------------ current state ------------

An assertion in Objects/rangeobject.c might fail:
    >>> type(iter(range(0)))
    <class 'range_iterator'>
    >>> type(iter(range(0)))(1, 1, 0)
    Assertion failed: step != 0, file ..\Objects\rangeobject.c, line 895

This is caused by the lack of a check whether 'step' is zero, during the creation of a range_iterator object, in rangeiter_new.

Note that during the creation of a range object, the function range_new does check that, by calling validate_step, which leads to the following behavior:
    >>> range(1, 1, 0)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: range() arg 3 must not be zero
    >>>

------------ proposed changes ------------
1. In Objects/rangeobject.c in rangeiter_new:
- in case 'step' is zero, raise a ValueError
- in error messages, replace 'rangeiter' with 'range_iterator', as the latter is the name of the type in Python code

2. In [Lib/test/test_range.py](https://github.com/python/cpython/blob/main/Lib/test/test_range.py), add tests for calling the range_iterator type (i.e. creating a range_iterator object)

------------ diff ------------
The proposed patches diff file is attached.

------------ tests ------------
I ran 'python_d.exe -m test -j3' (on my 64-bit Windows 10) with and without the patches, and got quite the same output. (That also means my new tests in test_range passed.)
The outputs of both runs are attached.

I'm not able to trigger an assertion error when running the latest trunk in debug mode. I get a "valid" range_iterator object though, and using reduce gives me direct access to range(0, 0, 0) (which is completely worthless).

Error or not, this should be fixed, and your patch LGTM. Thanks!

(I changed the type to 'behaviour' since I can't reproduce the assertion, but it doesn't make much of a difference)

patch is LGTM.

But there is one hidden inconsistency:

>>> r = range(2**100)
>>> type(iter(r))
<class 'longrange_iterator'>
>>> type(iter(r))(1, 1, 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 'longrange_iterator' instances

Should we have same tp_new method for longrange_iterator?

Good point Naoki. I think we can remove tp_new method from range_iterator in 3.7. Seems it is never used.

The patch LGTM for 3.5-3.6, but the test should be marked as CPython implementation detail (@support.cpython_only).

The diff files for 3.5 and 3.6 are attached (I only added @test.support.cpython_only, as you suggested).

The diff file for 3.7 is also attached. It contains:
- removing rangeiter_new
- tests to verify one cannot create instances of range_iterator or longrange_iterator (in CPython)
- added 'Iterator.register(longrange_iterator)' in Lib/_collections_abc.py

The output of 'python_d.exe -m test -j3' for each version is also attached (they were all successful).

P.S. If we remove the ability to create instances of range_iterator in 3.7, shouldn't we raise a DeprecationWarning in rangeiter_new in 3.5 and 3.6?

Raising a DeprecationWarning in 3.6 doesn't hurt.

rangeiter_new() was added in r55167.

And merged in a6eb6acfe04a.

The diff files for 3.6 and 3.7 are attached (named '*ver3.diff').
Changes:
- in 3.6, added:
* raising a DeprecationWarning in rangeiter_new
* a test to verify the DeprecationWarning is raised
- in 3.7:
* changed the tests so they would only verify a TypeError is raised when calling either range_iterator or longrange_iterator
* removed the test.support.cpython_only decorator of the test

The output of 'python_d.exe -m test -j3' for each version is also attached (they were both successful).

Thanks Oren. Patches for 3.5 and 3.7 LGTM, the patch for 3.6 should be fixed. Tests should pass with -Wa and -We.

No need to attach test output. Just check that your patch doesn't add a regression.

Alright, just added 'with test.support.check_warnings(('', DeprecationWarning)):'.
*ver4.diff is attached.
I ran the tests again (this time using 'python_d.exe -We -m test -j3'), and they were successful :)

That was totally my bad, of course, but anyway, ISTM that https://docs.python.org/devguide/index.html should say './python -We -m test -j3' in section 4 (Run the tests).

New changeset ee049edc3fff by Serhiy Storchaka in branch '3.5':
Issue bpo-28376: Fixed typos.
https://hg.python.org/cpython/rev/ee049edc3fff

New changeset e486f3d30e0e by Serhiy Storchaka in branch '3.5':
Issue bpo-28376: The constructor of range_iterator now checks that step is not 0.
https://hg.python.org/cpython/rev/e486f3d30e0e

New changeset 06f065a59751 by Serhiy Storchaka in branch '3.6':
Issue bpo-28376: Creating instances of range_iterator by calling range_iterator
https://hg.python.org/cpython/rev/06f065a59751

New changeset e099583400f3 by Serhiy Storchaka in branch 'default':
Issue bpo-28376: Creating instances of range_iterator by calling range_iterator
https://hg.python.org/cpython/rev/e099583400f3

New changeset ce4af7593e45 by Serhiy Storchaka in branch '3.5':
Issue bpo-28376: The type of long range iterator is now registered as Iterator.
https://hg.python.org/cpython/rev/ce4af7593e45

Thank you for your contribution Oren.

Thanks for the reviews and patience :)