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.
I have a few classes each of which has a number of attributes. What all of the attributes have in common is that they should be numeric properties. This seems to be an ideal place to use python's decorators, but I can't seem to wrap my mind around what the correct implementation would be. Here is a simple example:
class Junk(object):
def __init__(self, var):
self._var = var
#property
def var(self):
"""A numeric variable"""
return self._var
#var.setter
def size(self, value):
# need to make sure var is an integer
if not isinstance(value, int):
raise ValueError("var must be an integer, var = {}".format(value))
self._var = value
#var.deleter
def size(self):
raise RuntimeError("You can't delete var")
It seems to me that it should be possible to write a decorator that does everything so that the above can be transformed into:
def numeric_property(*args, **kwargs):
...
class Junk(object):
def __init__(self, var):
self._var = var
#numeric_property
def var(self):
"""A numeric variable"""
return self._var
That way the new numeric_property decorator can be used in many classes.
A #property is just a special case of Python's descriptor protocol, so you can certainly build your own custom versions. For your case:
class NumericProperty:
"""A property that must be numeric.
Args:
attr (str): The name of the backing attribute.
"""
def __init__(self, attr):
self.attr = attr
def __get__(self, obj, type=None):
return getattr(obj, self.attr)
def __set__(self, obj, value):
if not isinstance(value, int):
raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
setattr(obj, self.attr, value)
def __delete__(self, obj):
raise RuntimeError("You can't delete {}".format(self.attr))
class Junk:
var = NumericProperty('_var')
def __init__(self, var):
self.var = var
In use:
>>> j = Junk('hi')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/jonrsharpe/test.py", line 29, in __init__
self.var = var
File "/Users/jonrsharpe/test.py", line 17, in __set__
raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
ValueError: _var must be an integer, var = 'hi'
>>> j = Junk(1)
>>> del j.var
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/jonrsharpe/test.py", line 21, in __delete__
raise RuntimeError("You can't delete {}".format(self.attr))
RuntimeError: You can't delete _var
>>> j.var = 'hello'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/jonrsharpe/test.py", line 17, in __set__
raise ValueError("{} must be an integer, var = {!r}".format(self.attr, value))
ValueError: _var must be an integer, var = 'hello'
>>> j.var = 2
>>> j.var
2
Option 1: inherit from property
property is a descriptor. See Descriptor HowTo on python.org.
So, can inherit from property and override the relevant methods.
For example, to enforce int on setter:
class numeric_property(property):
def __set__(self, obj, value):
assert isinstance(value, int), "numeric_property requires an int"
super(numeric_property, self).__set__(obj, value)
class A(object):
#numeric_property
def x(self):
return self._x
#x.setter
def x(self, value):
self._x = value
And now you have integers enforced:
>>> a = A()
>>> a.x = 'aaa'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in __set__
AssertionError: numeric_property requires an int
Option 2: Create a better descriptor
On the other hand, it may be even better to implement a brand new descriptor which does not inherit from property, which would enable you to define the property in one go.
It would be nicer to have this kind of interface:
class A(object):
x = numeric_property('_x')
For that you would implement a descriptor which takes the attribute name:
class numeric_property(object):
def __init__(self, private_attribute_name, default=0):
self.private_attribute_name = private_attribute_name
self.default = default
def __get__(self, obj, typ):
if not obj: return self
return getattr(obj, self.private_attribute_name, self.default)
def __set__(self, obj, value):
assert isinstance(value, int), "numeric_property requires an int"
setattr(obj, self.private_attribute_name, value)
Disclaimer :)
I would rather not enforce strict typing in Pyhon, because Python is much more powerful without it.
You may just create a function that does it for you . As simple as it can get, no need to create a custom descriptor:
def numprop(name, privname):
#property
def _numprop(self):
return getattr(self, privname)
#_numprop.setter
def _numprop(self, value):
if not isinstance(value, int):
raise ValueError("{name} must be an integer, {name} = {}".format(value, name=name))
setattr(self, privname, value)
#_numprop.deleter
def _numprop(self):
raise RuntimeError("You can't delete var")
return _numprop
class Junk(object):
def __init__(self, var):
self._var = var
var = numprop("var", "_var")
This question already has answers here:
How to make an immutable object in Python?
