[Python-Dev] bug in grammar

[Jeremy Hylton]

As part of the implementation of PEP 227 (and in an attempt to reach
some low-hanging fruit Guido mentioned on the types-sig long ago), I
have been working on a compiler pass that generates a module-level
symbol table.  I recently discovered a bug in the handling of list
comprehensions that was giving me headaches.

I realize now that the problem is with the current grammar and/or
compiler.  Here's a simple demonstration; try it in your friendly
python 2.0 interpreter.

>>> [i for i in range(10)] = (1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: unpack list of wrong size

The generated bytecode is:

          0 SET_LINENO               0

          3 SET_LINENO               1
          6 LOAD_CONST               0 (1)
          9 LOAD_CONST               1 (2)
         12 LOAD_CONST               2 (3)
         15 BUILD_TUPLE              3
         18 UNPACK_SEQUENCE          1
         21 STORE_NAME               0 (i)
         24 LOAD_CONST               3 (None)
         27 RETURN_VALUE        

I assume this isn't intended :-).  The compiler is ignoring everything
after the initial atom in the list comprehension.  It's basically
compiling the code as if it were:

[i] = (1, 2, 3)

I'm not sure how to try and fix this.  Should the grammar allow one to
construct the example statement above?  If not, I'm not sure how to
fix the grammar.  If not, I suppose the compiler should detect that
the list comp is misplaced.  This seems fairly messy, since there are
about 10 nodes between the expr_stmt and the list_for.

Or is this a cool way to use list comprehensions to generate
ValueErrors?

Jeremy