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