slice notation as values?

Steven Bethard steven.bethard at gmail.com
Sat Dec 10 22:13:35 CET 2005


Antoon Pardon wrote:
> On 2005-12-10, Steven Bethard <steven.bethard at gmail.com> wrote:
> 
>>Antoon Pardon wrote:
>>
>>>So lets agree that tree['a':'b'] would produce a subtree. Then
>>>I still would prefer the possibility to do something like:
>>>
>>>  for key in tree.iterkeys('a':'b')
>>>
>>>Instead of having to write
>>>
>>>  for key in tree['a':'b'].iterkeys()
>>>
>>>Sure I can now do it like this:
>>>
>>>  for key in tree.iterkeys('a','b')
>>>
>>>But the way default arguments work, prevents you from having
>>>this work in an analague way as a slice.
>>
>>How so?  Can't you just pass the *args to the slice contstructor?  E.g.::
>>
>>     def iterkeys(self, *args):
>>         keyslice = slice(*args)
>>         ...
>>
>>Then you can use the slice object just as you would have otherwise.
> 
> This doesn't work for a number of reasons,
> 
> 1) 
>>>>slice()
> 
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
>   TypeError: slice expected at least 1 arguments, got 0

I wasn't sure whether or not the slice argument was optional. 
Apparently it's intended to be, so you have to make one special case:

def iterkeys(self, *args):
     keyslice = args and slice(*args) or slice(None, None, None)

> 2) It doens't give a clear way to indicate the following
>    kind of slice: tree.iterkeys('a':). Because of the
>    follwing:
> 
>>>>slice('a')
> slice(None, 'a', None)
> 
>    which would be equivallent to tree.iterkeys(:'a')

Well, it certainly gives a way to indicate it:

     tree.iterkeys(None, 'a')

Whether or not it's a "clear" way is too subjective of a topic for me to 
get into.  That's best left to Guido[1].  My point is that it *does* 
work, and covers (or can be slightly altered to cover) all the 
functionality you want.  That doesn't mean you have to like the API for 
it, of course.

STeVe

[1] By which I mean that you should submit a PEP on the idea, and let 
Guido decide which way is prettier.  Just be sure to give all the 
equivalent examples - i.e. calling the slice constructor with the 
appropriate arguments.



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