New submission from Barry A. Warsaw:
operator.index() is just a thin wrapper around PyNumber_Index(). The
documentation for operator.index() claims that it is equivalent to calling
obj.__index__() but for subclasses of int, this is not true. In fact,
PyNumber_Index() first does (e.g. in Python 3.3) a PyLong_Check() and if that
succeeds, the original object is returned *without* doing the moral equivalent
in C of calling obj.__index__(). An example:
class myint(int):
def __index__(self):
return int(self) + 1
>>> x = myint(7)
>>> x.__index__()
8
>>> from operator import index
>>> index(x)
7
The C API documents PyNumber_Index() as: "Returns the o converted to a Python
int on success or NULL with a TypeError exception raised on failure."
Because this has been the behavior of PyNumber_Index() since at least 2.7 (I
didn't check farther back), this probably cannot be classified as a bug
deserving to be fixed in the code for older Pythons. It might be worth fixing
for Python 3.4, i.e. by moving the index check before the type check. In the
meantime, this is probably a documentation bug.
The C API implies, but should be clearer that if o is an int subtype (int and
long in Python 2), it is returned unchanged. The operator.index()
documentation should be amended to describe this behavior for int/long
subclasses.
A different alternative would be to leave PyNumber_Index() unchanged, but with
the doco fix, and to augment operator.index() to do the PyIndex_Check() first,
before calling PyNumber_Index(). That's a little more redundant, but would
provide the documented behavior without changing the C API.
----------
assignee: docs@python
components: Documentation
messages: 185522
nosy: barry, docs@python
priority: normal
severity: normal
status: open
title: PyNumber_Index() is not int-subclass friendly
type: behavior
versions: Python 2.7, Python 3.1, Python 3.2, Python 3.3, Python 3.4
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue17576>
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