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python class instance variables and class variables
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Closed 4 years ago.
Sorry this is my first time asking a question, my formatting may be wrong. I am unsure about the syntax for calling functions from a class without creating an instance of the class. For the code:
class A_Class:
var = 10
def __init__(self):
self.num = 12
def print_12(self):
return 12
How come I am able to call
print(A_Class.var)
And have the console print out the value 10, but if I were to call
print(A_Class.num)
Then I get the error:
AttributeError: type object 'A_Class' has no attribute 'num'
And if I try to call
print(A_Class.print_12)
Then the console prints:
<function A_Class.print_12 at 0x039966F0>
And not the value 12
I am confused with how to call functions from classes.
var is a Class variable, while num is an instance variable, as an example:
class A_Class:
var = 10
def __init__(self):
self.num = 12
def print_12(self):
return 12
a = A_Class()
As a class variable, it belongs to the class and you are able to call it.
print(A_Class.var)
>> 10
As an instance variable, you have to instantiate it before you can access the values, this is why self (self has no special meaning and can be anything, but always the first argument for instance methods) is used and is initialized in the special __init__ method.
a = A_Class()
print(a.num)
>> 12
Finally, you want to print the returned value, and therefore will have to call it such as:
var = a.print_12()
print(var)
>> 12
As you were missing the parenthesis earlier, it is the instance method itself, and therefore did not return any value.
To expand on #BernardL excellent answer about the differences between a class variable and an instance variable, I wish to add this is from the PyTricks newsletter I get which may help answer your question about print(A_Class.print_12).
# #classmethod vs #staticmethod vs "plain" methods
# What's the difference?
class MyClass:
def method(self):
"""
Instance methods need a class instance and
can access the instance through `self`.
"""
return 'instance method called', self
#classmethod
def classmethod(cls):
"""
Class methods don't need a class instance.
They can't access the instance (self) but
they have access to the class itself via `cls`.
"""
return 'class method called', cls
#staticmethod
def staticmethod():
"""
Static methods don't have access to `cls` or `self`.
They work like regular functions but belong to
the class's namespace.
"""
return 'static method called'
# All methods types can be
# called on a class instance:
>>> obj = MyClass()
>>> obj.method()
('instance method called', <MyClass instance at 0x1019381b8>)
>>> obj.classmethod()
('class method called', <class MyClass at 0x101a2f4c8>)
>>> obj.staticmethod()
'static method called'
# Calling instance methods fails
# if we only have the class object:
>>> MyClass.classmethod()
('class method called', <class MyClass at 0x101a2f4c8>)
>>> MyClass.staticmethod()
'static method called'
>>> MyClass.method()
TypeError:
"unbound method method() must be called with MyClass "
"instance as first argument (got nothing instead)"
This is because what you define in the class root level is a static variable or method.
Also the methods within the class are objects themselfs, so if you print them this returns the object type and memory address as there is not way defined to print (or convert to string) the object (normally specified with __str__ otherwise).
I have a master class for a planet:
class Planet:
def __init__(self,name):
self.name = name
(...)
def destroy(self):
(...)
I also have a few classes that inherit from Planet and I want to make one of them unable to be destroyed (not to inherit the destroy function)
Example:
class Undestroyable(Planet):
def __init__(self,name):
super().__init__(name)
(...)
#Now it shouldn't have the destroy(self) function
So when this is run,
Undestroyable('This Planet').destroy()
it should produce an error like:
AttributeError: Undestroyable has no attribute 'destroy'
The mixin approach in other answers is nice, and probably better for most cases. But nevertheless, it spoils part of the fun - maybe obliging you to have separate planet-hierarchies - like having to live with two abstract classes each ancestor of "destroyable" and "non-destroyable".
First approach: descriptor decorator
But Python has a powerful mechanism, called the "descriptor protocol", which is used to retrieve any attribute from a class or instance - it is even used to ordinarily retrieve methods from instances - so, it is possible to customize the method retrieval in a way it checks if it "should belong" to that class, and raise attribute error otherwise.
