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Issue 643841: Including a new-style proxy base class in 2.6/3.0

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ncoghlan at gmail

May 20, 2008, 7:33 AM

Post #1 of 17 (791 views)
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Issue 643841: Including a new-style proxy base class in 2.6/3.0

One of the tasks where classic classes are currently far superior to
new-style classes is in writing proxy classes like weakref.proxy - cases
where nearly all operations need to be delegated to some other object,
with only a few being handled via the proxy type object itself.

With classic classes, this is trivial, since __getattr__ is always
consulted, even for retrieval of special methods.

With new-style classes, however, the __getattribute__ machinery can be
bypassed, meaning the only way to proxy an arbitrary instance is to
define all of the special methods that have C-level slots.

This issue was actually first raised five and a half years ago [1], but
has never been a particularly pressing problem, as anyone with any sense
that needed to write a proxy object just used a classic class instead of
a new-style one. In 3.0, with the demise of classic classes, that
workaround goes away.

So what do people think of including a ProxyBase implementation in 2.6
and 3.0 that explicitly delegates all of the C-level slots to a
designated target instance? For some proxy class implementers, it would
be possible to use this class as a base-class to get the special method
delegation 'for free', and for others with more esoteric needs, it would
at least provide a reference for which special methods needed to be
provided explicitly by the proxy type.

I attached a sample implementation to [1] which is essentially
equivalent to weakref.proxy, but with a strong reference to the target,
and written in Python rather than C.

I expect the target audience for such a feature to be quite small, but
without better support for it in the standard library, I also suspect it
could prove to be a show-stopper for the affected developers as far as
Py3k migration is concerned.

Cheers,
Nick.

[1] http://bugs.python.org/issue643841

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fuzzyman at voidspace

May 20, 2008, 7:38 AM

Post #2 of 17 (776 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Nick Coghlan wrote:
> One of the tasks where classic classes are currently far superior to
> new-style classes is in writing proxy classes like weakref.proxy -
> cases where nearly all operations need to be delegated to some other
> object, with only a few being handled via the proxy type object itself.
I've needed to do this a few times when wrapping libraries.

Michael Foord

>
> With classic classes, this is trivial, since __getattr__ is always
> consulted, even for retrieval of special methods.
>
> With new-style classes, however, the __getattribute__ machinery can be
> bypassed, meaning the only way to proxy an arbitrary instance is to
> define all of the special methods that have C-level slots.
>
> This issue was actually first raised five and a half years ago [1],
> but has never been a particularly pressing problem, as anyone with any
> sense that needed to write a proxy object just used a classic class
> instead of a new-style one. In 3.0, with the demise of classic
> classes, that workaround goes away.
>
> So what do people think of including a ProxyBase implementation in 2.6
> and 3.0 that explicitly delegates all of the C-level slots to a
> designated target instance? For some proxy class implementers, it
> would be possible to use this class as a base-class to get the special
> method delegation 'for free', and for others with more esoteric needs,
> it would at least provide a reference for which special methods needed
> to be provided explicitly by the proxy type.
>
> I attached a sample implementation to [1] which is essentially
> equivalent to weakref.proxy, but with a strong reference to the
> target, and written in Python rather than C.
>
> I expect the target audience for such a feature to be quite small, but
> without better support for it in the standard library, I also suspect
> it could prove to be a show-stopper for the affected developers as far
> as Py3k migration is concerned.
>
> Cheers,
> Nick.
>
> [1] http://bugs.python.org/issue643841
>

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greg.ewing at canterbury

May 20, 2008, 4:55 PM

Post #3 of 17 (765 views)
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Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Nick Coghlan wrote:
> So what do people think of including a ProxyBase implementation in 2.6
> and 3.0 that explicitly delegates all of the C-level slots to a
> designated target instance?

Sounds good to me.

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fdrake at acm

May 20, 2008, 5:39 PM

Post #4 of 17 (763 views)
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Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Nick Coghlan wrote:
> So what do people think of including a ProxyBase implementation in
> 2.6 and 3.0 that explicitly delegates all of the C-level slots to a
> designated target instance?

On May 20, 2008, at 7:55 PM, Greg Ewing wrote:
> Sounds good to me.

