I've been testing various 3rd-party python code against 3.3b1, and see
  ValueError: level must be >= 0
exceptions where C code is using PyImport_ImportModuleEx.

PyImport_ImportModuleEx reads as
    63 #define PyImport_ImportModuleEx(n, g, l, f) \
    64     PyImport_ImportModuleLevel(n, g, l, f, -1)
within http://hg.python.org/cpython/file/aaa68dce117e/Include/import.h as of now (2012-08-09)

Within PyImport_ImportModuleLevel there's this check:
  1280      if (level < 0) {
  1281          PyErr_SetString(PyExc_ValueError, "level must be >= 0");
  1282          goto error;
  1283      }
which thus always fires.

So it would seem that currently any usage of PyImport_ImportModuleEx will fail.

(FWIW, this was observed when compiling pygobject-3.3.4 against Python-3.3.0b1)

Can you please try my amazing patch?

Oh, I didn't realize that the documentation says that the default value is -1.
http://docs.python.org/library/functions.html#__import__

"level specifies whether to use absolute or relative imports. The default is -1 which indicates both absolute and relative imports will be attempted. 0 means only perform absolute imports. Positive values for level indicate the number of parent directories to search relative to the directory of the module calling __import__()."

We should probably tolerate -1, or just drop the exception.

On Thu, 2012-08-09 at 21:04 +0000, STINNER Victor wrote:
> STINNER Victor added the comment:
> 
> Oh, I didn't realize that the documentation says that the default value is -1.
> http://docs.python.org/library/functions.html#__import__
> 
> "level specifies whether to use absolute or relative imports. The default is -1 which indicates both absolute and relative imports will be attempted. 0 means only perform absolute imports. Positive values for level indicate the number of parent directories to search relative to the directory of the module calling __import__()."
That's the python 2 documentation

The 3.3 docs here:
  http://docs.python.org/dev/library/functions.html#__import__
say "Changed in version 3.3: Negative values for level are no longer
supported (which also changes the default value to 0)."

> We should probably tolerate -1, or just drop the exception.

Sounds like a rather annoying regression. Changing the macro's expansion would be good enough IMO.

> Sounds like a rather annoying regression.

PyImport_ImportModuleLevel() is part of the stable API. Is it an
acceptable to not only change the default value but also fail with the
previous *default* value?

Can't we just drop the check "level < 0"?

> Changing the macro's expansion would be good enough IMO.

Sounds good to me.

> PyImport_ImportModuleLevel() is part of the stable API...

From what I understand, as long as the function header has not changed, the stable ABI is still stable.

> Can't we just drop the check "level < 0"?

In Python 3 a negative value makes no sense, since there are no accommodations for implicit relative imports.  The fact that builtins.__import__() accommodated -1 still was an oversight that was corrected in 3.3.  Looks like this is just one bit that got missed.

OK, the macro expansion should get fixed, a versionchanged should probably be added to the C API docs (for PyImport_ImportModuleLevel()), and a line in What's New for porting C code should be added.

We can't go back to -1, as Eric said, because it makes no sense anymore since you can't syntactically do an import that has -1 level semantics in Python 3. The fact that __import__ accepted a negative level was a bug that went unnoticed up until this point since so few people import modules  programmatically and want implicit relative imports.

New changeset 9804aec74d4a by Brett Cannon in branch 'default':
Issue #15610: The PyImport_ImportModuleEx macro now calls
http://hg.python.org/cpython/rev/9804aec74d4a

Hopefully the 3rd-party code using PyImport_ImportModuleEx will work as expected with a 'level' of 0.