The descriptor protocol mandates that whenever you try to get any attribute from an instance object in Python, Python will check if the attribute exists in that object's class, and if so, if the attribute itself has a method named __get__. If it has, __get__ is called (with the instance and class where it is defined as parameters) - and whatever it returns is the attribute. Python uses this to implement methods: functions in Python 3 have a __get__ method that when called, will return another callable object that, in turn, when called will insert the self parameter in a call to the original function.
So, it is possible to create a class whose __get__ method will decide whether to return a function as a bound method or not depending on the outer class been marked as so - for example, it could check an specific flag non_destrutible. This could be done by using a decorator to wrap the method with this descriptor functionality
class Muteable:
def __init__(self, flag_attr):
self.flag_attr = flag_attr
def __call__(self, func):
"""Called when the decorator is applied"""
self.func = func
return self
def __get__(self, instance, owner):
if instance and getattr(instance, self.flag_attr, False):
raise AttributeError('Objects of type {0} have no {1} method'.format(instance.__class__.__name__, self.func.__name__))
return self.func.__get__(instance, owner)
class Planet:
def __init__(self, name=""):
pass
#Muteable("undestroyable")
def destroy(self):
print("Destroyed")
class BorgWorld(Planet):
undestroyable = True
And on the interactive prompt:
In [110]: Planet().destroy()
Destroyed
In [111]: BorgWorld().destroy()
...
AttributeError: Objects of type BorgWorld have no destroy method
In [112]: BorgWorld().destroy
AttributeError: Objects of type BorgWorld have no destroy method
Perceive that unlike simply overriding the method, this approach raises the error when the attribute is retrieved - and will even make hasattr work:
In [113]: hasattr(BorgWorld(), "destroy")
Out[113]: False
Although, it won't work if one tries to retrieve the method directly from the class, instead of from an instance - in that case the instance parameter to __get__ is set to None, and we can't say from which class it was retrieved - just the owner class, where it was declared.
In [114]: BorgWorld.destroy
Out[114]: <function __main__.Planet.destroy>
Second approach: __delattr__ on the metaclass:
While writting the above, it occurred me that Pythn does have the __delattr__ special method. If the Planet class itself implements __delattr__ and we'd try to delete the destroy method on specifc derived classes, it wuld nt work: __delattr__ gards the attribute deletion of attributes in instances - and if you'd try to del the "destroy" method in an instance, it would fail anyway, since the method is in the class.
However, in Python, the class itself is an instance - of its "metaclass". That is usually type . A proper __delattr__ on the metaclass of "Planet" could make possible the "disinheitance" of the "destroy" method by issuing a `del UndestructiblePlanet.destroy" after class creation.
Again, we use the descriptor protocol to have a proper "deleted method on the subclass":
class Deleted:
def __init__(self, cls, name):
self.cls = cls.__name__
self.name = name
def __get__(self, instance, owner):
raise AttributeError("Objects of type '{0}' have no '{1}' method".format(self.cls, self.name))
class Deletable(type):
def __delattr__(cls, attr):
print("deleting from", cls)
setattr(cls, attr, Deleted(cls, attr))
class Planet(metaclass=Deletable):
def __init__(self, name=""):
pass
def destroy(self):
print("Destroyed")
class BorgWorld(Planet):
pass
del BorgWorld.destroy
And with this method, even trying to retrieve or check for the method existense on the class itself will work:
In [129]: BorgWorld.destroy
...
AttributeError: Objects of type 'BorgWorld' have no 'destroy' method
In [130]: hasattr(BorgWorld, "destroy")
Out[130]: False
metaclass with a custom __prepare__ method.
Since metaclasses allow one to customize the object that contains the class namespace, it is possible to have an object that responds to a del statement within the class body, adding a Deleted descriptor.
