[issue5945] PyMapping_Check returns 1 for lists

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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status: open -> closed

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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Are these changes enough? Can this issue be closed now?

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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 93e9fb5664e56c02c9aa89098b556929735b35db by Serhiy Storchaka in 
branch '3.6':
[3.6] bpo-5945: Improve mappings and sequences C API docs. (GH-7029). (GH-7049)
https://github.com/python/cpython/commit/93e9fb5664e56c02c9aa89098b556929735b35db


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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue5945] PyMapping_Check returns 1 for lists

[miss-islington]

miss-islington  added the comment:


New changeset e1a78cacf65f007b1000966ce3aac6ac2eaa5cfc by Miss Islington (bot) 
in branch '3.7':
bpo-5945: Improve mappings and sequences C API docs. (GH-7029)
https://github.com/python/cpython/commit/e1a78cacf65f007b1000966ce3aac6ac2eaa5cfc


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[issue5945] PyMapping_Check returns 1 for lists

[miss-islington]

Change by miss-islington :


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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset f5b1183610d5888db3bbd639b1a0c945dbd8f8dd by Serhiy Storchaka in 
branch 'master':
bpo-5945: Improve mappings and sequences C API docs. (GH-7029)
https://github.com/python/cpython/commit/f5b1183610d5888db3bbd639b1a0c945dbd8f8dd


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[issue5945] PyMapping_Check returns 1 for lists

[Guido van Rossum]

Change by Guido van Rossum :


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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I propose an alternate PR 7029 which also adds other improvements to the 
documentation of mappings and sequences C API.

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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue5945] PyMapping_Check returns 1 for lists

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Proposed wording doesn't looks much informative to me. Maybe just say that 
PyMapping_Check() also returns 1 for sequences that support slicing? And 
recommend `PyMapping_Check() && !PySequence_Check()` for true mapping test?

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[issue5945] PyMapping_Check returns 1 for lists

[Mariatta Wijaya]

Changes by Mariatta Wijaya :


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versions:  -Python 3.4

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[issue5945] PyMapping_Check returns 1 for lists

[James R Barlow]

Changes by James R Barlow :


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[issue5945] PyMapping_Check returns 1 for lists

[Christian Barcenas]

Changes by Christian Barcenas christ...@cbarcenas.com:


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[issue5945] PyMapping_Check returns 1 for lists

[Ivan Levkivskyi]

Changes by Ivan Levkivskyi levkivs...@gmail.com:


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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


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[issue5945] PyMapping_Check returns 1 for lists

[Buck Golemon]

Buck Golemon added the comment:

We've hit this problem today.

What are we supposed to do in the meantime?

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[issue5945] PyMapping_Check returns 1 for lists

[Benjamin Peterson]

Benjamin Peterson added the comment:

Not use PyMapping_Check?

On Mon, Feb 2, 2015, at 14:02, Buck Golemon wrote:
 
 Buck Golemon added the comment:
 
 We've hit this problem today.
 
 What are we supposed to do in the meantime?
 
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[issue5945] PyMapping_Check returns 1 for lists

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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[issue5945] PyMapping_Check returns 1 for lists

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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[issue5945] PyMapping_Check returns 1 for lists

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Rather than introduce fixes that break code and hurt performance, I think it 
would be better to deprecate PyMapping_Check() and wait for a fast, clean C 
version of the ABCs (that is supposed to be our one obvious way to do it).

FWIW, the spreadsheet example has been around for years and I know of more than 
one private company that has made heavy use of code modeled on that example 
(not for spreadsheets, but as a hook for eval).  So, I don't think the new 
keys check should be backported.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

 Rather than introduce fixes that break code and hurt performance, I
 think it would be better to deprecate PyMapping_Check() and wait for a
 fast, clean C version of the ABCs (that is supposed to be our one
 obvious way to do it).

Do you also advocate deprecating PySequence_Check()? Both are useful
APIs to know what you're dealing with.

As for the clean C version of the ABCs, I'm afraid we could wait quite
a bit, since that's a lot more work and nobody seems really interested
in the matter.

 FWIW, the spreadsheet example has been around for years and I know of
 more than one private company that has made heavy use of code modeled
 on that example (not for spreadsheets, but as a hook for eval).  So, I
 don't think the new keys check should be backported.

Well, I'm not proposing to backport it, but to fix things in 3.2.

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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

 Do you also advocate deprecating PySequence_Check()? 

