[New-bugs-announce] [issue5945] PyMapping_Check returns 1 for lists

[John Millikin]

New submission from John Millikin <jmillikin at gmail.com>:

In Python 2, PyMapping_Check will return 0 for list objects. In Python
3, it returns 1. Obviously, this makes it rather difficult to
differentiate between mappings and other sized iterables. In addition,
it differs from the behavior of the ``collections.Mapping`` ABC --
isinstance([], collections.Mapping) returns False.

Since most of the PyMapping_* functions don't seem to work properly on
lists, I believe this behavior is erroneous.

The behavior can be seen from a C extension, or if you're lazy, using
ctypes:

Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> ctypes.CDLL('libpython2.6.so').PyMapping_Check(ctypes.py_object([]))
0

Python 3.0.1+ (r301:69556, Apr 15 2009, 15:59:22)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> ctypes.CDLL('libpython3.0.so').PyMapping_Check(ctypes.py_object([]))
1

----------
components: Library (Lib)
messages: 87291
nosy: jmillikin
severity: normal
status: open
title: PyMapping_Check returns 1 for lists
type: behavior
versions: Python 3.0

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<http://bugs.python.org/issue5945>
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