Related
I know that functions are just descriptors, like this:
def func(self):
print(self.name)
class C:
def __init__(self, name):
self.name = name
C.func = func
c = C("foo")
c.func()
I thought at first that c.func equals C.func.__get__(c)，yes，C.func.__get__(c) return a bound method. But when I set the __get__ of func to None, c.func still returns a bound method.
def func(self):
print(self.name)
class C:
def __init__(self, name):
self.name = name
func.__get__ = None
C.func = func
c = C("foo")
c.func
output:
<bound method func of <__main__.C object at 0x0000027EB23BF088>>
So I'm confused. Moreover, I found that when calling a function from an instance, Python actually calls the class's ___getAttribute__ method, which returns a bound method.
def func(self):
print(self.name)
func.__get__ = None
class C:
def __getattribute__(self, name):
r = super().__getattribute__(name)
print(r) # r is a bound method already
return r
def __init__(self, name):
self.name = name
C.func = func
c = C("foo")
c.func
output:
<bound method func of <__main__.C object at 0x0000027EB243D1C8>>
func.__get__ doesn't seem to have any effect. So, What happended in __getattribute__? How does Python turn a function into a method? I've Googled and done some research, but I still can't find the answer.
Maybe I'm making things complicated, In my understanding, function is itself a descriptor, but just like the code below, I set the func to None, it works normally:
class C:
def func(self):
print('hello world')
func.__get__ = None
c = C()
c.func()
but if it's a descriptor, it will raise TypeError:
class C:
class D:
def __get__(self, inst, cls):
if inst is None:
return self
return 'hello world'
D.__get__ = None
func = D()
c = C()
c.func
Well, if I understand correctly from what I found. (Since I didn't know the descriptors, that's exactly why I like to help, still learning)
First, let's look at __getattr__ and __getattribute__.
Let's have an empty class A
class A:
pass
If I initialize an object and try to call a property, because there is none at the moment, we get AttributeError.
a = A()
a.some_property
The following occurs:
Simple check of flow:
class FlowDemo:
def __init__(self):
self.inited_property = True
def __getattribute__(self, item):
if item in ('__class__', '__len__') : # For less spam of getting this attribute, if you want, you can remove condition.
print('Get Attribute', item)
# Call default behavior
return super().__getattribute__(item)
def __getattr__(self, item):
print('Get Attr', item)
if item == 'some_magic_name':
return "It's magic!"
raise AttributeError
fd = FlowDemo()
fd.inited_property
# Get Attribute inited_property
# True
fd.some_magic_property
# Get Attribute some_magic_name
# Get Attr some_magic_name
# "It's magic!"
fd.some_property
# Get Attribute some_property
# Get Attr some_property
# Traceback (most recent call last):
# File "<input>", line 1, in <module>
# File "stack-class-property-and-descriptors.py", line 67, in # __getattr__
# raise AttributeError
# AttributeError
This is probably understandable, including the use. But to be sure, I'll give an example. This logic is used as a dynamic representation of the result from the databases (mapping of attributes to ordinary dict, list, etc.).
But it can also be just logic for accessing an attribute (property), such as an access counter or validation (but this applies to __setattr__ and __setattribute__)
And what about descriptors?
First let's look at data-descriptors, they are easier for me to understand.
This is a class or decoder that has __get__ and one or both of __set__ and __delete__.
Once this is defined, python, when used in the property definition with it and then does not return a class but the value it obtains through __get__, does not overwrite an already declared class when declaring a value, but uses its __set__.
Example:
class WeekDayDescriptor:
def __init__(self):
self.__week_day = 0
def __get__(self, instance, owner=None):
return self.__week_day
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Value must be int')
if not (0 <= value < 6):
raise ValueError('Value must be in range 0 - 6')
self.__week_day = value
class Calendar:
week_day = WeekDayDescriptor()
def __init__(self, week_day):
self.week_day = week_day
Demo:
c = Calendar(9)
# ValueError: Value must be in range 0-6
c = Calendar('6')
# TypeError: Value must be int
c = Calendar(3)
c.week_day = 6
c.week_day = 10
# ValueError: Value must be in range 0-6
c.week_day = 'monday'
# TypeError: Value must be int
Decorator descriptor:
class Calendar:
#property
def week_day(self):
return self.__week_day
#week_day.setter
def week_day(self, week_day):
if not isinstance(week_day, int):
raise TypeError('Value must be int')
if not (0 <= week_day < 6):
raise ValueError('Value must be in range 0 - 6')
self.__week_day = week_day
def __init__(self, week_day):
self.week_day = week_day
pass
And now for non-data descriptors...
A non-data descriptor is one that has only __get__.
As I understand it, each method automatically has its own descriptor, thanks to which the functions get references to the object - self.
We can write our own descriptor for a function / method, but it's not that straightforward, we have to help ourselves and get around it a bit.
def function_as_method(self, value):
print(self, value)
class HelperDescriptor:
def __get__(self, instance, owner):
def wrapper(*args, **kwargs):
return function_as_method(instance, *args, **kwargs)
return wrapper
class Foo:
baz = HelperDescriptor()
>>> bar = Foo()
>>> bar.baz(1)
<__main__.Foo object at 0x7f64f7768b70> 1
Source of last code block, but in czech lang.
And finally, your mentioned problem, when we set __get__ to None and you still get a reference to the function.
It's simple, python doesn't directly distinguish between primitive data types and functions, it's all a variable (or attribute / property) that has a value. Whether it's just value or it's callable is a different matter.
def f(): return True
print(type(f), f())
# <class 'function'> True
f = 123
print(type(f), f)
# <class 'int'> 123
Therefore, when we ask for the obj.func method or call it obj.func() directly, the first two changed magic is called first - __getattribute__ and __getattr__.
And if we call a method, it is called only after we get a reference to a function in memory.
