[Python-3000] [Python-Dev] Python 3000 Status Update (Long!)

[Nick Coghlan]

Greg Ewing wrote:
> Russell E. Owen wrote:
>> I would personally be happy lose set comprehensions and just use 
>> generator expressions for all comprehension-like tasks.
> 
> One advantage of the comprehension syntaxes is that the
> body can be inlined instead of relegated to a lambda,
> saving the overhead of a Python function call per
> loop.

I'm not sure what you mean by "function call per loop" in this 
paragraph. There is no function call per loop even when using a 
generator expression - a generator function is implicit defined, and 
then called once to instantiate the generator. Iterating over this 
suspends and resumes the generating to retrieve each item, rather than 
making a Python function call as such - is that behaviour what you were 
referring to?

Regardless, what the list and set comprehension syntax saves you is that 
instead of having to suspend/resume a generator multiple times while 
iterating over it to fill the container, the implicitly defined function 
instead creates and populates the desired container type directly. These 
operations are also compiled to use special opcodes, so they should be 
significantly faster than the corresponding pure Python code would be.

(I'd provide some timing figures, but my Py3k checkout is somewhat 
stale, so the timeit module isn't working for me at the moment)

To get back to the original question, I believe the point of adding set 
literal and comprehension syntax is to make it possible to easily speed 
up membership tests for items in known groups - the existing list 
literals are fast to create, but slow to search. Using a set literal 
instead of a list literal is also a good way to make it explicit that 
the order in which the items are added to the container is arbitrary and 
coincidental, rather than having any significant meaning.

Cheers,
Nick.





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