Perhaps just document PyMapping_Check() as being less informative than before 
(now it has false positives for sequences).

 As for the clean C version of the ABCs, 
 I'm afraid we could wait quite a bit

That may be true.  I hope not.  The ABCs were meant to solve exactly this 
problem.  At this point, I would rather ignore the problem for 3.2 than to 
implement a less clean alternative.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

  As for the clean C version of the ABCs, 
  I'm afraid we could wait quite a bit
 
 That may be true.  I hope not.  The ABCs were meant to solve exactly
 this problem.  At this point, I would rather ignore the problem for
 3.2 than to implement a less clean alternative.

Also, please note that checking for ABCs would not solve the
test_builtin failure, since the class there doesn't implement the
Mapping ABC (and doesn't claim to either).

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

Actually, making PyMapping_Check() more robust produces a test failure in 
test_builtin, because of the following code:

# Verify locals stores (used by list comps)
eval('[locals() for i in (2,3)]', g, d)
eval('[locals() for i in (2,3)]', g, collections.UserDict())

class SpreadSheet:
Sample application showing nested, calculated lookups.
_cells = {}
def __setitem__(self, key, formula):
self._cells[key] = formula
def __getitem__(self, key):
return eval(self._cells[key], globals(), self)

ss = SpreadSheet()
ss['a1'] = '5'
ss['a2'] = 'a1*6'
ss['a3'] = 'a2*7'
self.assertEqual(ss['a3'], 210)


Should I fix the example to include a keys() method?

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

 Right, calling PyMapping_Check() was never particularly reliable, and
 extension modules depending on it probably always had subtle bugs. 

 Perhaps it would be nice if we provided a C API to at least some of
 the ABC package.

In the meantime, would it be reasonable to add the moral equivalent of 
`hasattr(type(op), 'items')` to PyMapping_Check()?

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[issue5945] PyMapping_Check returns 1 for lists

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

 In the meantime, would it be reasonable to add the moral equivalent
 of `hasattr(type(op), 'items')` to PyMapping_Check()?

That all depends on what it is used for.  Which is hard to say without someone 
following more of the links that Raymond posted.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

Modules/posixmodule.c: uses PyMapping_Size(), PyMapping_Keys() and 
PyMapping_Values() to parse an environment mapping (for execve() and friends).

PyFrame_New(): validates the locals argument in pydebug mode (only used for 
module-level code).
Note that functions in frameobject.c have assert(PyDict_Check(dict)) where 
dict is that same locals argument (copied into f_locals)...

PC/_subprocess.c: uses PyMapping_Size(), PyMapping_Keys() and 
PyMapping_Values() to parse an environment mapping (for CreateProcessW()).

Python/btninmodule.c: validates the locals argument to eval().


There are also a couple of places where the PyMapping API (such 
PyMapping_Keys()) is used (e.g. _json), but without calling PyMapping_Check 
first.

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[issue5945] PyMapping_Check returns 1 for lists

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

The question is, if PyMapping_Check() returns True, and a list is passed, will 
the code segfault or raise an exception?  A segfault would be unacceptable; an 
exception would be acceptable assuming that the code would have raised an 
exception anyway if PyMapping_Check() had returned False.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

Exceptions seem to be raised (for code that can be exercised in Python), but 
not very obvious ones:

 eval(x, {}, [])
Traceback (most recent call last):
  File stdin, line 1, in module
  File string, line 1, in module
TypeError: list indices must be integers, not str

$ ./python -c import os; os.execle('/bin/ls', 'ls', [])
Traceback (most recent call last):
  File string, line 1, in module
  File /home/antoine/py3k/__svn__/Lib/os.py, line 320, in execle
execve(file, args[:-1], env)
AttributeError: values

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[issue5945] PyMapping_Check returns 1 for lists

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

It looks like PyMapping_Check() already checks for the presence of a fairly 
arbitrary special operation (mp_subscript).  It sounds fine to replace that 
with a check for the presence of a keys() or items() method (I'm not sure which 
one is more representative). I wish the check can be done fast -- but I fear 
that it'll involve a dict lookup.  So be it.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

Guido, can we have your take on this?