Again a simple example:
def func(self, value):
print('Printing:', value)
class PrintDescriptor:
def __init__(self, name):
self.name = name
def __get__(self, instance, owner):
def wrapper(*args, **kwargs):
print(f"Calling '{self.name}' method")
return func(instance, *args, **kwargs)
return wrapper
class B:
foo = PrintDescriptor('foo')
bar = PrintDescriptor('bar')
def __getattribute__(self, item):
if item not in ('__len__', '__class__', '__dict__'):
print('Get Attribute', item)
return super().__getattribute__(item)
Demo:
b = B()
b.foo
# Get Attribute foo
# <function PrintDescriptor.__get__.<locals>.wrapper at 0x7f774a782ee0>
b.foo(2)
# Get Attribute foo
# Calling 'foo' method
# Printing: 2
b.bar(4)
# Get Attribute bar
# Calling 'bar' method
# Printing: 4
Sources:
https://www.datacamp.com/community/tutorials/python-descriptors#above1
https://blog.milde.cz/post/319-pokrocile-techniky-v-pythonu-deskriptory/
Python Doc, __get__
Python Docs, __getattribute__
Python Docs, __getattr__
I have given up memoization of a class as a bag-of-worms that I didn't want to explore and here is one example of why. The question I ask is "how does one extend or inherit from a memoized class" but it's very possible I have made a mistake. The memoize class below is a cut-down version of the one by brandizzi in How can I memoize a class instantiation in Python? and googling the subject finds more involved such classes.
class memoize(object):
def __init__(self, cls):
self.cls = cls
# I didn't understand why this was needed
self.__dict__.update(cls.__dict__)
# bit about static methods not needed
def __call__(self, *args):
try:
self.cls.instances
except:
self.cls.instances = {}
key = '//'.join(map(str, args))
if key not in self.cls.instances:
self.cls.instances[key] = self.cls(*args)
return self.cls.instances[key]
class Foo():
def __init__(self,val):
self.val = val
def __repr__(self):
return "{}<{},{}>".format(self.__class__.__name__,self.val,id(self))
class Bar(Foo):
def __init__(self,val):
super().__init__(val)
f1,f2,f3 = [Foo(i) for i in (0,0,1)]
print([f1,f2,f3])
b1,b2,b3 = [Bar(i) for i in (0,0,1)]
print([b1,b2,b3])
# produces exactly what I expect
# [Foo<0,3071981964>, Foo<0,3071982092>, Foo<1,3071982316>]
# [Bar<0,3071983340>, Bar<0,3071983404>, Bar<1,3071983436>]
Foo = memoize(Foo)
f1,f2,f3 = [Foo(i) for i in (0,0,1)]
print([f1,f2,f3])
b1,b2,b3 = [Bar(i) for i in (0,0,1)]
print([b1,b2,b3])
# and now Foo has been memoized so Foo(0) always produces the same object
# [Foo<0,3071725804>, Foo<0,3071725804>, Foo<1,3071726060>]
# [Bar<0,3071711916>, Bar<0,3071711660>, Bar<1,3071725644>]
# this produces a compilation error that I don't understand
class Baz(Foo):
def __init__(self,val):
super().__init__(val)
# Traceback (most recent call last):
# File "/tmp/foo.py", line 49, in <module>
# class Baz(Foo):
# TypeError: __init__() takes 2 positional arguments but 4 were given
This "recipe" is indeed a very bad idea - once you rebind Foo to memoize(Foo), Foo is a memoize instance and not class Foo anymore. This breaks all expectations wrt/ python's type and the whole object model. In this case, it about how the class statement works. Actually, this:
class Titi():
x = 42
def toto(self):
print(self.x)
is syntactic sugar for:
def toto(self):
print(self.x)
Titi = type("Titi", (object,), {x:42, toto:toto})
del toto
Note that this happens at runtime (like everything in Python except parsing / bytecode compilation), and that type is a class so calling type creates a new class which is a type instance (this is named a 'metaclass' - the class of a class - and type is the default metaclass).
So with Foo being now a memoize instance instead of a Type instance, and since memoize is not a proper metaclass (it's __init__ methods signature is incompatible), the whole thing just cannot work.
To get this to work, you'd have to make memoize a proper metaclass (this is a simplified example assuming a single arg named param but it can be generalized if you want to):
class FooType(type):
def __new__(meta, name, bases, attrs):
if "_instances" not in attrs:
attrs["_instances"] = dict()
return type.__new__(meta, name, bases, attrs)
def __call__(cls, param):
if param not in cls._instances:
cls._instances[param] = super(FooType, cls).__call__(param)
return cls._instances[param]
class Foo(metaclass=FooType):
def __init__(self, param):
self._param = param
print("%s init(%s)" % (self, param))
def __repr__(self):
return "{}<{},{}>".format(self.__class__.__name__, self._param, id(self))
class Bar(Foo):
pass
f1,f2,f3 = [Foo(i) for i in (0,0,1)]
print([f1,f2,f3])
b1,b2,b3 = [Bar(i) for i in (0,0,1)]
print([b1,b2,b3])
I have a class with two class methods (using the classmethod() function) for getting and setting what is essentially a static variable. I tried to use the property() function with these, but it results in an error. I was able to reproduce the error with the following in the interpreter:
class Foo(object):
_var = 5
#classmethod
def getvar(cls):
return cls._var
#classmethod
def setvar(cls, value):
cls._var = value
var = property(getvar, setvar)
I can demonstrate the class methods, but they don't work as properties:
>>> f = Foo()
>>> f.getvar()
5
>>> f.setvar(4)
>>> f.getvar()
4
>>> f.var
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable
>>> f.var=5
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable
Is it possible to use the property() function with #classmethod decorated functions?
3.8 < Python < 3.11
Can use both decorators together. See this answer.
Python < 3.9
A property is created on a class but affects an instance. So if you want a classmethod property, create the property on the metaclass.
>>> class foo(object):
... _var = 5
... class __metaclass__(type): # Python 2 syntax for metaclasses
... pass
... #classmethod
... def getvar(cls):
... return cls._var
... #classmethod
... def setvar(cls, value):
... cls._var = value
...
>>> foo.__metaclass__.var = property(foo.getvar.im_func, foo.setvar.im_func)
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
But since you're using a metaclass anyway, it will read better if you just move the classmethods in there.
>>> class foo(object):
... _var = 5
... class __metaclass__(type): # Python 2 syntax for metaclasses
... #property
... def var(cls):
... return cls._var
... #var.setter
... def var(cls, value):
... cls._var = value
...
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
or, using Python 3's metaclass=... syntax, and the metaclass defined outside of the foo class body, and the metaclass responsible for setting the initial value of _var:
>>> class foo_meta(type):
... def __init__(cls, *args, **kwargs):
... cls._var = 5
... #property
... def var(cls):
... return cls._var
... #var.setter
... def var(cls, value):
... cls._var = value
...
>>> class foo(metaclass=foo_meta):
... pass
...
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
In Python 3.9 You could use them together, but (as noted in #xgt's comment) it was deprecated in Python 3.11, so it is not recommended to use it.
Check the version remarks here:
https://docs.python.org/3.11/library/functions.html#classmethod
However, it used to work like so:
class G:
#classmethod
#property
def __doc__(cls):
return f'A doc for {cls.__name__!r}'
Order matters - due to how the descriptors interact, #classmethod has to be on top.
I hope this dead-simple read-only #classproperty decorator would help somebody looking for classproperties.
class classproperty(property):
def __get__(self, owner_self, owner_cls):
return self.fget(owner_cls)
class C(object):
#classproperty
def x(cls):
return 1
assert C.x == 1
assert C().x == 1
Reading the Python 2.2 release notes, I find the following.
