string issue

[Steven Bethard]

rbt wrote:
> John J. Lee wrote:
> 
>> Steve Holden <steve at holdenweb.com> writes:
>> [...]
>>
>>> You are modifying the list as you iterate over it. Instead, iterate
>>> over a copy by using:
>>>
>>> for ip in ips[:]:
>>>   ...
>>
>> Just to help popularise the alternative idiom, which IMO is
>> significantly less cryptic (sane constructors of mutable objects
>> almost always make a copy, and list is no exception: it's guaranteed
>> to do so):
>>
>> for ip in list(ips):
>>    ...
>>
>> Works back to at least Python 1.5.2.
> 
> I don't know that that approach is less cryptic. ips is already a 
> list... it looks cryptic to make it a list again, doesn't it? IMO, the 
> two are equally cryptic. The epitome of clarity would be copy(ips)... 
> now *that* makes sense, of course, ips[:] or list(ips) work equally well 
> to the programmer who has learned them.


This probably seems cryptic until you realize that almost all builtin 
mutable objects work this way:

py> def copy(obj):
...     return type(obj)(obj)
...

py> lst = range(5)
py> copy(lst)
[0, 1, 2, 3, 4]
py> lst is copy(lst)
False

py> dct = dict(a=1, b=2)
py> copy(dct)
{'a': 1, 'b': 2}
py> dct is copy(dct)
False

py> st = set(range(5))
py> copy(st)
set([0, 1, 2, 3, 4])
py> st is copy(st)
False

py> from collections import deque
py> dq = deque(range(5))
py> copy(dq)
deque([0, 1, 2, 3, 4])
py> dq is copy(dq)
False

If you're uncomfortable with this form, I'd suggest spending some time 
playing around with it -- Python is pretty consistent about this usage 
of a builtin type.

Steve