For the user (programmer) using this metaclass, it is almost the samething, but for the del statement been allowed into the class body itself:
class Deleted:
def __init__(self, name):
self.name = name
def __get__(self, instance, owner):
raise AttributeError("No '{0}' method on class '{1}'".format(self.name, owner.__name__))
class Deletable(type):
def __prepare__(mcls,arg):
class D(dict):
def __delitem__(self, attr):
self[attr] = Deleted(attr)
return D()
class Planet(metaclass=Deletable):
def destroy(self):
print("destroyed")
class BorgPlanet(Planet):
del destroy
(The 'deleted' descriptor is the correct form to mark a method as 'deleted' - in this method, though, it can't know the class name at class creation time)
As a class decorator:
And given the "deleted" descriptor, one could simply inform the methods to be removed as a class decorator - there is no need for a metaclass in this case:
class Deleted:
def __init__(self, cls, name):
self.cls = cls.__name__
self.name = name
def __get__(self, instance, owner):
raise AttributeError("Objects of type '{0}' have no '{1}' method".format(self.cls, self.name))
def mute(*methods):
def decorator(cls):
for method in methods:
setattr(cls, method, Deleted(cls, method))
return cls
return decorator
class Planet:
def destroy(self):
print("destroyed")
#mute('destroy')
class BorgPlanet(Planet):
pass
Modifying the __getattribute__ mechanism:
For sake of completeness - what really makes Python reach methods and attributes on the super-class is what happens inside the __getattribute__ call. n the object version of __getattribute__ is where the algorithm with the priorities for "data-descriptor, instance, class, chain of base-classes, ..." for attribute retrieval is encoded.
So, changing that for the class is an easy an unique point to get a "legitimate" attribute error, without need for the "non-existent" descritor used on the previous methods.
The problem is that object's __getattribute__ does not make use of type's one to search the attribute in the class - if it did so, just implementing the __getattribute__ on the metaclass would suffice. One have to do that on the instance to avoid instance lookp of an method, and on the metaclass to avoid metaclass look-up. A metaclass can, of course, inject the needed code:
def blocker_getattribute(target, attr, attr_base):
try:
muted = attr_base.__getattribute__(target, '__muted__')
except AttributeError:
muted = []
if attr in muted:
raise AttributeError("object {} has no attribute '{}'".format(target, attr))
return attr_base.__getattribute__(target, attr)
def instance_getattribute(self, attr):
return blocker_getattribute(self, attr, object)
class M(type):
def __init__(cls, name, bases, namespace):
cls.__getattribute__ = instance_getattribute
def __getattribute__(cls, attr):
return blocker_getattribute(cls, attr, type)
class Planet(metaclass=M):
def destroy(self):
print("destroyed")
class BorgPlanet(Planet):
__muted__=['destroy'] # or use a decorator to set this! :-)
pass
If Undestroyable is a unique (or at least unusual) case, it's probably easiest to just redefine destroy():
class Undestroyable(Planet):
# ...
def destroy(self):
cls_name = self.__class__.__name__
raise AttributeError("%s has no attribute 'destroy'" % cls_name)
From the point of view of the user of the class, this will behave as though Undestroyable.destroy() doesn't exist … unless they go poking around with hasattr(Undestroyable, 'destroy'), which is always a possibility.
If it happens more often that you want subclasses to inherit some properties and not others, the mixin approach in chepner's answer is likely to be more maintainable. You can improve it further by making Destructible an abstract base class:
from abc import abstractmethod, ABCMeta
class Destructible(metaclass=ABCMeta):
#abstractmethod
def destroy(self):
pass
class BasePlanet:
# ...
pass
class Planet(BasePlanet, Destructible):
def destroy(self):
# ...
pass
class IndestructiblePlanet(BasePlanet):
# ...
pass
This has the advantage that if you try to instantiate the abstract class Destructible, you'll get an error pointing you at the problem:
>>> Destructible()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Destructible with abstract methods destroy
… similarly if you inherit from Destructible but forget to define destroy():
class InscrutablePlanet(BasePlanet, Destructible):
pass
>>> InscrutablePlanet()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class InscrutablePlanet with abstract methods destroy
Rather than remove an attribute that is inherited, only inherit destroy in the subclasses where it is applicable, via a mix-in class. This preserves the correct "is-a" semantics of inheritance.
class Destructible(object):
def destroy(self):
pass
class BasePlanet(object):
...
class Planet(BasePlanet, Destructible):
...
class IndestructiblePlanet(BasePlanet): # Does *not* inherit from Destructible
...
