[code-quality] __len__ but no __getitem__

[Skip Montanaro]

Having demonstrated my confusion about early breaks from loops, I will
proceed to demonstrate my confusion about containers.

I have a queue-like class in which I implement __len__ but not
__getitem__. Pylint complains:

timeddata.py:79: [R0924(incomplete-protocol), TimedDataQueue] Badly
implemented Container, implements __len__ but not __getitem__

I can see where that would be a valid complaint for an array-like
container, but not all containers should support indexing even if they
have a measurable length. Python sets are one example:

>>> s = set(range(10))
>>> len(s)
10
>>> s[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'set' object does not support indexing

Queue-like containers seem similar.  While I would allow access to the
element at one end or the other (depending if I want queue-like or
stack-like behavior), I think it would violate the definition of those
types to allow indexing.

Should pylint really be this strict? Or am I expected to implement
everything necessary for an array-like containiner and just raise
exceptions in those methods the user really shouldn't access?

Thanks,

Skip