Many newbie questions regarding python

Ethan Furman ethan at
Mon Oct 11 03:36:27 CEST 2010

Peter Pearson wrote:
> On Sat, 09 Oct 2010 19:30:16 -0700, Ethan Furman <ethan at> wrote:
>>Steven D'Aprano wrote:
> [snip]
>>>But that doesn't mean that the list comp is the general purpose solution. 
>>>Consider the obvious use of the idiom:
>>>def func(arg, count):
>>>    # Initialise the list.
>>>    L = [arg for i in range(count)]
>>>    # Do something with it.
>>>    process(L, some_function)
>>>def process(L, f):
>>>    # Do something with each element.
>>>    for item in enumerate(L):
>>>        f(item)
>>>Looks good, right? But it isn't, because it will suffer the exact same 
>>>surprising behaviour if f modifies the items in place. Using a list comp 
>>>doesn't save you if you don't know what the object is.
>>I've only been using Python for a couple years on a part-time basis, so 
>>I am not aquainted with this obvious use -- could you give a more 
>>concrete example?  Also, I do not see what the list comp has to do with 
>>the problem in process() -- the list has already been created at that 
>>point, so how is it the list comp's fault?
> Well, 


> the unwary
> reader (including me, not too long ago) expects that
>   L = [arg for i in range(count)]
> will be equivalent to
>   L = [[], [], []]
> but it's not, because the three elements in the first L are three
> references to the *same* list.  Observe:


My question is more along the lines of: a mutable object was passed in 
to func()... what style of loop could be used to turn that one object 
into /n/ distinct objects?  A list comp won't do it, but neither will a 
for loop, nor a while loop.

Further, Steven stated "it will suffer the exact same surprising 
behaviour if f modifies the items in place" -- the damage has already 
been done by that point, as L has however many copies of the *same* object.

Seems to me you would have to use copy.copy or copy.deepcopy to be safe 
in such a circumstance, and the loop style is irrelevant.

If I've missed something, somebody please enlighten me.


More information about the Python-list mailing list