You can provide suitable definitions for destroy in any of Destructible, Planet, or any class that inherits from Planet.
Metaclasses and descriptor protocols are fun, but perhaps overkill. Sometimes, for raw functionality, you can't beat good ole' __slots__.
class Planet(object):
def __init__(self, name):
self.name = name
def destroy(self):
print("Boom! %s is toast!\n" % self.name)
class Undestroyable(Planet):
__slots__ = ['destroy']
def __init__(self,name):
super().__init__(name)
print()
x = Planet('Pluto') # Small, easy to destroy
y = Undestroyable('Jupiter') # Too big to fail
x.destroy()
y.destroy()
Boom! Pluto is toast!
Traceback (most recent call last):
File "planets.py", line 95, in <module>
y.destroy()
AttributeError: destroy
You cannot inherit only a portion of a class. Its all or nothing.
What you can do is to put the destroy function in a second level of the class, such you have the Planet-class without the destry-function, and then you make a DestroyablePlanet-Class where you add the destroy-function, which all the destroyable planets use.
Or you can put a flag in the construct of the Planet-Class which determines if the destroy function will be able to succeed or not, which is then checked in the destroy-function.
Python 3 doesn't allow you to reference a class inside its body (except in methods):
class A:
static_attribute = A()
def __init__(self):
...
This raises a NameError in the second line because 'A' is not defined.
Alternatives
I have quickly found one workaround:
class A:
#property
#classmethod
def static_property(cls):
return A()
def __init__(self):
...
Although this isn't exactly the same since it returns a different instance every time (you could prevent this by saving the instance to a static variable the first time).
Are there simpler and/or more elegant alternatives?
EDIT:
I have moved the question about the reasons for this restriction to a separate question
The expression A() can't be run until the class A has been defined. In your first block of code, the definition of A is not complete at the point you are trying to execute A().
Here is a simpler alternative:
class A:
def __init__(self):
...
A.static_attribute = A()
When you define a class, Python immediately executes the code within the definition. Note that's different than defining a function where Python compiles the code, but doesn't execute it.
That's why this will create an error:
class MyClass(object):
a = 1 / 0
But this won't:
def my_func():
a = 1 / 0
In the body of A's class definition, A is not yet defined, so you can't reference it until after it's been defined.
There are several ways you can accomplish what you're asking, but it's not clear to me why this would be useful in the first place, so if you can provide more details about your use case, it'll be easier to recommend which path to go down.
The simplest would be what khelwood posted:
class A(object):
pass
A.static_attribute = A()
Because this is modifying class creation, using a metaclass could be appropriate:
class MetaA(type):
def __new__(mcs, name, bases, attrs):
cls = super(MetaA, mcs).__new__(mcs, name, bases, attrs)
cls.static_attribute = cls()
return cls
class A(object):
__metaclass__ = MetaA
Or you could use descriptors to have the instance lazily created or if you wanted to customize access to it further:
class MyDescriptor(object):
def __get__(self, instance, owner):
owner.static_attribute = owner()
return owner.static_attribute
class A(object):
static_attribute = MyDescriptor()
Using the property decorator is a viable approach, but it would need to be done something like this:
class A:
_static_attribute = None
#property
def static_attribute(self):
if A._static_attribute is None:
A._static_attribute = A()
return A._static_attribute
def __init__(self):
pass
a = A()
print(a.static_attribute) # -> <__main__.A object at 0x004859D0>
b = A()
print(b.static_attribute) # -> <__main__.A object at 0x004859D0>
You can use a class decorator:
def set_static_attribute(cls):
cls.static_attribute = cls()
return cls
#set_static_attribute
class A:
pass
Now:
>>>> A.static_attribute
<__main__.A at 0x10713a0f0>
Applying the decorator on top of the class makes it more explicit than setting static_attribute after a potentially long class definition. The applied decorator "belongs" to the class definition. So if you move the class around in your source code you will more likely move it along than an extra setting of the attribute outside the class.
