[Patches] [ python-Patches-1660500 ] Hide iteration variable in list comprehensions

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Patches item #1660500, was opened at 2007-02-15 21:29
Message generated for change (Comment added) made by ncoghlan
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Category: Parser/Compiler
Group: Python 3000
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Nick Coghlan (ncoghlan)
Assigned to: Georg Brandl (gbrandl)
Summary: Hide iteration variable in list comprehensions

Initial Comment:
This patch hides the iteration variable in list comprehensions.
It adds new tests (modelled on the generator expression tests) and also removes some del statements from the standard library (where code previously cleaned up its own namespace).
The changes to symtable.[ch] are more significant than strictly necessary - I found it necessary to spend some time cleaning up the code in order to understand what was needed for the list comprehension changes. Given that the 2.x and 3.0 compilers have already diverged fairly significantly, I don't believe this will make the process of keeping them in sync any more difficult than it is already.

Assigning to Georg for initial review (as his set comprehensions patch provided a great deal of inspiration for this one).

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>Comment By: Nick Coghlan (ncoghlan)
Date: 2007-02-15 22:13

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For reference, the original set comprehensions patch & the py3k list
discussion:
  http://www.python.org/sf/1548388
  http://mail.python.org/pipermail/python-3000/2006-December/005188.html

Note that the current patch ended up looking a *lot* like the original one
(the main difference specific to list comprehensions is that the temporary
list is built in the inner scope and then returned rather than being passed
in as an argument. Additionally, the code has been unified only for the
symtable stage - the AST and compilation stages still use separate code for
listcomps and genexps).

It turns out that there are some really curly scoping problems that using
a nested function deals with automatically (see the new test_listcomps.py
in the patch for examples). Having a more efficient mechanism specifically
for 'transient' scopes like this is an interesting idea, but it's far from
easy (mainly because the variables in the scope still need to be properly
visible in scopes it *contains*).

Adding set comprehensions on top of this patch should be pretty
straightforward.

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Comment By: Nick Coghlan (ncoghlan)
Date: 2007-02-15 21:53

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Speed measurements show a significant speed up over trunk & Python 2.4 for
module/class level code:

(Python 2.4)$ python -m timeit -s "seq=range(1000)" "[[x for x in seq] for
y in seq]"
10 loops, best of 3: 239 msec per loop
(Python 2.x trunk)$ ./python -m timeit -s "seq=range(1000)" "[[x for x in
seq] for y in seq]"
10 loops, best of 3: 193 msec per loop
(Python 3000)$ ./python -m timeit -s "seq=range(1000)" "[[x for x in seq]
for y in seq]"
10 loops, best of 3: 176 msec per loop

This is almost certainly due to the variables and the list object becoming
function locals.

There is a slowdown inside a function (but we are still faster than Python
2.4):

(Python 2.4)$ python -m timeit -s "seq=range(1000)" -s "def f(): return
[[x for x in seq] for y in seq]" "f()"
10 loops, best of 3: 259 msec per loop
(Python 2.x trunk)$ ./python -m timeit -s "seq=range(1000)" -s "def f():
return [[x for x in seq] for y in seq]" "f()"
10 loops, best of 3: 176 msec per loop
(Python 3000)$ ./python -m timeit -s "seq=range(1000)" -s "def f(): return
[[x for x in seq] for y in seq]" "f()"
10 loops, best of 3: 185 msec per loop


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