With Neal Norwitz's help, I've developed this patch
which implements the with statement from PEP 343.  I
still need to flesh out the documentation, and the
contextmanager.py and nested.py files (copied almost
verbatim from the PEP) are very bare-bones, but the
with syntax itself is accompanied by a rather extensive
test.

Most of the patch reflects changes in automatically
generated files.  The most extensive changes were in
Python/ast.c and Python/compile.c, and the test resides
in Lib/test/with_test.py.

I'm very open to comments and suggestions, particularly
with regards to the contortions in compiler_with() in
Python/compile.c.

Also, a few questions regarding the PEP itself:

- Should the grammar be strict about not allowing
extraneous parentheses around "(NAME)" in the
with_var_name rule (Grammar/Grammar)?

- Should "(NAME ,)" (note the trailing comma) be a
valid VAR list?  Compared with the rule for global, the
current definition seems to suffice, but (according to
Neal) seems inconsistent in the light of typical tuple
syntax.

- Should VAR names that are not instantiated prior to
the with statement exist after exiting the scope of the
with statement?

Thanks,

Mike

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Thanks Mike!  I'm uploading a much improved version, mostly
for the benefit of Thomas.  I plan to check this in soon,
even though it doesn't have the __future__ statement or docs
yet.

I'm also moving nested.py and contextmanager.py into
Lib/test; the PEP was ambiguous but there was no plan to
make these part of the standard library yet.  (Only locks,
files and decimal will get context managers.)

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I'm closing this patch, since I've checked in most of the work.

Thomas is working on the __future__ statement.

We still need docs.  Mike, if you are working on those, just
email them to me or Neal for checkin.

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Sure, I'll work up some doc patches--hopefully by the end of
the week, this weekend fer sure.