Same here. There's an implementation in zope.proxy which might be
useful as a starting point:

http://pypi.python.org/pypi/zope.proxy/3.4.0

I'd be willing to write the required documentation, since I'm partly
to blame for the implementation.

The short description of the package on PyPI contains a typo; that's
been fixed in Subversion. :-)

-Fred

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ncoghlan at gmail

May 21, 2008, 2:41 AM

Post #5 of 17 (755 views)
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Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Fred Drake wrote:
> Nick Coghlan wrote:
>> So what do people think of including a ProxyBase implementation in 2.6
>> and 3.0 that explicitly delegates all of the C-level slots to a
>> designated target instance?
>
>
> On May 20, 2008, at 7:55 PM, Greg Ewing wrote:
>> Sounds good to me.
>
> Same here. There's an implementation in zope.proxy which might be
> useful as a starting point:

While a proxy class written in C would no doubt be faster than one
written in Python, one of the things I'm hoping to achieve is for the
stdlib generic proxy to serve as an example for people writing their own
new-style proxy classes in addition to being usable as a base class
(Adam Olsen suggested ProxyMixin instead of ProxyBase as a possible name).

One other issue is where to put it - none of the existing stdlib modules
seemed appropriate, so my sample implementation in the tracker issue
just invents a new 'typetools' module.

Cheers,
Nick.

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fdrake at acm

May 21, 2008, 4:36 AM

Post #6 of 17 (755 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

On May 21, 2008, at 5:41 AM, Nick Coghlan wrote:
> While a proxy class written in C would no doubt be faster than one
> written in Python, one of the things I'm hoping to achieve is for
> the stdlib generic proxy to serve as an example for people writing
> their own new-style proxy classes in addition to being usable as a
> base class (Adam Olsen suggested ProxyMixin instead of ProxyBase as
> a possible name).

The idea that it would serve as an example seems odd; the reason to
make things part of the standard library is because their
implementation needs to track the Python core closely. For a proxy,
it would be the need to reflect additional slot methods as they're
added. A Python implementation may be able to do this just fine, but
the performance penalty would make it uninteresting for many large
applications.

For what it's worth, zope.proxy does support subclassing.

-Fred

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ncoghlan at gmail

May 21, 2008, 7:30 AM

Post #7 of 17 (749 views)
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Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Fred Drake wrote:
> On May 21, 2008, at 5:41 AM, Nick Coghlan wrote:
>> While a proxy class written in C would no doubt be faster than one
>> written in Python, one of the things I'm hoping to achieve is for the
>> stdlib generic proxy to serve as an example for people writing their
>> own new-style proxy classes in addition to being usable as a base
>> class (Adam Olsen suggested ProxyMixin instead of ProxyBase as a
>> possible name).
>
>
> The idea that it would serve as an example seems odd; the reason to make
> things part of the standard library is because their implementation
> needs to track the Python core closely. For a proxy, it would be the
> need to reflect additional slot methods as they're added.

It does that too - the unit tests I just added to the tracker issue for
this proposal will actually start to fail if something is added to the
operator module without the unit tests for the proposed ProxyMixin being
updated appropriately (it would actually probably be a good thing to
have something similar in the weakref.proxy test suite to prevent a
repeat of our effort with forgetting to update that when the
operator.index slot was added)

> A Python
> implementation may be able to do this just fine, but the performance
> penalty would make it uninteresting for many large applications.

It should still be faster than the classic class __getattr__ based proxy
implementations it is primarily intended to replace. People that are
already using a C-based implementation like zope.proxy aren't going to
be affected by the removal of classic classes in 3.0, since they weren't
relying on them anyway.

In many ways, the TestProxyMixin test suite may prove more useful in the
long run than the ProxyMixin class itself, since the test suite provides
a template for how to test that a proxy class is doing its job, and also
to automatically detect when new C-level operations have been added to
the interpreter that the proxy class doesn't support.

> For what it's worth, zope.proxy does support subclassing.

But not in a mixin style - since zope.proxy is an extension class in its
own right, it can't be combined with other extension classes.

The Python version can be combined with anything.

Cheers,
Nick.