The get method [of a property] won't be called when
the property is accessed as a class
attribute (C.x) instead of as an
instance attribute (C().x). If you
want to override the __get__ operation
for properties when used as a class
attribute, you can subclass property -
it is a new-style type itself - to
extend its __get__ method, or you can
define a descriptor type from scratch
by creating a new-style class that
defines __get__, __set__ and
__delete__ methods.
NOTE: The below method doesn't actually work for setters, only getters.
Therefore, I believe the prescribed solution is to create a ClassProperty as a subclass of property.
class ClassProperty(property):
def __get__(self, cls, owner):
return self.fget.__get__(None, owner)()
class foo(object):
_var=5
def getvar(cls):
return cls._var
getvar=classmethod(getvar)
def setvar(cls,value):
cls._var=value
setvar=classmethod(setvar)
var=ClassProperty(getvar,setvar)
assert foo.getvar() == 5
foo.setvar(4)
assert foo.getvar() == 4
assert foo.var == 4
foo.var = 3
assert foo.var == 3
However, the setters don't actually work:
foo.var = 4
assert foo.var == foo._var # raises AssertionError
foo._var is unchanged, you've simply overwritten the property with a new value.
You can also use ClassProperty as a decorator:
class foo(object):
_var = 5
#ClassProperty
#classmethod
def var(cls):
return cls._var
#var.setter
#classmethod
def var(cls, value):
cls._var = value
assert foo.var == 5
Is it possible to use the property() function with classmethod decorated functions?
No.
However, a classmethod is simply a bound method (a partial function) on a class accessible from instances of that class.
Since the instance is a function of the class and you can derive the class from the instance, you can can get whatever desired behavior you might want from a class-property with property:
class Example(object):
_class_property = None
#property
def class_property(self):
return self._class_property
#class_property.setter
def class_property(self, value):
type(self)._class_property = value
#class_property.deleter
def class_property(self):
del type(self)._class_property
This code can be used to test - it should pass without raising any errors:
ex1 = Example()
ex2 = Example()
ex1.class_property = None
ex2.class_property = 'Example'
assert ex1.class_property is ex2.class_property
del ex2.class_property
assert not hasattr(ex1, 'class_property')
And note that we didn't need metaclasses at all - and you don't directly access a metaclass through its classes' instances anyways.
writing a #classproperty decorator
You can actually create a classproperty decorator in just a few lines of code by subclassing property (it's implemented in C, but you can see equivalent Python here):
class classproperty(property):
def __get__(self, obj, objtype=None):
return super(classproperty, self).__get__(objtype)
def __set__(self, obj, value):
super(classproperty, self).__set__(type(obj), value)
def __delete__(self, obj):
super(classproperty, self).__delete__(type(obj))
Then treat the decorator as if it were a classmethod combined with property:
class Foo(object):
_bar = 5
#classproperty
def bar(cls):
"""this is the bar attribute - each subclass of Foo gets its own.
Lookups should follow the method resolution order.
"""
return cls._bar
#bar.setter
def bar(cls, value):
cls._bar = value
#bar.deleter
def bar(cls):
del cls._bar
And this code should work without errors:
def main():
f = Foo()
print(f.bar)
f.bar = 4
print(f.bar)
del f.bar
try:
f.bar
except AttributeError:
pass
else:
raise RuntimeError('f.bar must have worked - inconceivable!')
help(f) # includes the Foo.bar help.
f.bar = 5
class Bar(Foo):
"a subclass of Foo, nothing more"
help(Bar) # includes the Foo.bar help!
b = Bar()
b.bar = 'baz'
print(b.bar) # prints baz
del b.bar
print(b.bar) # prints 5 - looked up from Foo!
if __name__ == '__main__':
main()
But I'm not sure how well-advised this would be. An old mailing list article suggests it shouldn't work.
Getting the property to work on the class:
The downside of the above is that the "class property" isn't accessible from the class, because it would simply overwrite the data descriptor from the class __dict__.
However, we can override this with a property defined in the metaclass __dict__. For example:
class MetaWithFooClassProperty(type):
#property
def foo(cls):
"""The foo property is a function of the class -
in this case, the trivial case of the identity function.
"""
return cls
And then a class instance of the metaclass could have a property that accesses the class's property using the principle already demonstrated in the prior sections:
class FooClassProperty(metaclass=MetaWithFooClassProperty):
#property
def foo(self):
"""access the class's property"""
return type(self).foo
And now we see both the instance
>>> FooClassProperty().foo
<class '__main__.FooClassProperty'>
and the class
>>> FooClassProperty.foo
<class '__main__.FooClassProperty'>
have access to the class property.
Python 3!
See #Amit Portnoy's answer for an even cleaner method in python >= 3.9
Old question, lots of views, sorely in need of a one-true Python 3 way.
Luckily, it's easy with the metaclass kwarg:
class FooProperties(type):
#property
def var(cls):
return cls._var
class Foo(object, metaclass=FooProperties):
_var = 'FOO!'
Then, >>> Foo.var
'FOO!'
There is no reasonable way to make this "class property" system to work in Python.
Here is one unreasonable way to make it work. You can certainly make it more seamless with increasing amounts of metaclass magic.
class ClassProperty(object):
def __init__(self, getter, setter):
self.getter = getter
self.setter = setter
def __get__(self, cls, owner):
return getattr(cls, self.getter)()
def __set__(self, cls, value):
getattr(cls, self.setter)(value)
class MetaFoo(type):
var = ClassProperty('getvar', 'setvar')
class Foo(object):
__metaclass__ = MetaFoo
_var = 5
#classmethod
def getvar(cls):
print "Getting var =", cls._var
return cls._var
#classmethod
def setvar(cls, value):
print "Setting var =", value
cls._var = value
x = Foo.var
print "Foo.var = ", x
Foo.var = 42
x = Foo.var
print "Foo.var = ", x
The knot of the issue is that properties are what Python calls "descriptors". There is no short and easy way to explain how this sort of metaprogramming works, so I must point you to the descriptor howto.
You only ever need to understand this sort of things if you are implementing a fairly advanced framework. Like a transparent object persistence or RPC system, or a kind of domain-specific language.
However, in a comment to a previous answer, you say that you
need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.
It seems to me, what you really want is an Observer design pattern.
