[Python-Dev] Baffled by PyArg_ParseTupleAndKeywords modification (original) (raw)

Jeremy Hylton jeremy at alum.mit.edu
Fri Feb 10 17:30:30 CET 2006

On 2/10/06, Guido van Rossum <guido at python.org> wrote:

OMG. Are we now adding 'const' modifiers to random places? I thought "const propagation hell" was a place we were happily avoiding by not falling for that meme. What changed?

I added some const to several API functions that take char* but typically called by passing string literals. In C++, a string literal is a const char* so you need to add a const_cast<> to every call site, which is incredibly cumbersome. After some discussion on python-dev, I made changes to a small set of API functions and chased the const-ness the rest of the way, as you would expect. There was nothing random about the places const was added.

I admit that I'm also puzzled by Jack's specific question. I don't understand why an array passed to PyArg_ParseTupleAndKeywords() would need to be declared as const. I observed the problem in my initial changes but didn't think very hard about the cause of the problem. Perhaps someone with better C/C++ standards chops can explain.

Jeremy

--Guido On 2/10/06, Jack Jansen <Jack.Jansen at cwi.nl> wrote: > I keep running into problems with the "const" modifications to > PyArgParseTupleAndKeywords() (rev. 41638 by Jeremy). > > I have lots of code of the form > char *kw[] = {"itself", 0}; > > if (PyArgParseTupleAndKeywords(args, kwds, "O&", kw, > CFTypeRefObjConvert, &itself)) ... > which now no longer compiles, neither with C nor with C++ (gcc4, both > MacOSX and Linux). Changing the kw declaration to "const char *kw[]" > makes it compile again. > > I don't understand why it doesn't compile: even though the > PyArgParseTupleAndKeywords signature promises that it won't change > the "kw" argument I see no reason why I shouldn't be able to pass a > non-const argument. > > And to make matters worse adding the "const" of course makes the code > non-portable to previous versions of Python (where the C compiler > rightly complains that I'm passing a const object through a non-const > parameter). > > Can anyone enlighten me? > -- > Jack Jansen, <Jack.Jansen at cwi.nl>, http://www.cwi.nl/~jack > If I can't dance I don't want to be part of your revolution -- Emma > Goldman > > _> ________________________ > Python-Dev mailing list > Python-Dev at python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org >

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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