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ncoghlan at gmail

May 22, 2008, 6:47 AM

Post #8 of 17 (734 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Fred Drake wrote:
> On May 21, 2008, at 5:41 AM, Nick Coghlan wrote:
>> While a proxy class written in C would no doubt be faster than one
>> written in Python, one of the things I'm hoping to achieve is for the
>> stdlib generic proxy to serve as an example for people writing their
>> own new-style proxy classes in addition to being usable as a base
>> class (Adam Olsen suggested ProxyMixin instead of ProxyBase as a
>> possible name).
>
>
> The idea that it would serve as an example seems odd; the reason to make
> things part of the standard library is because their implementation
> needs to track the Python core closely. For a proxy, it would be the
> need to reflect additional slot methods as they're added. A Python
> implementation may be able to do this just fine, but the performance
> penalty would make it uninteresting for many large applications.

I've added a documentation file to the tracker item and kicked it in
Barry's direction (I also bumped the priority to 'release blocker'
because I think we need an explicit decision on this one from Barry or
Guido before the first beta release of 3.0).

Here's what I included in the proposed documentation for ProxyMixin
(note that I modelled the implicit unwrapping behaviour directly on the
behaviour of weakref.proxy):

class:: ProxyMixin(target)

A proxy class that ensures all special method invocations which may
otherwise bypass the normal :method:`__getattribute__` lookup
process are correctly delegated to the specified target object.
Normal attribute manipulation operations are also delegated to the
specified target object.

All operations on a :class:`ProxyMixin` instance return an unproxied
result. Operations involving multiple :class:`ProxyMixin` instances
(e.g. addition) are permitted, and endeavour to return the same
result as would be calculated if the proxy objects were not involved.

Custom proxy class implementations may inherit from this class
in order to automatically delegate all such special methods
that the custom proxy class does not need to provide special
handling for. This may not be practical for more complex
delegation scenarios (e.g. a local interface to a remote
object, or delegating to a weakly referenced target as in
:code:`weakref.proxy`), but :class:`ProxyMixin` may still be used
as a reference when developing such classes to identify which methods
must be defined explicitly on the proxy type in order for them to be
delegated correctly by the Python interpreter.

Cheers,
Nick.

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armin.ronacher at active-4

May 26, 2008, 1:03 PM

Post #9 of 17 (686 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Hi,

Nick Coghlan <ncoghlan <at> gmail.com> writes:

> One of the tasks where classic classes are currently far superior to
> new-style classes is in writing proxy classes like weakref.proxy - cases
> where nearly all operations need to be delegated to some other object,
> with only a few being handled via the proxy type object itself.
Yes. I stumbled upon that multiple times when writing proxies for thread
locals. The solution I came up with is rather ugly but works at least:

http://dev.pocoo.org/projects/werkzeug/browser/werkzeug/local.py#L202

The main problem with such proxies is that especially builtin types perform
various type checks which break proxies sooner or later anyways (currently even
abstract base classes implementing a compatible interface). So as soon as you
start doing something like some_list + proxy_to_list you get a TypeError.

Regards,
Armin

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ncoghlan at gmail

May 26, 2008, 7:24 PM

Post #10 of 17 (682 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Armin Ronacher wrote:
> Hi,
>
> Nick Coghlan <ncoghlan <at> gmail.com> writes:
>
>> One of the tasks where classic classes are currently far superior to
>> new-style classes is in writing proxy classes like weakref.proxy - cases
>> where nearly all operations need to be delegated to some other object,
>> with only a few being handled via the proxy type object itself.
> Yes. I stumbled upon that multiple times when writing proxies for thread
> locals. The solution I came up with is rather ugly but works at least:
>
> http://dev.pocoo.org/projects/werkzeug/browser/werkzeug/local.py#L202
>
> The main problem with such proxies is that especially builtin types perform
> various type checks which break proxies sooner or later anyways (currently even
> abstract base classes implementing a compatible interface). So as soon as you
> start doing something like some_list + proxy_to_list you get a TypeError.

What version are you using, and are your proxies correctly implementing
all the __r*__ versions of the methods?

While there are still some cases where types in the standard library
raise TypeError directly instead of returning NotImplemented, they're
generally pretty good about playing well with others (see the
test_typetools.py file attached to the tracker item for #643841)

Cheers,
Nick.

