[Python-Dev] PEP 0424: A method for exposing a length hint

Mark Shannon mark at hotpy.org
Sun Jul 15 16:39:07 CEST 2012

Nick Coghlan wrote:
> Right, I agree on the value in being able to return something to say 
> "this cannot be converted to a concrete container".
> I still haven't seen a use case where the appropriate response to "I 
> don't know" differs from the appropriate response to a hint of zero - 
> that is, you don't preallocate, you just start iterating.

There seem to be 5 possible classes values of __length_hint__ that an
iterator object can provide:

1. Don't implement it at all.

2. Implement __length_hint__() but don't want to return any value.
    Either raise an exception (TypeError) -- As suggested in the PEP.
    or return NotImplemented -- my preferred option.

3. Return a "don't know" value:
    Returning 0 would be fine for this, but the VM might want to respond
    differently to "don't know" and 0.
	__length_hint__() == 0             container should be minimum size.
         __length_hint__() == "unknown"     container starts at default size.

4. Infinite iterator:
    Could return float('inf'), but given this is a "hint" then
    returning sys.maxsize or sys.maxsize + 1 might be OK.
    Alternatively raise an OverflowError

5. A meaningful length. No problem :)

Also, what are the allowable return types?

1. int only
2. Any number (ie any type with a __int__() method)?
3. Or any integer-like object (ie a type with a __index__() method)?

My suggestion:

a) Don't want to return any value or "don't know": return NotImplemented
b) For infinite iterators: raise an OverflowError
c) All other cases: return an int or a type with a __index__() method.


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