Explanation of this Python language feature? [x for x in x for x in x] (to flatten a nested list)

[Mark Lawrence]

On 24/03/2014 14:39, Steven D'Aprano wrote:
> On Mon, 24 Mar 2014 23:53:12 +1100, Chris Angelico wrote:
>
>> On Mon, Mar 24, 2014 at 11:36 PM, Marko Rauhamaa <marko at pacujo.net>
>> wrote:
>>>> def get_oneth_element_index(item):
>>>>      return item[1].index
>>>> L.sort(key=get_oneth_element_index)
>>>>
>>>> Or do you just shortcut the whole thing by inlining it?
>>>>
>>>> L.sort(key=lambda item:item[1].index)
>>>
>>> I still prefer the "def" variant. It even allows you to clarify the
>>> meaning of the tuple slot by using a nicer name.
>>
>> It's the index of element 1. What more do you need to know? Is it
>> actually any help to give that a name? All you gain is a chance for the
>> name, the purpose, and the functionality to come into disagreement.
>
>
>
> # Magic constants are wicked. Never use a constant without naming it.
> ELEMENT_TO_USE_FOR_INDEXING_WHEN_SORTING_L = 1
>
> # <summary>
> #     key func used when sorting L, returns item's 1th elem index method
> # </summary>
> # <function_name>
> #    _get_oneth_element_index_to_use_as_keyfunc_when_sorting_L
> # </function_name>
> # <author>Steven D'Aprano</author>
> # <date_created>2014-03-25</date_created>
> # <date_modified>2014-03-25</date_modified>
> # <revision>1</revision>
> # <param name="item">item to be sorted</param>
> # <returns>index method of the oneth element</returns>
> # <raises>NameError</raises>  # FIXME can this fail any other way?
> def _get_oneth_element_index_to_use_as_keyfunc_when_sorting_L(item):
>      """Private key function for sorting list L.
>
>      Returns the index method of element 1 of the given item.
>
>      Example of use:
>
>      >>> item = (None, '')
>      >>> _get_oneth_element_index_to_use_as_keyfunc_when_sorting_L(item)
>      <built-in method index of str object at ...>
>
>      Relies on global constant ELEMENT_TO_USE_FOR_INDEXING_WHEN_SORTING_L.
>      May raise NameError if that constant is missing.
>
>      Warning: do not use this for anything else.
>      """
>      return item[ELEMENT_TO_USE_FOR_INDEXING_WHEN_SORTING_L].index
>
> L.sort(key=_get_oneth_element_index_to_use_as_keyfunc_when_sorting_L)
> del _get_oneth_element_index_to_use_as_keyfunc_when_sorting_L
> # Better to be safe than sorry.
> del ELEMENT_TO_USE_FOR_INDEXING_WHEN_SORTING_L
>
> Definitely being paid by the line :-)
>

One of the finest examples of extracting the urine I've ever read. 
Please keep up the good work.  Ah but wait, down voted on the grounds 
that there are no unit tests and, far more importantly, it doesn't fit 
on one line (unless you use gg that is).  There is also no proof that 
it's been committed to your source control system after going through 
its code review.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

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