Setting it only on the meta class doesn't help if you want to access the class property via an instantiated object, in this case you need to install a normal property on the object as well (which dispatches to the class property). I think the following is a bit more clear:
#!/usr/bin/python
class classproperty(property):
def __get__(self, obj, type_):
return self.fget.__get__(None, type_)()
def __set__(self, obj, value):
cls = type(obj)
return self.fset.__get__(None, cls)(value)
class A (object):
_foo = 1
#classproperty
#classmethod
def foo(cls):
return cls._foo
#foo.setter
#classmethod
def foo(cls, value):
cls.foo = value
a = A()
print a.foo
b = A()
print b.foo
b.foo = 5
print a.foo
A.foo = 10
print b.foo
print A.foo
Half a solution, __set__ on the class does not work, still. The solution is a custom property class implementing both a property and a staticmethod
class ClassProperty(object):
def __init__(self, fget, fset):
self.fget = fget
self.fset = fset
def __get__(self, instance, owner):
return self.fget()
def __set__(self, instance, value):
self.fset(value)
class Foo(object):
_bar = 1
def get_bar():
print 'getting'
return Foo._bar
def set_bar(value):
print 'setting'
Foo._bar = value
bar = ClassProperty(get_bar, set_bar)
f = Foo()
#__get__ works
f.bar
Foo.bar
f.bar = 2
Foo.bar = 3 #__set__ does not
Because I need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.
Do you have access to at least one instance of the class? I can think of a way to do it then:
class MyClass (object):
__var = None
def _set_var (self, value):
type (self).__var = value
def _get_var (self):
return self.__var
var = property (_get_var, _set_var)
a = MyClass ()
b = MyClass ()
a.var = "foo"
print b.var
Give this a try, it gets the job done without having to change/add a lot of existing code.
>>> class foo(object):
... _var = 5
... def getvar(cls):
... return cls._var
... getvar = classmethod(getvar)
... def setvar(cls, value):
... cls._var = value
... setvar = classmethod(setvar)
... var = property(lambda self: self.getvar(), lambda self, val: self.setvar(val))
...
>>> f = foo()
>>> f.var
5
>>> f.var = 3
>>> f.var
3
The property function needs two callable arguments. give them lambda wrappers (which it passes the instance as its first argument) and all is well.
Here's a solution which should work for both access via the class and access via an instance which uses a metaclass.
In [1]: class ClassPropertyMeta(type):
...: #property
...: def prop(cls):
...: return cls._prop
...: def __new__(cls, name, parents, dct):
...: # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
...: dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
...: dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
...: return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)
...:
In [2]: class ClassProperty(object):
...: __metaclass__ = ClassPropertyMeta
...: _prop = 42
...: def __getattr__(self, attr):
...: raise Exception('Never gets called')
...:
In [3]: ClassProperty.prop
Out[3]: 42
In [4]: ClassProperty.prop = 1
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-4-e2e8b423818a> in <module>()
----> 1 ClassProperty.prop = 1
AttributeError: can't set attribute
In [5]: cp = ClassProperty()
In [6]: cp.prop
Out[6]: 42
In [7]: cp.prop = 1
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-7-e8284a3ee950> in <module>()
----> 1 cp.prop = 1
<ipython-input-1-16b7c320d521> in <lambda>(cls, attr, val)
6 # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
7 dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
----> 8 dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
9 return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)
AttributeError: can't set attribute
This also works with a setter defined in the metaclass.
I found one clean solution to this problem. It's a package called classutilities (pip install classutilities), see the documentation here on PyPi.
Consider example:
import classutilities
class SomeClass(classutilities.ClassPropertiesMixin):
_some_variable = 8 # Some encapsulated class variable
#classutilities.classproperty
def some_variable(cls): # class property getter
return cls._some_variable
#some_variable.setter
def some_variable(cls, value): # class property setter
cls._some_variable = value
You can use it on both class level and instance level:
# Getter on class level:
value = SomeClass.some_variable
print(value) # >>> 8
# Getter on instance level
inst = SomeClass()
value = inst.some_variable
print(value) # >>> 8
# Setter on class level:
new_value = 9
SomeClass.some_variable = new_value
print(SomeClass.some_variable) # >>> 9
print(SomeClass._some_variable) # >>> 9
# Setter on instance level
inst = SomeClass()
inst.some_variable = new_value
print(SomeClass.some_variable) # >>> 9
print(SomeClass._some_variable) # >>> 9
print(inst.some_variable) # >>> 9
print(inst._some_variable) # >>> 9
As you can see, it works correctly under all circumstances.
Based on https://stackoverflow.com/a/1800999/2290820
class MetaProperty(type):
def __init__(cls, *args, **kwargs):
super()
#property
def praparty(cls):
return cls._var
#praparty.setter
def praparty(cls, val):
cls._var = val
class A(metaclass=MetaProperty):
_var = 5
print(A.praparty)
A.praparty = 6
print(A.praparty)
For a functional approach pre Python 3.9 you can use this:
def classproperty(fget):
return type(
'classproperty',
(),
{'__get__': lambda self, _, cls: fget(cls), '__module__': None}
)()
class Item:
a = 47
#classproperty
def x(cls):
return cls.a
Item.x
After searching different places, I found a method to define a classproperty
valid with Python 2 and 3.
from future.utils import with_metaclass
class BuilderMetaClass(type):
#property
def load_namespaces(self):
return (self.__sourcepath__)
class BuilderMixin(with_metaclass(BuilderMetaClass, object)):
__sourcepath__ = 'sp'
print(BuilderMixin.load_namespaces)
Hope this can help somebody :)
A code completion friendly solution for Python < 3.9
from typing import (
Callable,
Generic,
TypeVar,
)
T = TypeVar('T')
class classproperty(Generic[T]):
"""Converts a method to a class property.
"""
def __init__(self, f: Callable[..., T]):
self.fget = f
def __get__(self, instance, owner) -> T:
return self.fget(owner)
Here is my solution that also caches the class property
class class_property(object):
# this caches the result of the function call for fn with cls input
# use this as a decorator on function methods that you want converted
# into cached properties
def __init__(self, fn):
self._fn_name = fn.__name__
if not isinstance(fn, (classmethod, staticmethod)):
fn = classmethod(fn)
self._fn = fn
def __get__(self, obj, cls=None):
if cls is None:
cls = type(obj)
if (
self._fn_name in vars(cls) and
type(vars(cls)[self._fn_name]).__name__ != "class_property"
):
return vars(cls)[self._fn_name]
else:
value = self._fn.__get__(obj, cls)()
setattr(cls, self._fn_name, value)
return value
Here's my suggestion. Don't use class methods.
Seriously.
What's the reason for using class methods in this case? Why not have an ordinary object of an ordinary class?
If you simply want to change the value, a property isn't really very helpful is it? Just set the attribute value and be done with it.
A property should only be used if there's something to conceal -- something that might change in a future implementation.
Maybe your example is way stripped down, and there is some hellish calculation you've left off. But it doesn't look like the property adds significant value.
