[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon  added the comment:

After 6 years and no really movement I don't think we are going to bother to 
change the semantics for this. :)

--
resolution:  -> wont fix
stage: test needed -> resolved
status: open -> closed

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[issue18275] Make isinstance() work with super type instances

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com:


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[issue18275] Make isinstance() work with super type instances

[Antoine Pitrou]

Antoine Pitrou added the comment:

Does anyone actually check for instance-ness of super objects? I would never 
have thought of such an idiom.

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon added the comment:

So I have a use case for this actually. =)

http://bugs.python.org/issue17621 is about trying to finally add a lazy mixin 
to importlib so people stop asking for it. I have a version already written 
externally at 
https://code.google.com/p/importers/source/browse/importers/lazy.py. Part of 
the trick of making it work is having a way to drop the lazy mixin from the 
loader MRO for all future uses (which is important for reloads) is to assign 
__loader__ to super(Mixin, self). The problem is that if you use isinstance on 
that loader with any of the ABCs in importlib.abc it will always come back 
false no matter whether the interface is actually implemented or not.

While I consider it a marginal thing for people to do (importlib.abc is for 
making sure people implement the right methods and providing default 
implementations, not for interface checking), I'm sure someone will do it and I 
would like one less possible warning in any future docs about this lazy mixin 
regarding isinstance checks.

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priority: normal - low

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[issue18275] Make isinstance() work with super type instances

[Antoine Pitrou]

Antoine Pitrou added the comment:

 http://bugs.python.org/issue17621 is about trying to finally add a
 lazy mixin to importlib so people stop asking for it. I have a
 version already written externally at
 https://code.google.com/p/importers/source/browse/importers/lazy.py.
 Part of the trick of making it work is having a way to drop the lazy
 mixin from the loader MRO for all future uses (which is important
 for reloads) is to assign __loader__ to super(Mixin, self).

That sounds like a better job for a proxy rather than a mixin.

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon added the comment:

On Fri, Jun 21, 2013 at 10:02 AM, Antoine Pitrou rep...@bugs.python.orgwrote:


 Antoine Pitrou added the comment:

  http://bugs.python.org/issue17621 is about trying to finally add a
  lazy mixin to importlib so people stop asking for it. I have a
  version already written externally at
  https://code.google.com/p/importers/source/browse/importers/lazy.py.
  Part of the trick of making it work is having a way to drop the lazy
  mixin from the loader MRO for all future uses (which is important
  for reloads) is to assign __loader__ to super(Mixin, self).

 That sounds like a better job for a proxy rather than a mixin.


I'm attracted to the idea of making a loader lazy by simply doing something
like ``class LazySourceFileLoader(LazyMixin, SourceFileLoader): ...``. The
key thing is that it doesn't require proxying the constructor in situations
like PathEntryFinder where the finder is going to be making your loader
instance (or at least calling the object). I mean you could override
__call__ to store the arguments and then simply use them when needed to
construct the real loader, but I don't know if that makes the code simpler
without actually coding it up.

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[issue18275] Make isinstance() work with super type instances

[Antoine Pitrou]

Antoine Pitrou added the comment:

 I'm attracted to the idea of making a loader lazy by simply doing
 something
 like ``class LazySourceFileLoader(LazyMixin, SourceFileLoader):
 ...``.

But then you have to make a specific Lazy subclass for each delegated
loader implementation. With a proxy class you would simply proxy each
loader instance:

existing_loader = SourceFileLoader(...)
lazy_loader = LazyLoader(existing_loader)
...

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon added the comment:

On Fri, Jun 21, 2013 at 11:11 AM, Antoine Pitrou rep...@bugs.python.orgwrote:


 Antoine Pitrou added the comment:

  I'm attracted to the idea of making a loader lazy by simply doing
  something
  like ``class LazySourceFileLoader(LazyMixin, SourceFileLoader):
  ...``.

 But then you have to make a specific Lazy subclass for each delegated
 loader implementation. With a proxy class you would simply proxy each
 loader instance:

 existing_loader = SourceFileLoader(...)
 lazy_loader = LazyLoader(existing_loader)
 ...

But the point I tried to make earlier is that approach doesn't work
directly when you are creating the loader instances dynamically within a
finder. E.g. FileFinder (
http://docs.python.org/3/library/importlib.html#importlib.machinery.FileFinder)
instantiates the loader for you.

Now you could tweak things to make this work, but it isn't quite as
straightforward as you suggest because of this need to have a callable for
finders to use to create new loaders:

class LazyProxy(importlib.abc.Loader):

  def __init__(self, loader):
self.loader = loader

  def __call__(self, *args, **kwargs):
self.args = args
self.kwargs = kwargs
return self

  def load_module(self, fullname):
# XXX ignoring sys.modules details, e.g. if module already existed.
lazy_module = LazyModule(fullname, proxy=self, name=fullname)
sys.modules[fullname] = lazy_module
return lazy_module

class LazyModule(types.ModuleType):

def __init__(*args, proxy, name, **kwargs):
  self.__proxy = proxy
  self.__name = name
  super().__init__(*args, **kwargs)

def __getattribute__(self, attr):
  self.__class__ = Module
  state = self.__dict__.copy()
  loader = self.__proxy.loader(*self.proxy.args, **self.proxy.kwargs)
  # XXX ignoring sys.modules details, e.g. removing module on load
failure.
  loader.load_module(self.__name)
  self.__dict__.update(state)
  return getattr(module, attr)

That's all totally untested (not even verified to compile) but I think that
would do what you are suggesting.

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[issue18275] Make isinstance() work with super type instances

[Raymond Hettinger]

Raymond Hettinger added the comment:

I'm not sure this is a theoretically sound idea.  Do you know what CLOS does in 
the same circumstance?

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon added the comment:

I have no idea what CLOS does in this situation.

I'm fine with this never being changed, but I wanted to at least file the bug 
to have the discussion in case it ever comes up again.

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[issue18275] Make isinstance() work with super type instances

[Raymond Hettinger]

Raymond Hettinger added the comment:

 I have no idea what CLOS does in this situation.
No worries, I was just curious whether you knew whether this was a solved 
problem in Dylan or CLOS or some other language.

When faced with a diamond diagram, what are the semantics for 
isinstance(super_instance, cls)?

I worked on it for a little bit and came up with the following:

def super_isinstance(super_inst, cls):
'Is the cls in the mro somewhere after the current class?'
mro = super_inst.__self__.__class__.__mro__
thisclass = super_inst.__thisclass__
return cls in mro and mro.index(thisclass)  mro.index(cls)

--
Added file: http://bugs.python.org/file30663/simple_diamond_example.py

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

Brett Cannon added the comment:

That solution looks right. Problem is that it would need to either go directly 
into isinstance() or type would need to grow an __instancecheck__ to deal with 
this case since you can't override isinstance from the instance end. That I'm 
not sure people are okay with.

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[issue18275] Make isinstance() work with super type instances

[Brett Cannon]

New submission from Brett Cannon:

class A: pass
class B(A): pass

sup = super(B, B())
isinstance(sup, A)  # returns False

Why is that? Is there an explicit reason to prevent that isinstance check from 
working? How about just for ABCs? Either way it's annoying that at least for 
ABCs you can't check against a super object.

--
components: Interpreter Core
messages: 191550
nosy: brett.cannon
priority: normal
severity: normal
stage: test needed
status: open
title: Make isinstance() work with super type instances
type: behavior
versions: Python 3.4

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