 'inky', ''], ['', 'bb', 'binky', ''], ['', 'cc', 'pinky', ''], ['', 'dd',
> 'clyde', '']]

> Out[83]:

> [['a', 'b', 'c', 'd'],
>  ['A', 'B', 'C', 'D'],
>  ['', 'aa', 'inky', ''],
>  ['', 'bb', 'binky', ''],
>  ['', 'cc', 'pinky', ''],
>  ['', 'dd', 'clyde', '']]

Dont quite know what you are trying to do.
Does this serve your purpose?

>>> proc_file = [['a','b','c','d']]
>>> proc_file.append(['A','B','C','D'])
>>> blank_r = ['','','','']
>>> [[i,j0,j1, ""] for (i, (j0, j1)) in enumerate(zip(qq,rr))]
[[0, 'aa', 'inky', ''], [1, 'bb', 'binky', ''], [2, 'cc', 'pinky', ''], [3, 'dd', 'clyde', '']]

Or if you prefer:
>>> proc_file + [[i,j0,j1, ""] for (i, (j0, j1)) in enumerate(zip(qq,rr))]
[['a', 'b', 'c', 'd'], [0, 'aa', 'inky', ''], [1, 'bb', 'binky', ''], [2, 'cc', 'pinky', ''], [3, 'dd', 'clyde', '']]



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