(26 answers)
Closed 8 years ago.
I have read a lot about this subject here but i still can't find an appropriate answer.
I have a class like:
class A(object):
def __init__(self, first, second):
self.first = first
self.second = second
def __eq__(self, other):
return ****
def __str__(self):
return *****
def __repr__(self):
return ****
a = A("a", "b")
How can i forbid a.first = "c" for example ?
You can override __setattr__ to either prevent any changes:
def __setattr__(self, name, value):
raise AttributeError('''Can't set attribute "{0}"'''.format(name))
or prevent adding new attributes:
def __setattr__(self, name, value):
if not hasattr(self, name):
raise AttributeError('''Can't set attribute "{0}"'''.format(name))
# Or whatever the base class is, if not object.
# You can use super(), if appropriate.
object.__setattr__(self, name, value)
You can also replace hasattr with a check against a list of allowed attributes:
if name not in list_of_allowed_attributes_to_change:
raise AttributeError('''Can't set attribute "{0}"'''.format(name))
Another approach is to use properties instead of plain attributes:
class A(object):
def __init__(self, first, second):
self._first = first
self._second = second
#property
def first(self):
return self._first
#property
def second(self):
return self._second
You can disable __setattr__ as the last step of initializing the object.
class A(object):
def __init__(self, first, second):
self.first = first
self.second = second
self.frozen = True
def __setattr__(self, name, value):
if getattr(self, 'frozen', False):
raise AttributeError('Attempt to modify immutable object')
super(A, self).__setattr__(name, value)
>>> a = A(1, 2)
>>> a.first, a.second
(1, 2)
>>> a.first = 3
Traceback (most recent call last):
File "<pyshell#46>", line 1, in <module>
a.first = 3
File "<pyshell#41>", line 10, in __setattr__
raise AttributeError('Attempt to modify immutable object')
AttributeError: Attempt to modify immutable object
Edit: This answer has a flaw, which I'm sure is shared by every other solution: if the members themselves are mutable, nothing protects them. If your object contains a list for example, it's all over. This is in contrast to i.e. C++ where declaring an object const extends to all its members recursively.
Consider the following python code:
class Foo(object):
def __init__(self, value):
self._value = value
#property
def value(self):
return "value: {v}".format(v=self._value)
#value.setter
def value(self, value):
self._value = value
class Bar(object):
def __init__(self):
self.foo = Foo('foo')
def __getattr__(self, attr, *args, **kwargs):
"""
Intercepts attribute calls, and if we don't have it, look at the
webelement to see if it has the attribute.
"""
# Check first to see if it looks like a method, if not then just return
# the attribute the way it is.
# Note: this has only been tested with variables, and methods.
if not hasattr(getattr(self.foo, attr), '__call__'):
return getattr(self.foo, attr)
def callable(*args, **kwargs):
'''
Returns the method from the webelement module if found
'''
return getattr(self.foo, attr)(*args, **kwargs)
return callable
>>> b = Bar()
>>> b.foo
<__main__.Foo object at 0x819410>
>>> b.foo.value
'value: foo'
>>> b.foo.value = '2'
>>> b.foo.value
'value: 2'
>>> b.value
'value: 2'
>>> b.value = '3'
>>> b.value
'3'
That last part, I want it to be 'value: 3' instead of '3' because now my property 'value' is now an attribute instead.
Is it possible, and if it is how would I would I do that.
Your __getattr__ returns the property value, not the property itself. When you access getattr(self.foo, attr) it does the equivalent of self.foo.value and returns that, and the property is called at that time.
You thus need to implement a __setattr__ method too, to mirror the __getattr__ and pass on the value setting to the contained foo object.
Under the hood, Python implements properties as descriptors; their __get__() method is called by the lower-level __getattribute__ method, which causes them to return their computed value. It is never the property object itself that is returned.
