Turn slice into range

Question

I'm using Python 3.3. I want to get a slice object and use it to make a new range object.

It goes something like that:

>>> class A:     def __getitem__(self, item):         if isinstance(item, slice):             return list(range(item.start, item.stop, item.step))  >>> a = A() >>> a[1:5:2] # works fine [1, 3] >>> a[1:5] # won't work :( Traceback (most recent call last):   File "<pyshell#18>", line 1, in <module>     a[1:5] # won't work :(   File "<pyshell#9>", line 4, in __getitem__     return list(range(item.start, item.stop, item.step)) TypeError: 'NoneType' object cannot be interpreted as an integer 

Well, the problem is obvious here - range doesn't accept None as a value:

>>> range(1, 5, None) Traceback (most recent call last):   File "<pyshell#19>", line 1, in <module>     range(1, 5, None) TypeError: 'NoneType' object cannot be interpreted as an integer 

But what is not obvious (to me) is the solution. How will I call range so it will work in every case? I'm searching for a nice pythonic way to do it.

Answer 1

There's an easier way to do this (at least in 3.4, I don't have 3.3 at the moment, and I don't see it in the changelog).

Assuming your class already has a known length you can just slice a range of that size:

>>> range(10)[1:5:2] range(1, 5, 2) >>> list(range(10)[1:5:2]) [1, 3] 

If you don't know the length a priori you'll have to do:

>>> class A:     def __getitem__(self, item):         if isinstance(item, slice):             return list(range(item.stop)[item]) >>> a = A() >>> a[1:5:2] [1, 3] >>> a[1:5] [1, 2, 3, 4] 

Answer 2

Try

class A:     def __getitem__(self, item):         ifnone = lambda a, b: b if a is None else a         if isinstance(item, slice):             if item.stop is None:                 # do something with itertools.count()             else:                 return list(range(ifnone(item.start, 0), item.stop, ifnone(item.step, 1)))         else:             return item 

This will reinterpret .start and .step appropriately if they are None.

---

Another option could be the .indices() method of a slice. It is called with the number of entries and reinterprets None to the appropriate values and wraps negative values around the given length parameter:

>>> a=slice(None, None, None) >>> a.indices(1) (0, 1, 1) >>> a.indices(10) (0, 10, 1) >>> a=slice(None, -5, None) >>> a.indices(100) (0, 95, 1) 

It depends what you intend to do with negative indices...