[issue13290] get vars for object with __slots__

[Raymond Hettinger]

Raymond Hettinger added the comment:

> On any case, feel free to mark this as "rejected" or "wont fix"

Okay, will do.

And though I think the idea had some significant downsides, it is was creative 
and we appreciate the suggestion.

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status: open -> closed

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[issue13290] get vars for object with __slots__

[João Bernardo]

João Bernardo added the comment:

Being the OP, at that time I felt it was important to have a vars() function 
that worked on __slots__ to ease something I was developing. The truth for me 
is: __slots__ is not really relevant anymore. The benefits it brings are not 
enough to make the feature usable because of these incompatibilities with 
normal classes. Also, it does not bring real benefits even for most people who 
think they need this.

If you believe having vars() working on __slots__ is bad because people may 
want to update it, why would locals() exist at all?

Also, your argument that having vars() working on __slots__ classes may mask 
the wrong use of the attribute, why would someone use vars() on __slots__ if it 
doesn't work right now? If it is not useful for identifying this problem now, 
then I am ok with it not being useful in the future as well.

On any case, feel free to mark this as "rejected" or "wont fix" because if in 6 
years no one cared enough to have it working means that either it is not 
important and/or __slots__ classes are also not relevant for others as well.

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[issue13290] get vars for object with __slots__

[Raymond Hettinger]

Raymond Hettinger added the comment:

I think this API expansion should not be done.

As a teacher of Python, I find that vars(o) is easily understood as a short-cut 
for o.__dict__ in much the same way as len(o) is a short-cut for o.__len__().  
The proposed change makes this tool harder to teach.

The vars() function is often used for more than viewing a dictionary, it is 
also used in situations where the dictionary can be modified.  The proposed 
feature makes it harder to distinguish cases where it acts like a regular, 
updateable dict.

Further, I believe the addition of invisible magic to vars() will actively 
promote a misunderstanding of how Python works.  When applied to an instance 
variable, it creates an illusion that a dictionary is present when to whole 
point of __slots__ is to suppress an instance dictionary.  

In fact, the actual dictionary entries are the member objects at the class 
level.  This fact would be hidden by the proposed change.  In addition it 
creates confusion by treating __slots__ differently from other class-level 
descriptor entries such as property objects

Another issue is that right now, if a user misspells __slots__ (perhaps as 
__slots_ por __slot__), the indication that something went wrong is that 
running vars() on an instance returns a dictionary rather than raising 
"TypeError: vars() argument must have __dict__ attribute".   The proposed 
change will take away this active affirmation that __slots__ is working and 
will make debugging more difficult (and it will break any existing __slots__ 
tests that use assertRaises(TypeError) with vars() to verify that __slots__ is 
working).

Overall, my assessment is that this proposed API expansion impairs the 
learnabilty and usability of vars().

In the entire long history of vars(), this feature has never been requested.  
Even the OP said, I just realized that it isn't built-out for inclusion 
__slots__; rather than saying, I really think this would have helped me with a 
real debugging problem.  We have zero evidence that this proposed feature would 
be of any value and I have given reasons why it would make the language worse. 

Guido choose not to include this behavior when he added __slots__ back in 
Python 2.2 (when many introspection builtins were changed to accommodate 
new-style classes).  IMO, no reason has surfaced in the last 15 years to 
warrant reversing his decision.

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[issue13290] get vars for object with __slots__

[Jim Fasarakis-Hilliard]

Changes by Jim Fasarakis-Hilliard :


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[issue13290] get vars for object with __slots__

[Terry J. Reedy]

Terry J. Reedy added the comment:

Michelle (maker)'s patch was reviewed by Serhiy and Richard in msg171867 and 
following.  It appears that stage should be 'needs revised patch'.

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[issue13290] get vars for object with __slots__

[Steven D'Aprano]

Steven D'Aprano added the comment:

I independently raised this on Python-Ideas and the initial responses are that 
vars() should support objects with slots too.

https://mail.python.org/pipermail/python-ideas/2016-December/043965.html

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Oh, this was incorrect example. The correct one is:

$ ./python -m timeit -s from http.cookiejar import parse_ns_headers -- 
parse_ns_headers(['foo=bar; path=/; version=1; Expires=Thu, 01 Jan 1970 
00:00:10 GMT'])
Before: 1 loops, best of 3: 177 usec per loop
After:  1 loops, best of 3: 104 usec per loop

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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[issue13290] get vars for object with __slots__

[Eric Snow]

Changes by Eric Snow ericsnowcurren...@gmail.com:


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[issue13290] get vars for object with __slots__

[Michele Orrù]

Michele Orrù added the comment:

As a reference, linking the discussion on python-dev. 
http://mail.python.org/pipermail/python-dev/2012-October/122011.html

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[issue13290] get vars for object with __slots__

[Michele Orrù]

Michele Orrù added the comment:

Patch + unittests + documentation attached.

$ ./python -m test -R 3:2 test_builtin
[1/1] test_builtin
beginning 5 repetitions
12345
.
1 test OK.
[158296 refs]

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Added file: http://bugs.python.org/file27395/issue13290.patch

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I added some comments in Rietveld.