Here's an example __setattr__:
def __setattr__(self, attr, value):
if hasattr(self, 'foo') and hasattr(self.foo, attr):
setattr(self.foo, attr, value)
return
super(Bar, self).__setattr__(attr, value)
Note: because your __init__ sets self.foo, you need to test if foo exists on your class (hasattr(self, 'foo'). You also need to call the original __setattr__ implementation to make sure that things like self.foo = Foo() work still.
I want to be able use python descriptors in a class which has the slots optimization:
class C(object):
__slots__ = ['a']
a = MyDescriptor('a')
def __init__(self, val):
self.a = val
The problem I have is how to implement the descriptor class in order to be able to store values in the class instance which invokes the descriptor object. The usual solution would look like the one below but will not work since "dict" is no longer defined when "slots" is invoked in the C class:
class MyDescriptor(object):
__slots__ = ['name']
def __init__(self, name_):
self.name = name_
def __get__(self, instance, owner):
if self.name not in instance.__dict__:
raise AttributeError, self.name
return instance.__dict__[self.name]
def __set__(self, instance, value):
instance.__dict__[self.name] = value
Don't declare the same name as a slot and as an instance method. Use different names, and access the slot as an attribute, not via __dict__.
class MyDescriptor(object):
__slots__ = ['name']
def __init__(self, name_):
self.name = name_
def __get__(self, instance, owner):
return getattr(instance, self.name)
def __set__(self, instance, value):
setattr(instance, self.name, value)
class C(object):
__slots__ = ['_a']
a = MyDescriptor('_a')
def __init__(self, val):
self.a = val
foo = C(1)
print foo.a
foo.a = 2
print foo.a
Though of dubious value, the following trick will work, if it is ok to put the first __slots__ in a subclass.
class A( object ):
__slots__ = ( 'a', )
class B( A ):
__slots__ = ()
#property
def a( self ):
try:
return A.a.__get__( self )
except AttributeError:
return 'no a set'
#a.setter
def a( self, val ):
A.a.__set__( self, val )
(You can use your own descriptor rather than property.) With these definitions:
>>> b = B()
>>> b.a
'no a set'
>>> b.a = 'foo'
>>> b.a
'foo'
As far as I understand, __slots__ is implemented with its own descriptor, so another descriptor after __slots__ in the same class would just overwrite. If you want to elaborate this technique, you could search for a candidate descriptor in self.__class__.__mro__ (or starting with instance in your own __get__).
Postscript
Ok ... well if you really want to use one class, you can use the following adaptation:
class C( object ):
__slots__ = ( 'c', )
class MyDescriptor( object ):
def __init__( self, slots_descriptor ):
self.slots_descriptor = slots_descriptor
def __get__( self, inst, owner = None ):
try:
return self.slots_descriptor.__get__( inst, owner )
except AttributeError:
return 'no c'
def __set__( self, inst, val ):
self.slots_descriptor.__set__( inst, val )
C.c = MyDescriptor( C.c )
If you insist on inscrutability, you can make the assignment in a metaclass or a class decorator.
The #Glenn Maynard's answer is the good one.
But I would like to point at a problem in the OP's question (I can't add a comment to his question since I havn't enough reputation yet):
The following test is raising an error when the instance hasn't a __dict__ variable:
if self.name not in instance.__dict__:
So, here is an a generic solution that tries to acces to the __dict__ variable first (which is the default anyway) and, if it fails, use getattr and setattr:
class WorksWithDictAndSlotsDescriptor:
def __init__(self, attr_name):
self.attr_name = attr_name
def __get__(self, instance, owner):
try:
return instance.__dict__[self.attr_name]
except AttributeError:
return getattr(instance, self.attr_name)
def __set__(self, instance, value):
try:
instance.__dict__[self.attr_name] = value
except AttributeError:
setattr(instance, self.attr_name, value)
(Works only if the attr_name is not the same as the real instance variable's name, or you will have a RecursionError as pointed to in the accepted answer)
(Won't work as expected if there is both __slots__ AND __dict__)
Hope this helps.