semantics of [:]

[Ethan Furman]

Dave Angel wrote:
> Esmail wrote:
>>> Esmail schrieb:
>>>
>>>> Could someone help confirm/clarify the semantics of the [:] operator
>>>> in Python?
>>>>
>>>> a = range(51,55)
>>>>
>>>> ############# 1 ##################
>>>> b = a[:] # b receives a copy of a, but they are independent
>>
>> Semi-aside, if I wanted to make local copy of a list sent to me as a
>> parameter, which of these is the most appropriate to use (I don't want
>> changes to change the original list sent).
>>
> (1) is most appropriate in that case.  In fact, unless there is other 
> code mixed in  between, it's always the most appropriate.

Be aware: this is only a shallow copy -- if your copied list contains 
mutable objects that you mutate, those changes *will* show up in the 
original list.

Shallow copies are useful when, for example, you won't be mutating 
objects in the copied list but are adding, removing, or reordering the 
copied list.

Hope this helps.

~Ethan~