ReST documentation should be updated too.

vars() returns modifiable dict. The patch should be much harder if we want to 
preserve this behavior. Otherwise, the difference must be explicitly documented.

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

There's another thing. __slots__ value is converted to a tuple and saved as 
ht_slots field in PyHeapTypeObject. You can use fast PyTuple_GET_ITEM() for 
item access.

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[issue13290] get vars for object with __slots__

[Richard Oudkerk]

Richard Oudkerk added the comment:

The patch does not seem to walk the mro to look for slots in base classes.  
Also, an instance with a __dict__ attribute may also have attributes stored in 
slots.

BTW, copyreg._slotnames(cls) properly calculates the slot names for cls and 
tries to cache them as cls.__slotnames__.  Pickle does the equivalent of

try:
slotnames = cls.__slotnames__
except AttributeError:
slotnames = copyreg._slotnames(cls)

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[issue13290] get vars for object with __slots__

[Michele Orrù]

Michele Orrù added the comment:

 The patch does not seem to walk the mro to look for slots in base 
 classes.  Also, an instance with a __dict__ attribute may also have 
attributes stored in slots.
Well, what I am doing is more or less the equivalent of 

return object.__slots__ if hasattr(object, '__slots') else object.__dict__

and this is coherent with the updated documentation. The one you proposed is an 
alternative behavior; am I supposed to follow that one?


 BTW, copyreg._slotnames(cls) properly calculates the slot names for cls and 
 tries to cache them as cls.__slotnames__.  Pickle does the equivalent of

 try:
 slotnames = cls.__slotnames__
 except AttributeError:
 slotnames = copyreg._slotnames(cls)
thanks! I'll take into account. 

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[issue13290] get vars for object with __slots__

[Richard Oudkerk]

Richard Oudkerk added the comment:

 Well, what I am doing is more or less the equivalent of 
 
 return object.__slots__ if hasattr(object, '__slots') else object.__dict__
 
 and this is coherent with the updated documentation. The one you 
 proposed is an alternative behavior; am I supposed to follow that one?

Ignoring some slots but not others would be confusing.  I would be inclined to 
just leave vars() alone.  Maybe a Python implementation could be put in 
inspect.py instead.

A possible implementation (which can't be used to modify the object) might be:

import copyreg

def fullvars(obj):
cls = type(obj)
try:
slotnames = cls.__dict__['__slotnames__']
except (KeyError, AttributeError):
slotnames = copyreg._slotnames(cls)
try:
d = vars(obj).copy()
except TypeError:
d = {}
for name in slotnames:
try:
d[name] = getattr(obj, name)
except AttributeError:
pass
return d

class A:
__slots__ = 'x', 'y'

class B(A):
__slots__ = 'u', 'v'

class C(B):
pass

a = A()
a.x = 1
print(fullvars(a))  # {'x': 1}

b = B()
b.x = 2; b.u = 3
print(fullvars(b))  # {'u': 3, 'x': 2}

c = C()
c.y = 4; c.r = 5
print(fullvars(c))  # {'y': 4, 'r': 5}



BTW, I before should have written

try:
slotnames = cls.__dict__['__slotnames__']
except (KeyError, AttributeError):
slotnames = copyreg._slotnames(cls)

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 The patch does not seem to walk the mro to look for slots in base classes. 
 Also, an instance with a __dict__ attribute may also have attributes
 stored in slots.
 
 BTW, copyreg._slotnames(cls) properly calculates the slot names for cls and
 tries to cache them as cls.__slotnames__.

This is from Python side. Did ht_slots field of PyHeapTypeObject does not 
contain properly calculated slot names?

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[issue13290] get vars for object with __slots__

[Richard Oudkerk]

Richard Oudkerk added the comment:

 This is from Python side. Did ht_slots field of PyHeapTypeObject does not 
 contain properly calculated slot names?

Looking at the code, it appears that ht_slots does *not* include inherited 
slots.

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[issue13290] get vars for object with __slots__

[Terry J. Reedy]

Terry J. Reedy added the comment:

As I understand it, var(ob) is pretty much a synonym for ob.__dict__. Without 
knowing the internal details, I think vars should do with slots what it would 
do if slots were not used and normal dicts were used. In particular, if a class 
is changed by adding __slots__, vars(class) and vars(instances) should be as 
much the same as possible, at least for read-only uses. This allows iteration 
by keys, values, or items for introspection. That modifying the dict has no 
effect on the object is okay. It is the same for locals() inside a function.

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[issue13290] get vars for object with __slots__

[Richard Oudkerk]

Richard Oudkerk added the comment:

 That modifying the dict has no effect on the object is okay.

I have written vars(obj).update(...) before.  I don't think it would be okay 
to break that.

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[issue13290] get vars for object with __slots__

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I see a special warning in the documentation:

  Note: The returned dictionary should not be modified: the effects on the 
corresponding symbol table are undefined. [2]

  [2] In the current implementation, local variable bindings cannot normally be 
affected this way, but variables retrieved from other scopes (such as modules) 
can be. This may change.