The Java-influenced "privacy" techniques (in Python, attribute names that begin with _) aren't really very helpful. Private from whom? The point of private is a little nebulous when you have the source (as you do in Python.)
The Java-influenced EJB-style getters and setters (often done as properties in Python) are there to facilitate Java's primitive introspection as well as to pass muster with the static language compiler. All those getters and setters aren't as helpful in Python.
This question already has answers here:
Using property() on classmethods
(19 answers)
Closed 3 years ago.
In python I can add a method to a class with the #classmethod decorator. Is there a similar decorator to add a property to a class? I can better show what I'm talking about.
class Example(object):
the_I = 10
def __init__( self ):
self.an_i = 20
#property
def i( self ):
return self.an_i
def inc_i( self ):
self.an_i += 1
# is this even possible?
#classproperty
def I( cls ):
return cls.the_I
#classmethod
def inc_I( cls ):
cls.the_I += 1
e = Example()
assert e.i == 20
e.inc_i()
assert e.i == 21
assert Example.I == 10
Example.inc_I()
assert Example.I == 11
Is the syntax I've used above possible or would it require something more?
The reason I want class properties is so I can lazy load class attributes, which seems reasonable enough.
Here's how I would do this:
class ClassPropertyDescriptor(object):
def __init__(self, fget, fset=None):
self.fget = fget
self.fset = fset
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
return self.fget.__get__(obj, klass)()
def __set__(self, obj, value):
if not self.fset:
raise AttributeError("can't set attribute")
type_ = type(obj)
return self.fset.__get__(obj, type_)(value)
def setter(self, func):
if not isinstance(func, (classmethod, staticmethod)):
func = classmethod(func)
self.fset = func
return self
def classproperty(func):
if not isinstance(func, (classmethod, staticmethod)):
func = classmethod(func)
return ClassPropertyDescriptor(func)
class Bar(object):
_bar = 1
#classproperty
def bar(cls):
return cls._bar
#bar.setter
def bar(cls, value):
cls._bar = value
# test instance instantiation
foo = Bar()
assert foo.bar == 1
baz = Bar()
assert baz.bar == 1
# test static variable
baz.bar = 5
assert foo.bar == 5
# test setting variable on the class
Bar.bar = 50
assert baz.bar == 50
assert foo.bar == 50
The setter didn't work at the time we call Bar.bar, because we are calling
TypeOfBar.bar.__set__, which is not Bar.bar.__set__.
Adding a metaclass definition solves this:
class ClassPropertyMetaClass(type):
def __setattr__(self, key, value):
if key in self.__dict__:
obj = self.__dict__.get(key)
if obj and type(obj) is ClassPropertyDescriptor:
return obj.__set__(self, value)
return super(ClassPropertyMetaClass, self).__setattr__(key, value)
# and update class define:
# class Bar(object):
# __metaclass__ = ClassPropertyMetaClass
# _bar = 1
# and update ClassPropertyDescriptor.__set__
# def __set__(self, obj, value):
# if not self.fset:
# raise AttributeError("can't set attribute")
# if inspect.isclass(obj):
# type_ = obj
# obj = None
# else:
# type_ = type(obj)
# return self.fset.__get__(obj, type_)(value)
Now all will be fine.
If you define classproperty as follows, then your example works exactly as you requested.
class classproperty(object):
def __init__(self, f):
self.f = f
def __get__(self, obj, owner):
return self.f(owner)
The caveat is that you can't use this for writable properties. While e.I = 20 will raise an AttributeError, Example.I = 20 will overwrite the property object itself.
[answer written based on python 3.4; the metaclass syntax differs in 2 but I think the technique will still work]
You can do this with a metaclass...mostly. Dappawit's almost works, but I think it has a flaw:
class MetaFoo(type):
#property
def thingy(cls):
return cls._thingy
class Foo(object, metaclass=MetaFoo):
_thingy = 23
This gets you a classproperty on Foo, but there's a problem...
print("Foo.thingy is {}".format(Foo.thingy))
# Foo.thingy is 23
# Yay, the classmethod-property is working as intended!
foo = Foo()
if hasattr(foo, "thingy"):
print("Foo().thingy is {}".format(foo.thingy))
else:
print("Foo instance has no attribute 'thingy'")
# Foo instance has no attribute 'thingy'
# Wha....?
What the hell is going on here? Why can't I reach the class property from an instance?
I was beating my head on this for quite a while before finding what I believe is the answer. Python #properties are a subset of descriptors, and, from the descriptor documentation (emphasis mine):
The default behavior for attribute access is to get, set, or delete the
attribute from an object’s dictionary. For instance, a.x has a lookup chain
starting with a.__dict__['x'], then type(a).__dict__['x'], and continuing
through the base classes of type(a) excluding metaclasses.
So the method resolution order doesn't include our class properties (or anything else defined in the metaclass). It is possible to make a subclass of the built-in property decorator that behaves differently, but (citation needed) I've gotten the impression googling that the developers had a good reason (which I do not understand) for doing it that way.
That doesn't mean we're out of luck; we can access the properties on the class itself just fine...and we can get the class from type(self) within the instance, which we can use to make #property dispatchers:
class Foo(object, metaclass=MetaFoo):
_thingy = 23
#property
def thingy(self):
return type(self).thingy
Now Foo().thingy works as intended for both the class and the instances! It will also continue to do the right thing if a derived class replaces its underlying _thingy (which is the use case that got me on this hunt originally).
This isn't 100% satisfying to me -- having to do setup in both the metaclass and object class feels like it violates the DRY principle. But the latter is just a one-line dispatcher; I'm mostly okay with it existing, and you could probably compact it down to a lambda or something if you really wanted.
If you use Django, it has a built in #classproperty decorator.
from django.utils.decorators import classproperty
For Django 4, use:
from django.utils.functional import classproperty
I think you may be able to do this with the metaclass. Since the metaclass can be like a class for the class (if that makes sense). I know you can assign a __call__() method to the metaclass to override calling the class, MyClass(). I wonder if using the property decorator on the metaclass operates similarly.
Wow, it works:
class MetaClass(type):
def getfoo(self):
return self._foo
foo = property(getfoo)
#property
def bar(self):
return self._bar
class MyClass(object):
__metaclass__ = MetaClass
_foo = 'abc'
_bar = 'def'
print MyClass.foo
print MyClass.bar
Note: This is in Python 2.7. Python 3+ uses a different technique to declare a metaclass. Use: class MyClass(metaclass=MetaClass):, remove __metaclass__, and the rest is the same.
As far as I can tell, there is no way to write a setter for a class property without creating a new metaclass.
