[Python-3000] Python-3000 Digest, Vol 9, Issue 27

[Nick Coghlan]

George Sakkis wrote:
> On 11/14/06, Fredrik Lundh <fredrik at pythonware.com> wrote:
> 
>> BJ?rn Lindqvist wrote:
>>
>>> But why is both the dict and list protocol so fat then? Is it hard to
>>> create your own dict or list-derived types in Python?
>> don't confuse things like lists and dictionaries with things like
>> sequences and mappings.  iterators and iterables belong to the second
>> category.
> 
> This doesn't answer my last question: why do we need itertools when we
> can live without sequencetools, mappingtools, fileliketools, etc. ?

Because the iterator protocol is simple enough to be easily composable - there 
are a wide variety of things that can be done with an object when all you know 
about it is that it is some form of iterable. The more assumptions you have to 
start making about the iterable, the less widely applicable the resulting 
operation will be.

And I think the number one reason we don't see a compelling need for the extra 
tool libraries you describe is the fact that sequences, mappings and files are 
themselves all iterables.

That said, there are actually a number of modules for working with sequences 
in the standard library, like bisect and heapq. They just aren't lumped into 
one place the way itertools and functools are.

Having a rich method API vs having a narrow method API and duck-typed support 
functions is a design trade-off. In the case of sequences and mappings, the 
trade-off went towards a richer API because the de facto reference 
implementations were the builtin dict and list classes (which is why DictMixin 
and ListMixin are so useful when implementing your own containers). In the 
case of iterables and iterators, the trade-off went towards the narrow API so 
that the interface could be used in a wide variety of situations (lines in a 
file, records in a database, characters in a string, bytes from a serial port, 
frames in a bowling game, active players in a MMORPG, etc, etc, etc).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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