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If the `step` parameter for `itertools.count()` rounds to 1 (f.i., 1.1, 1.437643, 1.99999), then it fallsback to fast (integer) mode and increases the counter by 1. Here's an example:
Python 3.6.0a4+ (default:ddc95a9bc2e0+, Sep 8 2016, 14:46:19)
>>> import itertools
>>> for i in itertools.count(1, step=1.5):
... print(i)
... if i > 10:
... break
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Here's a first approach on fixing this bug. I'm not sure how to handle the case where step=1.0.

I think the fix nails it; all the problem was that the "fast" mode was wrongly detected, and all the problems (counting badly, or a bad repr, etc) is a problem after setting cnt into PY_SSIZE_T_MAX.
IIUC there is nothing special to do when step=1.0, as later on the original objects are used (they don't suffer a wrong cast to integer anymore).

New patch following comments from SilentGhost.

PyLong_AsLong() not just converts Python integer to C long, but it also converts Python float to Python integer (with the lost of the fractional part). We need to check that the value is Python integer.
Here is a patch that fixes this issue and few other bugs.

New changeset b23f963a7b45 by Serhiy Storchaka in branch '3.5':
Issue #28019: itertools.count() no longer rounds non-integer step in range
https://hg.python.org/cpython/rev/b23f963a7b45
New changeset 74667320778e by Serhiy Storchaka in branch '2.7':
Issue #28019: Backported additional tests for itertools.count().
https://hg.python.org/cpython/rev/74667320778e
New changeset 51dfab4f28e7 by Serhiy Storchaka in branch 'default':
Issue #28019: itertools.count() no longer rounds non-integer step in range
https://hg.python.org/cpython/rev/51dfab4f28e7