Therefore it's okay that modifying the returned dict has no effect on the 
object.

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[issue13290] get vars for object with __slots__

[Richard Oudkerk]

Richard Oudkerk added the comment:

A search of googlecode shows 42 examples of vars(...).update compared to 3000 
for .__dict__.update.  I don't know if that is enough to worry about.

http://code.google.com/codesearch#search/q=vars\%28[A-Za-z0-9_]%2B\%29\.update%20lang:^python$type=cs

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[issue13290] get vars for object with __slots__

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

 You should attach diffs (context diffs, I believe)
Nope, these are hard to read :)  The universal diff format is the unified one.  
That’s also what Mercurial uses (see the devguide for more info on 
contributing).

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[issue13290] get vars for object with __slots__

[Éric Araujo]

Changes by Éric Araujo mer...@netwok.org:


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[issue13290] get vars for object with __slots__

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

New features only go in future versions: Given the doc: With a module, class 
or class instance object as argument (or anything else that has a __dict__ 
attribute), return that attribute. So you are proposing a change in the 
definition of vars() input.

I agree that the proposal is a good idea as far as it goes. Vars() predates 
__slots__ (introduced in 2.2). In particular, adding or deleting a __slots__ 
binding to a user-defined class should not change the behavior of vars().

I am thinking that the doc should also be tweaked a bit. It implies that all 
class instances have a __dict__ attribute. Before 2.2, when 'class' meant a 
user-defined old style class, that was true. But with 'types' becoming 
new-style classes, it is not.

Perhaps the domain of vars() should be extended to all objects, returning {} if 
nothing more. I may bring that idea to python-ideas list.

You should attach diffs (context diffs, I believe) rather than files so we can 
see what you changed. The diff module can create such. I believe your changes 
come after the following:

d = PyObject_GetAttrString(v, __dict__);
if (d == NULL) {

The statement '''slots = PyObject_GetAttrString(v, __slots__)''' gets the 
attribute from the object itself. Like most special attributes other than 
__dict__, it should be gotten from the object's class (or superclasses). [I 
don't know the C version of doing this.] For example, suppose v is a class with 
a custom metaclass and type(v).__slots__ == ('__slots__', '__init__') whereas 
v.__slots__ == ('a', 'b') (and v.__init__ is not assigned). Then vars(v) should 
return {'__slots__': ('__slots__', '__init__')}.

The comment /* Find attributes out of __slots__ */ seems misleading. I believe 
it should be /* Convert string to iterable of 1 string */

New code for 3.3 should use the new unicode api from pep 393. Though I could be 
wrong, I believe PyUnicode_Check() has been replaced.

Your example __slots__ = {'some': 'mapping'} is somewhat strange. It works, 
but only because iterating dicts iterates the keys, and the key is a string. As 
far as I know, the value is ignored and useless, so I hope no one does this. 
Perhaps your only point was to test a non-sequence iterable of strings.

A patch should include a patch to the appropriate Lib/test/testxxx.py file.

--
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stage:  - needs patch
versions:  -Python 3.2

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[issue13290] get vars for object with __slots__

[João Bernardo]

João Bernardo jbv...@gmail.com added the comment:

Oh, sorry for the full file. Yes, I only changed after 

d = PyObject_GetAttrString(v, __dict__);
if (d == NULL) {

I was searching for uses of slots other than __slots__ = (a, b) and I saw a guy 
saying that dicts may have special meaning the future. So, something like

  __slots__ = {'my_var': int}

could be an annotation (or hint for IDE's). In the present implementation, the 
value of each key is just ignored.

If slots is just a_single_string it should not be treated as an iterable, so 
I used PyUnicode_Check because I didn't knew a better method.

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[issue13290] get vars for object with __slots__

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

PyUnicode_Check may still be correct. I have not examined PEP393 in detail.

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[issue13290] get vars for object with __slots__

[João Bernardo]

New submission from João Bernardo jbv...@gmail.com:

I just realized the builtin function `vars` can't handle custom objects without 
the __dict__ attribute.

It would be nice if it worked with objects that have __slots__ to make them 
look more like normal objects and to make debugging easier.

I changed the source of Python/bltinmodule.c to accept this new case, but, as 
I'm fairly new to the C API, it may not be much good.

I'm attaching the whole file to this Issue and the `builtin_vars` function was 
modified (lines 1859 to 1921) to work with:
__slots__ = ('a', 'b', 'c')
__slots__ = 'single_name'
__slots__ = {'some': 'mapping'}

The output is a dict (just like in the __dict__ case).

#Example
 class C:
   __slots__ = ['x', 'y']

 c = C()
 c.x = 123
 vars(c)
{'x': 123}

--
components: Interpreter Core
files: bltinmodule.c
messages: 146598
nosy: JBernardo
priority: normal
severity: normal
status: open
title: get vars for object with __slots__
type: feature request
versions: Python 3.2, Python 3.3
Added file: http://bugs.python.org/file23546/bltinmodule.c

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