I have found that the following method works. Define a metaclass with all of the class properties and setters you want. IE, I wanted a class with a title property with a setter. Here's what I wrote:
class TitleMeta(type):
#property
def title(self):
return getattr(self, '_title', 'Default Title')
#title.setter
def title(self, title):
self._title = title
# Do whatever else you want when the title is set...
Now make the actual class you want as normal, except have it use the metaclass you created above.
# Python 2 style:
class ClassWithTitle(object):
__metaclass__ = TitleMeta
# The rest of your class definition...
# Python 3 style:
class ClassWithTitle(object, metaclass = TitleMeta):
# Your class definition...
It's a bit weird to define this metaclass as we did above if we'll only ever use it on the single class. In that case, if you're using the Python 2 style, you can actually define the metaclass inside the class body. That way it's not defined in the module scope.
def _create_type(meta, name, attrs):
type_name = f'{name}Type'
type_attrs = {}
for k, v in attrs.items():
if type(v) is _ClassPropertyDescriptor:
type_attrs[k] = v
return type(type_name, (meta,), type_attrs)
class ClassPropertyType(type):
def __new__(meta, name, bases, attrs):
Type = _create_type(meta, name, attrs)
cls = super().__new__(meta, name, bases, attrs)
cls.__class__ = Type
return cls
class _ClassPropertyDescriptor(object):
def __init__(self, fget, fset=None):
self.fget = fget
self.fset = fset
def __get__(self, obj, owner):
if self in obj.__dict__.values():
return self.fget(obj)
return self.fget(owner)
def __set__(self, obj, value):
if not self.fset:
raise AttributeError("can't set attribute")
return self.fset(obj, value)
def setter(self, func):
self.fset = func
return self
def classproperty(func):
return _ClassPropertyDescriptor(func)
class Bar(metaclass=ClassPropertyType):
__bar = 1
#classproperty
def bar(cls):
return cls.__bar
#bar.setter
def bar(cls, value):
cls.__bar = value
bar = Bar()
assert Bar.bar==1
Bar.bar=2
assert bar.bar==2
nbar = Bar()
assert nbar.bar==2
I happened to come up with a solution very similar to #Andrew, only DRY
class MetaFoo(type):
def __new__(mc1, name, bases, nmspc):
nmspc.update({'thingy': MetaFoo.thingy})
return super(MetaFoo, mc1).__new__(mc1, name, bases, nmspc)
#property
def thingy(cls):
if not inspect.isclass(cls):
cls = type(cls)
return cls._thingy
#thingy.setter
def thingy(cls, value):
if not inspect.isclass(cls):
cls = type(cls)
cls._thingy = value
class Foo(metaclass=MetaFoo):
_thingy = 23
class Bar(Foo)
_thingy = 12
This has the best of all answers:
The "metaproperty" is added to the class, so that it will still be a property of the instance
Don't need to redefine thingy in any of the classes
The property works as a "class property" in for both instance and class
You have the flexibility to customize how _thingy is inherited
In my case, I actually customized _thingy to be different for every child, without defining it in each class (and without a default value) by:
def __new__(mc1, name, bases, nmspc):
nmspc.update({'thingy': MetaFoo.services, '_thingy': None})
return super(MetaFoo, mc1).__new__(mc1, name, bases, nmspc)
If you only need lazy loading, then you could just have a class initialisation method.
EXAMPLE_SET = False
class Example(object):
#classmethod
def initclass(cls):
global EXAMPLE_SET
if EXAMPLE_SET: return
cls.the_I = 'ok'
EXAMPLE_SET = True
def __init__( self ):
Example.initclass()
self.an_i = 20
try:
print Example.the_I
except AttributeError:
print 'ok class not "loaded"'
foo = Example()
print foo.the_I
print Example.the_I
But the metaclass approach seems cleaner, and with more predictable behavior.
Perhaps what you're looking for is the Singleton design pattern. There's a nice SO QA about implementing shared state in Python.
I have a class with two class methods (using the classmethod() function) for getting and setting what is essentially a static variable. I tried to use the property() function with these, but it results in an error. I was able to reproduce the error with the following in the interpreter:
class Foo(object):
_var = 5
#classmethod
def getvar(cls):
return cls._var
#classmethod
def setvar(cls, value):
cls._var = value
var = property(getvar, setvar)
I can demonstrate the class methods, but they don't work as properties:
>>> f = Foo()
>>> f.getvar()
5
>>> f.setvar(4)
>>> f.getvar()
4
>>> f.var
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable
>>> f.var=5
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable
Is it possible to use the property() function with #classmethod decorated functions?
3.8 < Python < 3.11
Can use both decorators together. See this answer.
Python < 3.9
A property is created on a class but affects an instance. So if you want a classmethod property, create the property on the metaclass.
>>> class foo(object):
... _var = 5
... class __metaclass__(type): # Python 2 syntax for metaclasses
... pass
... #classmethod
... def getvar(cls):
... return cls._var
... #classmethod
... def setvar(cls, value):
... cls._var = value
...
>>> foo.__metaclass__.var = property(foo.getvar.im_func, foo.setvar.im_func)
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
But since you're using a metaclass anyway, it will read better if you just move the classmethods in there.
>>> class foo(object):
... _var = 5
... class __metaclass__(type): # Python 2 syntax for metaclasses
... #property
... def var(cls):
... return cls._var
... #var.setter
... def var(cls, value):
... cls._var = value
...
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
or, using Python 3's metaclass=... syntax, and the metaclass defined outside of the foo class body, and the metaclass responsible for setting the initial value of _var:
>>> class foo_meta(type):
... def __init__(cls, *args, **kwargs):
... cls._var = 5
... #property
... def var(cls):
... return cls._var
... #var.setter
... def var(cls, value):
... cls._var = value
...
>>> class foo(metaclass=foo_meta):
... pass
...
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3
In Python 3.9 You could use them together, but (as noted in #xgt's comment) it was deprecated in Python 3.11, so it is not recommended to use it.
Check the version remarks here:
https://docs.python.org/3.11/library/functions.html#classmethod
However, it used to work like so:
class G:
#classmethod
#property
def __doc__(cls):
return f'A doc for {cls.__name__!r}'
Order matters - due to how the descriptors interact, #classmethod has to be on top.
I hope this dead-simple read-only #classproperty decorator would help somebody looking for classproperties.
class classproperty(property):
def __get__(self, owner_self, owner_cls):
return self.fget(owner_cls)
class C(object):
#classproperty
def x(cls):
return 1
assert C.x == 1
assert C().x == 1
Reading the Python 2.2 release notes, I find the following.
The get method [of a property] won't be called when
the property is accessed as a class
attribute (C.x) instead of as an
instance attribute (C().x). If you
want to override the __get__ operation
for properties when used as a class
attribute, you can subclass property -
it is a new-style type itself - to
extend its __get__ method, or you can
define a descriptor type from scratch
by creating a new-style class that
defines __get__, __set__ and
__delete__ methods.
NOTE: The below method doesn't actually work for setters, only getters.
