The 2.5 compiler forgets how to count linenumbers in
certain situations:

>>> s255 = "".join(["\n"] * 255 + ["spam"])
>>> s256 = "".join(["\n"] * 256 + ["spam"])

>>> exec s255
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 256, in <module>
NameError: name 'spam' is not defined

>>> exec s256
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'spam' is not defined

Notice the 'line 1' linenumber in the case of 256 blank
lines. The same happens for 'pass' statements or 'if 0'
blocks instead of blank lines. The problem is in the
actual code objects created:

>>> dis.disco(compile(s255, "", "exec"))
256           0 LOAD_NAME                0 (spam)
              3 POP_TOP             
              4 LOAD_CONST               0 (None)
              7 RETURN_VALUE        
>>> dis.disco(compile(s256, "", "exec"))
  1           0 LOAD_NAME                0 (spam)
              3 POP_TOP             
              4 LOAD_CONST               0 (None)
              7 RETURN_VALUE        

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Committed revision 50500.

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Unfortunately, it isn't quite fixed. It's fixed for code in
the global scope, but not for functions:

>>> s255 = "def foo():\n " + "".join(["\n "] * 254 + ["
spam\n"])
>>> exec s255
>>> dis.dis(foo)
.256           0 LOAD_GLOBAL              0 (spam)
.              3 POP_TOP             
.              4 LOAD_CONST               0 (None)
.              7 RETURN_VALUE        

>>> s256 = "def foo():\n " + "".join(["\n "] * 255 + ["
spam\n"])
>>> exec s256
>>> dis.dis(foo)
.  1           0 LOAD_GLOBAL              0 (spam)
.              3 POP_TOP             
.              4 LOAD_CONST               0 (None)
.              7 RETURN_VALUE        

I haven't tried figuring out for what else it's broken like
this, sorry :)

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Try the attached patch on for size.  Let me know if you can
find any other holes.

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Yep, the patch seems to work, also for the other case
someone pointed out to me, that was still broken after the
last fix:

>>> exec "import os\ns='''" + "\n"*256 + "'''\n1/0\n"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

I tried a couple of other ways to get large linenumbers and
complex(er) parsing structures, and they all seem to work right.

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Committed revision 50673.

Hopefully I'm really done this time. :-)