Raymond Hettinger added the comment:
The different data types make different choices:
>>> from decimal import Decimal
>>> from fractions import Fraction
>>> (1).__round__(None)
Traceback (most recent call last):
File "<pyshell#39>", line 1, in <module>
(1).__round__(None)
TypeError: 'NoneType' object cannot be interpreted as an integer
>>> (1.0).__round__(None)
1
>>> Decimal(1).__round__(None)
Traceback (most recent call last):
File "<pyshell#41>", line 1, in <module>
Decimal(1).__round__(None)
TypeError: optional arg must be an integer
>>> Fraction(1, 1).__round__(None)
1
>>> from _pydecimal import Decimal
>>> Decimal(1).__round__(None)
1
For Fraction and _pydecimal, the behavior comes from using None as a
placeholder (which is common and normal in pure python code). For float there
is explicit code to test for the None case. For int, the None test was
omitted (perhaps a mistake) and the error is raised by PyNumber_Index.
Looking through tests, only Lib/test/test_float.py tests for None being
allowable. Elsewhere, it seems to be an implementation detail.
The old Python 2 version of the round() function never let None be passed in
(because it used PyArg_ParseTupleAndKeywords(args, kwds, "d|i:round"). That
logic seems to get lost in the Python 3 version when __round__ was introduced.
To resolve the differences, I think the round() function should explicitly
check for None and replace it with zero before calling the underling __round__
functions where we would allow variable according the needs of the
implementation.
----------
nosy: +rhettinger
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue27936>
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