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This is probably harmless, but Modules/_csv.c has the following code:
 Py_INCREF(&Dialect_Type);
 if (PyModule_AddObject(module, "Dialect", (PyObject *)&Dialect_Type))
 return NULL;
However, PyModule_AddObject returns only -1 and 0. It also doesn't decref Dialect_Type if it returns -1 so I guess more correct code should be:
 Py_INCREF(&Dialect_Type);
 if (PyModule_AddObject(module, "Dialect", (PyObject *)&Dialect_Type) == -1) {
 Py_DECREF(&Dialect_Type);
 return NULL;
 }
The same pattern can be found in a few more modules.

Testing returned value of PyModule_AddObject() is correct. This is a matter of style what to use: `if (...)`, `if (... == -1)` or `if (... < 0)`.
But the problem with a leak is more general. I have opened a discussion on Python-Dev: http://comments.gmane.org/gmane.comp.python.devel/157545 .

Thanks for the write-up, Serhiy.
It looks like "... == -1" is more popular in the codebase (for PyModule_AddObject, "... < 0" is the most popular style).
Here is a patch to document the current behavior of PyModule_AddObject.

Added comments on Rietveld.

As for Modules/_csv.c, I think it is better to change the behavior of PyModule_AddObject() (issue26871). This will fix similar issues in all modules.

Thanks for the review Serhiy. Here is an updated patch.

Serhiy, do you have further comments about issue26868_v2.diff?

issue26868_v2.diff LGTM.

It looks like the idiom of calling PyModule_AddObject without Py_DECREF'ing on the error condition (or even checking for it at all) has spread quite a bit since this first reported. I'm preparing a PR to fix the other call sites.

New changeset 224b8aaa7e8f67f748e8b7b6a4a77a25f6554651 by Stéphane Wirtel (Brandt Bucher) in branch 'master':
bpo-26868: Fix example usage of PyModule_AddObject. (#15725)
https://github.com/python/cpython/commit/224b8aaa7e8f67f748e8b7b6a4a77a25f6554651

New changeset 535863e3f599a6ad829204d83f144c91e44de443 by Miss Islington (bot) in branch '3.8':
bpo-26868: Fix example usage of PyModule_AddObject. (GH-15725)
https://github.com/python/cpython/commit/535863e3f599a6ad829204d83f144c91e44de443

New changeset c8d1338441114fbc504625bc66607e7996018a5d by Miss Islington (bot) in branch '3.7':
bpo-26868: Fix example usage of PyModule_AddObject. (GH-15725)
https://github.com/python/cpython/commit/c8d1338441114fbc504625bc66607e7996018a5d