The code

def x(((y))):pass 

crashes the compiler (?) in 2.5c2, on Windows.

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The problem is in Python/ast.c around line 666.  See the
comment:

/* def foo((x)): setup for checking NAME below. */

The code is not sufficient, we need a loop and need to
handle various combinations of:

def f(((((x))),)),))): pass

I don't know if the parens above match, but the general idea
is that there could be a bunch of parens and commas at
various places.  I'm not sure how the above should be
interpreted.

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I guess what 2.4 does is the most reasonable behavior:

>>> def f((((((x)),),),)): pass
>>> dis.dis(f)
  1           0 LOAD_FAST                0 (.0)
              3 UNPACK_SEQUENCE          1
              6 UNPACK_SEQUENCE          1
              9 UNPACK_SEQUENCE          1
             12 STORE_FAST               1 (x)
             15 LOAD_CONST               0 (None)
             18 RETURN_VALUE        

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The attached patch seems to fix the ((((x)))) problem.  I
didn't run in debug mode, so I'm not positive the assert
works as I expect.

However now my test case below doesn't work, it puts in 5
UNPACK_SEQUENCES rather than 3.  This looks like a different
problem in compiler_complex_args().

Not sure how much farther I'll get tonight.

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I think patch v2 might fix both problems.  I'm not sure it's
correct.  We really need a lot of tests for this stuff.

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Attaching a new patch with tests.  There probably should be
more tests, but this is all I can think of at this point.  I
think I've covered the cases of the recursive definition
properly now.

Conceptually this is a very small patch.  I've added a bunch
of asserts and broken some complex lines up though.

The first chunk deals with complex_args and elides
superfluous parens around (x).  The second chunk does the
same eliding but for non-complex args.  Any number of extra
parens should be elided properly now.

I will apply to head and 2.5.1 later.

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Committed revision 51972. (2.6)
Still needs backport.

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Backported in rev. 51999.