Lie Ryan, 01.09.2010 15:46:
On 09/01/10 00:09, Aahz wrote:
However, I think there are some rock-bottom basic guarantees we can make
regardless of implementation. Does anyone seriously think that an
implementation would be accepted that had anything other than O(1) for
index access into tuples and lists? Dicts that were not O(1) for access
with non-pathological hashing? That we would accept sets having O()
performance worse than dicts?
I suggest that we should agree on these guarantees and document them in
the core.
While I think documenting them would be great for all programmers that
care about practical and theoretical execution speed; I think including
these implementation details in core documentation as a "guarantee"
would be a bad idea for the reasons Terry outlined.
One way of resolving that is by having two documentations (or two
separate sections in the documentation) for:
- Python -- the language -- documenting Python as an abstract language,
this is the documentation which can be shared across all Python
implementations. This will also be the specification for Python Language
which other implementations will be measured to.
- CPython -- the Python interpreter -- documents implementation details
and performance metrics. It should be properly noted that these are not
part of the language per se. This will be the playground for CPython
experts that need to fine tune their applications to the last drop of
blood and don't mind their application going nuts with the next release
of CPython.
I disagree. I think putting the "obvious" guarantees right into the normal
documentation will actually make programmers aware that there *are*
different implementations (and differences between implementations), simply
because it wouldn't just say "O(1)" but "the CPython implementation of this
method has an algorithmic complexity of O(1), other Python implementations
are known to perform alike at the time of this writing". Maybe without the
last half of the sentence if we really don't know how other implementations
work here, or if we expect that there may well be a reason they may choose
to behave different, but in most cases, it shouldn't be hard to make that
complete statement.
After all, we basically know what other implementations there are, and we
also know that they tend to match the algorithmic complexities at least for
the major builtin types. It seems quite clear to me as a developer that the
set of builtin types and "collections" types was chosen in order to cover a
certain set of algorithmic complexities and not just arbitrary interfaces.
Stefan
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