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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Here's a code search link:

 http://www.google.com/codesearch?as_q=PyMapping_Check

And here's how it currently used internal to the Python codebase:

$ ack --cc PyMapping_CheckInclude/abstract.h
1125: PyAPI_FUNC(int) PyMapping_Check(PyObject *o);

Modules/posixmodule.c
3585:if (!PyMapping_Check(env)) {
3764:if (!PyMapping_Check(env)) {
3950:if (!PyMapping_Check(env)) {

Objects/abstract.c
1987:PyMapping_Check(PyObject *o)

Objects/frameobject.c
605:(locals != NULL  !PyMapping_Check(locals))) {

PC/_subprocess.c
340:if (! PyMapping_Check(environment)) {

Python/bltinmodule.c
700:if (locals != Py_None  !PyMapping_Check(locals)) {
705:PyErr_SetString(PyExc_TypeError, PyMapping_Check(globals) ?
794:if (!PyMapping_Check(locals)) {

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[issue5945] PyMapping_Check returns 1 for lists

[Guido van Rossum]

Guido van Rossum gu...@python.org added the comment:

 Why did the list implementation get changed in Py3.x?

Because we decided to get rid of the sq_slice and sq_ass_slice slots
in PySequenceMethods, and that in turn was because we got rid of the
slice-related opcodes and the separate __getslice__ and __setslice__
magic methods.

 Is it now necessary for any subscripting type to put the same method
 in both the sequence methods and mapping methods?

Yes, if the type wants to support slicing.  The reason is that while
we changed many things, we didn't want to change the signature of the
methods that we kept, and the sq_item/sq_ass_item signatures have
arguments that make it impossible to pass the info necessary for a
slice.

 Was this change necessary?

The changes are briefly mentioned in PEP 3100 without any motivation.
However I think the rationale was the observation that the old
sq_slice / sq_ass_slice took only integers (really ssize_t), meaning
that it was impossible to implement a sequence type taking a slice
with arguments outside the range supported by ssize_t (e.g. a custom
range type supporting huge longs).  Or with non-integral arguments.
This problem never existed for non-slice __get__ since one could
always implement mp_subscript / mp_ass_subscript.

Was it necessary?  I'm not sure -- but that's water under the bridge.   

Was it a good change?  From the POV of Python code, yes.  The old
approach caused someodd problems for classes implementing
__getslice__ / __setslice__ (since those were invoked after the
arguments had been pushed through the sq_slice / sq_ass_slice API).

 Personally, I think PyMapping_Check and PySequence_Check should be
 deprecated and removed. Like their Python counterparts,
 operator.isMappingType() and operation.isSequenceType(), they are
 unreliable at best in the face of not LBYL and abcs.

Right, calling PyMapping_Check() was never particularly reliable, and
extension modules depending on it probably always had subtle bugs. 

Perhaps it would be nice if we provided a C API to at least some of
the ABC package.

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[issue5945] PyMapping_Check returns 1 for lists

[Georg Brandl]

Changes by Georg Brandl ge...@python.org:


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[issue5945] PyMapping_Check returns 1 for lists

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

I understand this is indeed unintuitive. The reason list objects support
the C mapping protocol in 3.x is that it is how slicing of lists (and
tuples, for that matter) is implemented. Perhaps the documentation
should carry a warning about this.

Unfortunately, right now there is no easy way in C to check that an
object implements a given ABC.

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[issue5945] PyMapping_Check returns 1 for lists

[Benjamin Peterson]

Benjamin Peterson benja...@python.org added the comment:

Personally, I think PyMapping_Check and PySequence_Check should be
deprecated and removed. Like their Python counterparts,
operator.isMappingType() and operation.isSequenceType(), they are
unreliable at best in the face of not LBYL and abcs.

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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Why did the list implementation get changed in Py3.x?  Is it now
necessary for any subscripting type to put the same method in both the
sequence methods and mapping methods?  Was this change necessary?

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

 Why did the list implementation get changed in Py3.x?  Is it now
 necessary for any subscripting type to put the same method in both the
 sequence methods and mapping methods?  Was this change necessary?

I think it's a case of foolish consistency. In py3k there are no opcodes
dedicated to slicing anymore, instead the slice object is passed to the
mapping's getitem method.

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[issue5945] PyMapping_Check returns 1 for lists

[Antoine Pitrou]

Changes by Antoine Pitrou pit...@free.fr:


--
priority:  - critical
versions: +Python 3.1

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[issue5945] PyMapping_Check returns 1 for lists

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


--
assignee:  - rhettinger
nosy: +rhettinger

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[issue5945] PyMapping_Check returns 1 for lists

[John Millikin]

New submission from John Millikin jmilli...@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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