Therefore, I believe the prescribed solution is to create a ClassProperty as a subclass of property.
class ClassProperty(property):
def __get__(self, cls, owner):
return self.fget.__get__(None, owner)()
class foo(object):
_var=5
def getvar(cls):
return cls._var
getvar=classmethod(getvar)
def setvar(cls,value):
cls._var=value
setvar=classmethod(setvar)
var=ClassProperty(getvar,setvar)
assert foo.getvar() == 5
foo.setvar(4)
assert foo.getvar() == 4
assert foo.var == 4
foo.var = 3
assert foo.var == 3
However, the setters don't actually work:
foo.var = 4
assert foo.var == foo._var # raises AssertionError
foo._var is unchanged, you've simply overwritten the property with a new value.
You can also use ClassProperty as a decorator:
class foo(object):
_var = 5
#ClassProperty
#classmethod
def var(cls):
return cls._var
#var.setter
#classmethod
def var(cls, value):
cls._var = value
assert foo.var == 5
Is it possible to use the property() function with classmethod decorated functions?
No.
However, a classmethod is simply a bound method (a partial function) on a class accessible from instances of that class.
Since the instance is a function of the class and you can derive the class from the instance, you can can get whatever desired behavior you might want from a class-property with property:
class Example(object):
_class_property = None
#property
def class_property(self):
return self._class_property
#class_property.setter
def class_property(self, value):
type(self)._class_property = value
#class_property.deleter
def class_property(self):
del type(self)._class_property
This code can be used to test - it should pass without raising any errors:
ex1 = Example()
ex2 = Example()
ex1.class_property = None
ex2.class_property = 'Example'
assert ex1.class_property is ex2.class_property
del ex2.class_property
assert not hasattr(ex1, 'class_property')
And note that we didn't need metaclasses at all - and you don't directly access a metaclass through its classes' instances anyways.
writing a #classproperty decorator
You can actually create a classproperty decorator in just a few lines of code by subclassing property (it's implemented in C, but you can see equivalent Python here):
class classproperty(property):
def __get__(self, obj, objtype=None):
return super(classproperty, self).__get__(objtype)
def __set__(self, obj, value):
super(classproperty, self).__set__(type(obj), value)
def __delete__(self, obj):
super(classproperty, self).__delete__(type(obj))
Then treat the decorator as if it were a classmethod combined with property:
class Foo(object):
_bar = 5
#classproperty
def bar(cls):
"""this is the bar attribute - each subclass of Foo gets its own.
Lookups should follow the method resolution order.
"""
return cls._bar
#bar.setter
def bar(cls, value):
cls._bar = value
#bar.deleter
def bar(cls):
del cls._bar
And this code should work without errors:
def main():
f = Foo()
print(f.bar)
f.bar = 4
print(f.bar)
del f.bar
try:
f.bar
except AttributeError:
pass
else:
raise RuntimeError('f.bar must have worked - inconceivable!')
help(f) # includes the Foo.bar help.
f.bar = 5
class Bar(Foo):
"a subclass of Foo, nothing more"
help(Bar) # includes the Foo.bar help!
b = Bar()
b.bar = 'baz'
print(b.bar) # prints baz
del b.bar
print(b.bar) # prints 5 - looked up from Foo!
if __name__ == '__main__':
main()
But I'm not sure how well-advised this would be. An old mailing list article suggests it shouldn't work.
Getting the property to work on the class:
The downside of the above is that the "class property" isn't accessible from the class, because it would simply overwrite the data descriptor from the class __dict__.
However, we can override this with a property defined in the metaclass __dict__. For example:
class MetaWithFooClassProperty(type):
#property
def foo(cls):
"""The foo property is a function of the class -
in this case, the trivial case of the identity function.
"""
return cls
And then a class instance of the metaclass could have a property that accesses the class's property using the principle already demonstrated in the prior sections:
class FooClassProperty(metaclass=MetaWithFooClassProperty):
#property
def foo(self):
"""access the class's property"""
return type(self).foo
And now we see both the instance
>>> FooClassProperty().foo
<class '__main__.FooClassProperty'>
and the class
>>> FooClassProperty.foo
<class '__main__.FooClassProperty'>
have access to the class property.
Python 3!
See #Amit Portnoy's answer for an even cleaner method in python >= 3.9
Old question, lots of views, sorely in need of a one-true Python 3 way.
Luckily, it's easy with the metaclass kwarg:
class FooProperties(type):
#property
def var(cls):
return cls._var
class Foo(object, metaclass=FooProperties):
_var = 'FOO!'
Then, >>> Foo.var
'FOO!'
There is no reasonable way to make this "class property" system to work in Python.
Here is one unreasonable way to make it work. You can certainly make it more seamless with increasing amounts of metaclass magic.
class ClassProperty(object):
def __init__(self, getter, setter):
self.getter = getter
self.setter = setter
def __get__(self, cls, owner):
return getattr(cls, self.getter)()
def __set__(self, cls, value):
getattr(cls, self.setter)(value)
class MetaFoo(type):
var = ClassProperty('getvar', 'setvar')
class Foo(object):
__metaclass__ = MetaFoo
_var = 5
#classmethod
def getvar(cls):
print "Getting var =", cls._var
return cls._var
#classmethod
def setvar(cls, value):
print "Setting var =", value
cls._var = value
x = Foo.var
print "Foo.var = ", x
Foo.var = 42
x = Foo.var
print "Foo.var = ", x
The knot of the issue is that properties are what Python calls "descriptors". There is no short and easy way to explain how this sort of metaprogramming works, so I must point you to the descriptor howto.
You only ever need to understand this sort of things if you are implementing a fairly advanced framework. Like a transparent object persistence or RPC system, or a kind of domain-specific language.
However, in a comment to a previous answer, you say that you
need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.
It seems to me, what you really want is an Observer design pattern.
