[C++-sig] vector_indexing_suite too defensive ...

[Mike Thompson]

I've found a small difference between the behaviour of python lists and
vector's wrapped by using the talented vector_indexing_suite.

Let's say I write a python function which forces a list to be a particular size
by padding it (if below the required size), or chopping it down (if above the
required size):

>>> def padOrChop(lst, size, padValue=0):
...     diff = size- len(lst)
...     lst[size:] = [padValue] * diff
...

This function work nicely with normal python lists ....

>>>
>>> L = [2, 3, 4, 5]
>>> padOrChop(L, 10)
>>> L
[2, 3, 4, 5, 0, 0, 0, 0, 0, 0]
>>> padOrChop(L, 2)
>>> L
[2, 3]
>>>

but it will fail with an object created through vector_indexing_suite whenever
it has to pad:

>>> L = IntVec()        # really a vector<int>
>>> L[:] = [2, 3, 4, 5]
>>> padOrChop(L, 10)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in padOrChop
IndexError: Index out of range
>>> padOrChop(L, 2)
>>> [i for i in L]
[2, 3]
>>>

The IndexError arises because vector_indexing_suite defends against an slice
index being too big, rather than the python list approach which regards a slice
index > len(L) as equal to len(L).  So, the line of padOrChop() which says:

    lst[size:] = [padValue] * diff

causes an exception when size > len(lst).

Should vector_indexing_suite be altered to give behaviour more like that of
lists in this regard?

--
Mike