On Thu, Aug 20, 2015 at 03:00:57AM -0400, Joseph Jevnik wrote:

You can define a property on the metaclass for this effect.

True, but then you can't look up the property on an instance of the class, only on the class itself. That makes it much less useful. -- Steve

On Thu, Aug 20, 2015 at 12:20 AM, Andrew Barnert via Python-ideas wrote:

Modules are a more serious problem, because there's no immediate way to specify the type for a module object. You can work around it by, e.g., declaring a module subclass, copying the module's dict to an instance of that subclass, then replacing the entry in sys.modules, but this is more than a little ugly.

There have been proposals to allow modules to specify a type (e.g., something similar to the way __metaclass__ worked in 2.x), and to make it easier to hook the import machinery to create modules of a custom type, and probably other variations on this. You might want to search this list and -dev for previous ideas, to find one you think should be reconsidered.

This is already done :-). Python 3.5 allows a module to contain code like: class MetaModule(types.ModuleType): @property def some_attribute(self): print("Accessing module.some_attribute") return 1 sys.modules[__name__].__class__ = MetaModule See also https://pypi.python.org/pypi/metamodule for utility code and a polyfill back to ancient Pythons. -n -- Nathaniel J. Smith -- http://vorpus.org

How are metaclasses unpythonic? This is one of the more defining features of python. There will be no way to properly implement this without moving the descriptor up to the metaclass. When thinking about metaclasses, it is often best to just think of them as classes and other types as instances. On Thu, Aug 20, 2015 at 11:20 AM, Sven R. Kunze wrote:

Sounds more like Thomas is looking for an Pythonic solution without messing around with sys.modules or metaclasses.

I personally like the proposals on StackOverflow and don't see why it couldn't be made to work out of the box:

class MyClass: @classproperty def prop(cls): return '42'

# module property @property def prop(mod): return '42'

On 20.08.2015 09:47, Nathaniel Smith wrote:

...

On Thu, Aug 20, 2015 at 12:20 AM, Andrew Barnert via Python-ideas wrote:

...

Modules are a more serious problem, because there's no immediate way to specify the type for a module object. You can work around it by, e.g., declaring a module subclass, copying the module's dict to an instance of that subclass, then replacing the entry in sys.modules, but this is more than a little ugly.

There have been proposals to allow modules to specify a type (e.g., something similar to the way __metaclass__ worked in 2.x), and to make it easier to hook the import machinery to create modules of a custom type, and probably other variations on this. You might want to search this list and -dev for previous ideas, to find one you think should be reconsidered.

This is already done :-). Python 3.5 allows a module to contain code like:

class MetaModule(types.ModuleType): @property def some_attribute(self): print("Accessing module.some_attribute") return 1

sys.modules[__name__].__class__ = MetaModule

See also

https://pypi.python.org/pypi/metamodule

for utility code and a polyfill back to ancient Pythons.

-n

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I think that syntax would be fine, but then `type.__new__` would need to do something special with that to implement the setter protocol for the types. On Thu, Aug 20, 2015 at 11:34 AM, Thomas Güttler < guettliml@thomas-guettler.de> wrote:

Am 20.08.2015 um 17:27 schrieb Joseph Jevnik:

...

How are metaclasses unpythonic? This is one of the more defining features of python. There will be no way to properly implement this without moving the descriptor up to the metaclass. When thinking about metaclasses, it is often best to just think of them as classes and other types as instances.

Yes, metaclasses are a great feature for experts.

For me this syntax is the most readable:

class MyClass:

...

@classproperty def prop(cls): return '42'--

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On 20.08.2015 17:31, Thomas Güttler wrote:

The following solution provides an @classproperty: http://stackoverflow.com/a/5191224/633961

I have not tried setting the value yet, but getting the value works very fine.

It would be great to have "@classproperty" even in Python2. Or at least installable via pip.

I am just a user in this case. I don't know about the best, cleanest, most efficient way to implement this.

Reminds me of cached_property: https://pypi.python.org/pypi/cached-property/0.1.5 I feel it could be useful to have a holistic solution (property + cached + instance,classes,modules) in the stdlib/Python somehow. Best, Sven

On Thu, Aug 20, 2015 at 11:57 AM, Sven R. Kunze wrote:

On 20.08.2015 17:31, Thomas Güttler wrote:

...

The following solution provides an @classproperty: http://stackoverflow.com/a/5191224/633961

I have not tried setting the value yet, but getting the value works very fine.

It would be great to have "@classproperty" even in Python2. Or at least installable via pip.

I am just a user in this case. I don't know about the best, cleanest, most efficient way to implement this.

Reminds me of cached_property: https://pypi.python.org/pypi/cached-property/0.1.5

I feel it could be useful to have a holistic solution (property + cached + instance,classes,modules) in the stdlib/Python somehow.

Having something like a cached-property (or the lazy=True option in my classproperty implementation) is rather nice, but as Guido points out it's out of scope for a general implementation of @classproperty (it's also easy enough to implement memoization patterns outside the stdlib). However, for (settable) @classproperty to work seamlessly, without having to involve custom metaclasses, would require a CPython implementation and probably a PEP. For in order for this to work it would have to be special-cased, since assignments to a class attribute don't normally go through <descriptor>.__set__. Erik

Am 20.08.2015 um 18:42 schrieb Sven R. Kunze:

What about 'staticproperties'?

Yes, that sound good. In my case a read-only classproperty is all I need. I guess that is what you mean with "staticproperty". Regards, Thomas Güttler -- Thomas Guettler http://www.thomas-guettler.de/

Am 20.08.2015 um 17:29 schrieb Guido van Rossum:

I think it's reasonable to propose @classproperty as a patch to CPython. It needs to be C code. Not sure about the writable version. The lazy=True part is not appropriate for th he stdlib (it's just a memoize pattern).

What's the next step? My knowledge of the programming language C is very limited. I am not able to write a patch for CPython. I could write a patch which looks like this: {{{ # From http://stackoverflow.com/a/5192374/633961 class classproperty(object): def __init__(self, f): self.f = f def __get__(self, obj, owner): return self.f(owner) }}} -- Thomas Guettler http://www.thomas-guettler.de/

Here is the created issue: http://bugs.python.org/issue24941 Please let me know if something is missing. Thomas Güttler Am 24.08.2015 um 19:17 schrieb Terry Reedy:

On 8/24/2015 6:07 AM, Thomas Güttler wrote:

...

Am 20.08.2015 um 17:29 schrieb Guido van Rossum:

...

I think it's reasonable to propose @classproperty as a patch to CPython. It needs to be C code. Not sure about the writable version. The lazy=True part is not appropriate for th he stdlib (it's just a memoize pattern).

What's the next step?

Open an issue on the tracker. Quote Guido's message above with list name, date, and thread name -- or pipermail archive url. Add python code below, or revision thereof, for someone to translate to C.

...

My knowledge of the programming language C is very limited. I am not able to write a patch for CPython.

I could write a patch which looks like this:

{{{ # From http://stackoverflow.com/a/5192374/633961

class classproperty(object): def __init__(self, f): self.f = f def __get__(self, obj, owner): return self.f(owner)

}}}

-- Thomas Guettler http://www.thomas-guettler.de/