Setting it only on the meta class doesn't help if you want to access the class property via an instantiated object, in this case you need to install a normal property on the object as well (which dispatches to the class property). I think the following is a bit more clear:
#!/usr/bin/python
class classproperty(property):
def __get__(self, obj, type_):
return self.fget.__get__(None, type_)()
def __set__(self, obj, value):
cls = type(obj)
return self.fset.__get__(None, cls)(value)
class A (object):
_foo = 1
#classproperty
#classmethod
def foo(cls):
return cls._foo
#foo.setter
#classmethod
def foo(cls, value):
cls.foo = value
a = A()
print a.foo
b = A()
print b.foo
b.foo = 5
print a.foo
A.foo = 10
print b.foo
print A.foo
Half a solution, __set__ on the class does not work, still. The solution is a custom property class implementing both a property and a staticmethod
class ClassProperty(object):
def __init__(self, fget, fset):
self.fget = fget
self.fset = fset
def __get__(self, instance, owner):
return self.fget()
def __set__(self, instance, value):
self.fset(value)
class Foo(object):
_bar = 1
def get_bar():
print 'getting'
return Foo._bar
def set_bar(value):
print 'setting'
Foo._bar = value
bar = ClassProperty(get_bar, set_bar)
f = Foo()
#__get__ works
f.bar
Foo.bar
f.bar = 2
Foo.bar = 3 #__set__ does not
Because I need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.
Do you have access to at least one instance of the class? I can think of a way to do it then:
class MyClass (object):
__var = None
def _set_var (self, value):
type (self).__var = value
def _get_var (self):
return self.__var
var = property (_get_var, _set_var)
a = MyClass ()
b = MyClass ()
a.var = "foo"
print b.var
Give this a try, it gets the job done without having to change/add a lot of existing code.
>>> class foo(object):
... _var = 5
... def getvar(cls):
... return cls._var
... getvar = classmethod(getvar)
... def setvar(cls, value):
... cls._var = value
... setvar = classmethod(setvar)
... var = property(lambda self: self.getvar(), lambda self, val: self.setvar(val))
...
>>> f = foo()
>>> f.var
5
>>> f.var = 3
>>> f.var
3
The property function needs two callable arguments. give them lambda wrappers (which it passes the instance as its first argument) and all is well.
Here's a solution which should work for both access via the class and access via an instance which uses a metaclass.
In [1]: class ClassPropertyMeta(type):
...: #property
...: def prop(cls):
...: return cls._prop
...: def __new__(cls, name, parents, dct):
...: # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
...: dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
...: dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
...: return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)
...:
In [2]: class ClassProperty(object):
...: __metaclass__ = ClassPropertyMeta
...: _prop = 42
...: def __getattr__(self, attr):
...: raise Exception('Never gets called')
...:
In [3]: ClassProperty.prop
Out[3]: 42
In [4]: ClassProperty.prop = 1
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-4-e2e8b423818a> in <module>()
----> 1 ClassProperty.prop = 1
AttributeError: can't set attribute
In [5]: cp = ClassProperty()
In [6]: cp.prop
Out[6]: 42
In [7]: cp.prop = 1
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-7-e8284a3ee950> in <module>()
----> 1 cp.prop = 1
<ipython-input-1-16b7c320d521> in <lambda>(cls, attr, val)
6 # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
7 dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
----> 8 dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
9 return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)
AttributeError: can't set attribute
This also works with a setter defined in the metaclass.
I found one clean solution to this problem. It's a package called classutilities (pip install classutilities), see the documentation here on PyPi.
Consider example:
import classutilities
class SomeClass(classutilities.ClassPropertiesMixin):
_some_variable = 8 # Some encapsulated class variable
#classutilities.classproperty
def some_variable(cls): # class property getter
return cls._some_variable
#some_variable.setter
def some_variable(cls, value): # class property setter
cls._some_variable = value
You can use it on both class level and instance level:
# Getter on class level:
value = SomeClass.some_variable
print(value) # >>> 8
# Getter on instance level
inst = SomeClass()
value = inst.some_variable
print(value) # >>> 8
# Setter on class level:
new_value = 9
SomeClass.some_variable = new_value
print(SomeClass.some_variable) # >>> 9
print(SomeClass._some_variable) # >>> 9
# Setter on instance level
inst = SomeClass()
inst.some_variable = new_value
print(SomeClass.some_variable) # >>> 9
print(SomeClass._some_variable) # >>> 9
print(inst.some_variable) # >>> 9
print(inst._some_variable) # >>> 9
As you can see, it works correctly under all circumstances.
Based on https://stackoverflow.com/a/1800999/2290820
class MetaProperty(type):
def __init__(cls, *args, **kwargs):
super()
#property
def praparty(cls):
return cls._var
#praparty.setter
def praparty(cls, val):
cls._var = val
class A(metaclass=MetaProperty):
_var = 5
print(A.praparty)
A.praparty = 6
print(A.praparty)
For a functional approach pre Python 3.9 you can use this:
def classproperty(fget):
return type(
'classproperty',
(),
{'__get__': lambda self, _, cls: fget(cls), '__module__': None}
)()
class Item:
a = 47
#classproperty
def x(cls):
return cls.a
Item.x
After searching different places, I found a method to define a classproperty
valid with Python 2 and 3.
from future.utils import with_metaclass
class BuilderMetaClass(type):
#property
def load_namespaces(self):
return (self.__sourcepath__)
class BuilderMixin(with_metaclass(BuilderMetaClass, object)):
__sourcepath__ = 'sp'
print(BuilderMixin.load_namespaces)
Hope this can help somebody :)
A code completion friendly solution for Python < 3.9
from typing import (
Callable,
Generic,
TypeVar,
)
T = TypeVar('T')
class classproperty(Generic[T]):
"""Converts a method to a class property.
"""
def __init__(self, f: Callable[..., T]):
self.fget = f
def __get__(self, instance, owner) -> T:
return self.fget(owner)
Here is my solution that also caches the class property
class class_property(object):
# this caches the result of the function call for fn with cls input
# use this as a decorator on function methods that you want converted
# into cached properties
def __init__(self, fn):
self._fn_name = fn.__name__
if not isinstance(fn, (classmethod, staticmethod)):
fn = classmethod(fn)
self._fn = fn
def __get__(self, obj, cls=None):
if cls is None:
cls = type(obj)
if (
self._fn_name in vars(cls) and
type(vars(cls)[self._fn_name]).__name__ != "class_property"
):
return vars(cls)[self._fn_name]
else:
value = self._fn.__get__(obj, cls)()
setattr(cls, self._fn_name, value)
return value
Here's my suggestion. Don't use class methods.
Seriously.
What's the reason for using class methods in this case? Why not have an ordinary object of an ordinary class?
If you simply want to change the value, a property isn't really very helpful is it? Just set the attribute value and be done with it.
A property should only be used if there's something to conceal -- something that might change in a future implementation.
Maybe your example is way stripped down, and there is some hellish calculation you've left off. But it doesn't look like the property adds significant value.
The Java-influenced "privacy" techniques (in Python, attribute names that begin with _) aren't really very helpful. Private from whom? The point of private is a little nebulous when you have the source (as you do in Python.)
The Java-influenced EJB-style getters and setters (often done as properties in Python) are there to facilitate Java's primitive introspection as well as to pass muster with the static language compiler. All those getters and setters aren't as helpful in Python.