--
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armin.ronacher at active-4

May 27, 2008, 5:18 AM

Post #11 of 17 (675 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Hi,

Nick Coghlan <ncoghlan <at> gmail.com> writes:

> What version are you using, and are your proxies correctly implementing
> all the __r*__ versions of the methods?
The link to the ugly proxy is in the mail :-) And no, I'm currently not
providing any __r*__ methods as I was too lazy to test on each call if the
method that is proxied is providing an __rsomething__ or not, and if not come up
with an ad-hoc implementation by calling __something__ and reversing the
arguments passed.

> While there are still some cases where types in the standard library
> raise TypeError directly instead of returning NotImplemented, they're
> generally pretty good about playing well with others (see the
> test_typetools.py file attached to the tracker item for #643841)
I also think that the stdlib should mention NotImplemented with a big warning.
I see countless classes raising TypeError()s if __add__ or something fails which
seem to work alright as long as someone tries to __radd__ it.

Regards,
Armin

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ncoghlan at gmail

May 27, 2008, 5:56 AM

Post #12 of 17 (675 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Armin Ronacher wrote:
> Hi,
>
> Nick Coghlan <ncoghlan <at> gmail.com> writes:
>
>> What version are you using, and are your proxies correctly implementing
>> all the __r*__ versions of the methods?
> The link to the ugly proxy is in the mail :-) And no, I'm currently not
> providing any __r*__ methods as I was too lazy to test on each call if the
> method that is proxied is providing an __rsomething__ or not, and if not come up
> with an ad-hoc implementation by calling __something__ and reversing the
> arguments passed.

I had another look - indeed, it is almost certainly the missing __r*__
methods which are causing problems for you when interacting with
unproxied objects.

The other differences between what you're doing and the proposed
typetools.ProxyMixin are that:
- ProxyMixin delegates the whole __getattribute__ operation to the
target object so that descriptors work correctly (invoking
object.__getattribute__ explicitly when it needs to retrieve the proxy
object's _target attribute).
- ProxyMixin correctly delegates in-place assignment operations via the
__i*__ methods (note however that using in-place assignment will remove
the proxy wrapper, just as it does for weakref.proxy)
- ProxyMixin implicitly unwraps other ProxyMixin instances when it
encounters them as an argument to an explicitly proxied operation
- ProxyMixin doesn't support deprecated operations like __getslice__ or
__methods__

>
>> While there are still some cases where types in the standard library
>> raise TypeError directly instead of returning NotImplemented, they're
>> generally pretty good about playing well with others (see the
>> test_typetools.py file attached to the tracker item for #643841)
> I also think that the stdlib should mention NotImplemented with a big warning.
> I see countless classes raising TypeError()s if __add__ or something fails which
> seem to work alright as long as someone tries to __radd__ it.

Returning NotImplemented is already mentioned in the documentation for
the binary special methods, but I agree that it could be made more
obvious that explicitly raising a TypeError from these methods is almost
certainly the wrong thing to do.

Cheers,
Nick.

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fuzzyman at voidspace

May 27, 2008, 6:09 AM

Post #13 of 17 (675 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Nick Coghlan wrote:
> Armin Ronacher wrote:
>> Hi,
>>
>> Nick Coghlan <ncoghlan <at> gmail.com> writes:
>>
>>> What version are you using, and are your proxies correctly
>>> implementing all the __r*__ versions of the methods?
>> The link to the ugly proxy is in the mail :-) And no, I'm currently not
>> providing any __r*__ methods as I was too lazy to test on each call
>> if the
>> method that is proxied is providing an __rsomething__ or not, and if
>> not come up
>> with an ad-hoc implementation by calling __something__ and reversing the
>> arguments passed.
>
> I had another look - indeed, it is almost certainly the missing __r*__
> methods which are causing problems for you when interacting with
> unproxied objects.
>
> The other differences between what you're doing and the proposed
> typetools.ProxyMixin are that:
> - ProxyMixin delegates the whole __getattribute__ operation to the
> target object so that descriptors work correctly (invoking
> object.__getattribute__ explicitly when it needs to retrieve the proxy
> object's _target attribute).
> - ProxyMixin correctly delegates in-place assignment operations via
> the __i*__ methods (note however that using in-place assignment will
> remove the proxy wrapper, just as it does for weakref.proxy)

Couldn't in place operations wrap the return value with a proxy?

