[Python-ideas] Add a datatype collections.ListView

[Terry Reedy]

On 4/17/2014 11:49 AM, David Mertz wrote:

> What I'd really like is a "ListView" that acts something like NumPy's
> non-copying slices.

Consider a more generic SeqView, especially if you do not intend to 
mutate through the view. I also suggest that you use a range object 
instead of .start, .stop, (and .step). Range gives you a lot for free, 
including slicing.  Untested:

class SeqView:
     def __init__(self, seq, rng)
         self.seq = seq
         self.rng = rng

     def __getitem__(self, index):
         if isinstance(index, slice):
             return Seqview(self.seq, self.rng[index])
         else:
             return self.seq[self.rng[index]]  # see below
     def __iter__(self):
         seq, rng = self.seq, self.rng
         for i in rng:
             yield seq[i]
     ...

 >>> r = range(3, 50, 3)
 >>> r[4]
15

 >>> r[500]
Traceback (most recent call last):
   File "<pyshell#2>", line 1, in <module>
     r[500]
IndexError: range object index out of range
# You would want to catch this and change 'range' to SeqView

 >>> r[1:3]
range(6, 12, 3)
 >>> r[slice(1,3)]
range(6, 12, 3)
 >>> r[slice(1,3,2)]
range(6, 12, 6)

-- 
Terry Jan Reedy