Shadowing built-ins [was Re: filter a list of strings]

[Steven D'Aprano]

On Wednesday 09 December 2015 09:58, Thomas 'PointedEars' Lahn wrote:

> Mark Lawrence wrote:
> 
>> On 03/12/2015 01:15, c.buhtz at posteo.jp wrote:
>>> I would like to know how this could be done more elegant/pythonic.
>>>
>>> I have a big list (over 10.000 items) with strings (each 100 to 300
>>> chars long) and want to filter them.
>>>
>>> list = .....
>>> […]
[...]
> However, “list” is a built-in class/constructor that would be overwritten
> this way.  One should choose another identifier than “list” for one’s
> variables.

Correct, well mostly correct.

The original built-in list is *not* overridden, as such, but shadowed: it 
still exists, but access to it is blocked by the local variable "list". The 
list *type* still exists, only the *name* list is shadowed. So you can 
easily recover from accidentally shadowing the name by deleting the local:

del list

To beginners, accidental shadowing of built-ins is a serious source of 
confusion. But to experts, intentional shadowing can be useful.

There's nothing wrong with (say):

def sort_and_pop(list):
    list.sort()
    list.pop(0)

Yes, the local "list" shadows the built-in list. So what?

More usefully, one can monkey-patch a module by shadowing a built-in in that 
module:

# Python 3 only.
import builtins
def print(*args, **kwargs):
    log_something()
    builtins.print(*args, **kwargs)




-- 
Steve