Michael Foord

> - ProxyMixin implicitly unwraps other ProxyMixin instances when it
> encounters them as an argument to an explicitly proxied operation
> - ProxyMixin doesn't support deprecated operations like __getslice__
> or __methods__
>
>
>>
>>> While there are still some cases where types in the standard library
>>> raise TypeError directly instead of returning NotImplemented,
>>> they're generally pretty good about playing well with others (see
>>> the test_typetools.py file attached to the tracker item for #643841)
>> I also think that the stdlib should mention NotImplemented with a big
>> warning. I see countless classes raising TypeError()s if __add__ or
>> something fails which
>> seem to work alright as long as someone tries to __radd__ it.
>
> Returning NotImplemented is already mentioned in the documentation for
> the binary special methods, but I agree that it could be made more
> obvious that explicitly raising a TypeError from these methods is
> almost certainly the wrong thing to do.
>
> Cheers,
> Nick.
>

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ncoghlan at gmail

May 27, 2008, 6:32 AM

Post #14 of 17 (676 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Michael Foord wrote:
> Nick Coghlan wrote:
>> - ProxyMixin correctly delegates in-place assignment operations via
>> the __i*__ methods (note however that using in-place assignment will
>> remove the proxy wrapper, just as it does for weakref.proxy)
>
> Couldn't in place operations wrap the return value with a proxy?

They could, but I tried to copy as much behaviour as possible from
weakref.proxy which loses the proxy wrapper when you do an in-place
operation:

>>> from weakref import proxy as p
>>> class MyInt(int): pass
...
>>> x = MyInt(1)
>>> px = p(x)
>>> px
<weakproxy at 0xb768f11c to MyInt at 0xb768d86c>
>>> px += 1
>>> px
2

>>> class MyList(list): pass
...
>>> y = MyList([])
>>> py = p(y)
>>> py
<weakproxy at 0xb768f16c to MyList at 0xb768f0f4>
>>> py += []
>>> py
[]

That said, it wouldn't be hard (and might make more sense) to redefine
the proxied in-place operations along the following lines:

# See tracker item for _deref/_unwrap definitions
def __iadd__(self, other):
target = _deref(self)
result = target + _unwrap(other)
if result is target:
# Mutated in-place, keep same proxy
result = self
else:
# Returned a different object, make a new proxy
result = type(self)(result)
return result

However, if we did something like that, I'd prefer to see the
weakref.proxy implementation change to provide similar semantics.

Cheers,
Nick.

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greg.ewing at canterbury

May 27, 2008, 5:30 PM

Post #15 of 17 (663 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Armin Ronacher wrote:
> I'm currently not
> providing any __r*__ methods as I was too lazy to test on each call if the
> method that is proxied is providing an __rsomething__ or not, and if not come up
> with an ad-hoc implementation by calling __something__ and reversing the
> arguments passed.

I don't see why you should have to do that, as the reversed
method of the proxy will only be called if a prior non-reversed
call failed.

All the proxy should have to do is delegate any lookups of its
own reversed methods to corresponding methods of the proxied
object, no differently from any other method.

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Greg
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greg.ewing at canterbury

May 27, 2008, 5:38 PM

Post #16 of 17 (666 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Nick Coghlan wrote:

> else:
> # Returned a different object, make a new proxy
> result = type(self)(result)

You might want to check that the result has the
same type as the proxied object before doing that.

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Greg
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fuzzyman at voidspace

May 28, 2008, 3:07 AM

Post #17 of 17 (647 views)
Permalink

Re: Issue 643841: Including a new-style proxy base class in 2.6/3.0 [In reply to]

Greg Ewing wrote:
> Nick Coghlan wrote:
>
>> else:
>> # Returned a different object, make a new proxy
>> result = type(self)(result)
>
> You might want to check that the result has the
> same type as the proxied object before doing that.
>
Yep - and I really think it would need to do this. To have a proxy where:

proxy_instance += 1

unwraps the proxy is really no good! (At least not for my use cases...)

Michael Foord

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