Why instantiating a class does not instantiate all its attributes ? I think a very simple example could explain my problem better:
class Example:
def example(self):
self.var=10
if __name__=='__main__':
E=Example()
# In this case, the attribute var is not instantiated
try:
attr=getattr(E,"var")
print(attr) # <-- it does not exist even if E instantiated Example class
except AttributeError:
print("Object not having this attribute") # <-- This is the output
Why the object E does not have all its attributes instantiated (namely the attribute var) ?
Unlike Java, in Python, the initiator function is called __init__; naming the method like the name of the class does not create a constructor.
So, when you instantiate an object of class Example, the method example is not called and your attribute doesn't exist. You'll have to call it explicitly, like this:
>>> e = Example()
>>> e.example()
>>> e.var
10
To have the attribute available to all objects at instantiation, modify your class and create a __init__ method, like this:
class Example:
def __init__(self):
self.var = 10
Now, it will work as you expect:
>>> e = Example()
>>> e.var
10
in your class if you need to initialize var you need to call it explicitly by calling the example method in the example class
but if if you write __init__ method it it automatically initialize the variable at the time of object creation
The init method (init for initialise) is called when the object is instantiated. Instantiation is done by (effectively) calling the
class.Here a new instance is created. Then its init method is called .it call the example method to initialize var in the exapme
class Example:
def __init__(self):
self.var=0 # or you can directly give self.var=10
self.example()
def example(self):
self.var=10
if __name__=='__main__':
E=Example()
# In this case, the attribute var is not instantiated
try:
attr=getattr(E,"var")
print(attr) # <-- it does not exist even if E instantiated Example class
except AttributeError:
print("Object not having this attribute")
other way of doing is
when you intalized class object call the method in it
E=Example()
E.example()#explicitly call example method in the class
You not called example method which sets var to self.
try
if __name__=='__main__':
E=Example()
E.example()
# In this case, the attribute var is not instantiated
try:
attr=getattr(E,"var")
print(attr)
except AttributeError:
print("Object not having this attribute")
So I have a .py file containing a class where its subclasses can be accessed as properties. All these subclasses are defined beforehand. I also need all the subclasses to have the same ability (having their own subclasses be accessible as properties). The biggest problem I've been facing is that I don't know how to access the current class within my implementation of __getattr__(), so that'd be a good place to start.
Here's some Python+Pseudocode with what I've tried so far. I'm pretty sure it won't work since __getattr__() seems to be only working with instances of a class. If that is case, sorry, I am not as familiar with OOP in Python as I would like.
class A(object):
def __getattr__(self, name):
subclasses = [c.__name__ for c in current_class.__subclasses__()]
if name in subclasses:
return name
raise AttributeError
If I've understood your question properly, you can do what you want by using a custom metaclass that adds a classmethod to its instances. Here's an example:
class SubclassAttributes(type):
def __getattr__(cls, name): # classmethod of instances
for subclass in cls.__subclasses__():
if subclass.__name__ == name:
return subclass
else:
raise TypeError('Class {!r} has no subclass '
'named {!r}'.format(cls.__name__, name))
class Base(object):
__metaclass__ = SubclassAttributes # Python 2 metaclass syntax
#class Base(object, metaclass=SubclassAttributes): # Python 3 metaclass syntax
# """ nothing to see here """
class Derived1(Base): pass
class Derived2(Base): pass
print(Base.Derived1) # -> <class '__main__.Derived1'>
print(Base.Derived2) # -> <class '__main__.Derived2'>
print(Base.Derived3) # -> TypeError: Class 'Base' has no subclass named 'Derived3'
For something that works in both Python 2 and 3, define the class as shown below. Derives Base from a class that has SubclassAttributes as its metaclass. The is similar to what the six module's with_metaclass() function does:
class Base(type.__new__(type('TemporaryMeta', (SubclassAttributes,), {}),
'TemporaryClass', (), {})): pass
class A(object):
def __getattr__(self, key):
for subclass in self.__class__.__subclasses__():
if (subclass.__name__ == key):
return subclass
raise AttributeError, key
Out of curiosity, what is this designed to be used for?
>>> class A(object):
... pass
...
>>> foo = A()
>>> foo.__class__
<class '__main__.A'>