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How to access superclass class attributes in Python?

Take a look at the following code:

class A(object):
    defaults = {'a': 1}

    def __getattr__(self, name):
        print('A.__getattr__')
        return self.get_default(name)

    @classmethod
    def get_default(cls, name):
        # some debug output
        print('A.get_default({}) - {}'.format(name, cls))
        try:
            print(super(cls, cls).defaults) # as expected
        except AttributeError: #except for the base object class, of course
            pass

        # the actual function body
        try:
            return cls.defaults[name]
        except KeyError:
            return super(cls, cls).get_default(name) # infinite recursion
            #return cls.__mro__[1].get_default(name) # this works, though

class B(A):
    defaults = {'b': 2}

class C(B):
    defaults = {'c': 3}


c = C()
print('c.a =', c.a)

I have a class hierarchy, each with its own dictionary containing some default values. If the class instance does not have a specific attribute, you should return the default value instead. If the default value for the attribute is not contained in the current class dictionary defaults, look for the superclass dictionary defaults.

I am trying to implement this using a recursive class method get_default. Unfortunately, the program is stuck in infinite recursion. My understanding is super()clearly absent. Having accessed __mro__, I can make it work properly, but I'm not sure if this is the right solution.

, - , . , ?

edit: __getattr__ self.base. None, . . , __getattribute__. ?

edit 2: , . __mro__ (unutbu , ). - , . , .

class A(object):
    defaults = {'a': 1}

    def __init__(self, name, base=None):
        self.name = name
        self.base = base

    def __repr__(self):
        return self.name

    def __getattr__(self, name):
        print(" '{}' attribute not present in '{}'".format(name, self))
        if self.base is not None:
            print("  getting '{}' from base ({})".format(name, self.base))
            return getattr(self.base, name)
        else:
            print("  base = None; returning default value")
            return self.get_default(name)

    def get_default(self, name):
        for cls in self.__class__.__mro__:
            try:
                return cls.defaults[name]
            except KeyError:
                pass
        raise KeyError

class B(A):
    defaults = {'b': 2}

class C(B):
    defaults = {'c': 3}


c1 = C('c1')
c1.b = 55

print('c1.a = ...'); print('   ...', c1.a) # 1
print(); print('c1.b = ...'); print('   ...', c1.b) # 55
print(); print('c1.c = ...'); print('   ...', c1.c) # 3

c2 = C('c2', base=c1)
c2.c = 99

print(); print('c2.a = ...'); print('   ...', c2.a) # 1
print(); print('c2.b = ...'); print('   ...', c2.b) # 55
print(); print('c2.c = ...'); print('   ...', c2.c) # 99

:

c1.a = ...
 'a' attribute not present in 'c1'
  base = None; returning default value
   ... 1

c1.b = ...
   ... 55

c1.c = ...
 'c' attribute not present in 'c1'
  base = None; returning default value
   ... 3

c2.a = ...
 'a' attribute not present in 'c2'
  getting 'a' from base (c1)
 'a' attribute not present in 'c1'
  base = None; returning default value
   ... 1

c2.b = ...
 'b' attribute not present in 'c2'
  getting 'b' from base (c1)
   ... 55

c2.c = ...
   ... 99
+5
6

, , - , , . unutbu , __mro__ IMO (. ).

0

, :

, , .

defaults dict , ? .

class A:
    a = 1

class B(A):
    b = 2

class C(B):
    c = 3


c = C()
print('c.a =', c.a)

EDIT:

, , , __getattribute__ :

def __getattribute__(self, name):
    try:
        return object.__getattribute__(self.base, name)
    except AttributeError:
        return object.__getattribute__(self, name)
+8

, super().

http://docs.python.org/library/functions.html#super

, ( ) super() Python . , C get_default, , C get_default . , .

, , A. :

class DefaultsClass(type):
    def __init__(cls, name, bases, dct):

        def get_default(self, name):
            # some debug output
            print('A.get_default(%s) - %s' % (name, cls))
            try:
                print(cls.defaults) # as expected
            except AttributeError: #except for the base object class, of course
                pass

            # the actual function body
            try:
                return cls.defaults[name]
            except KeyError:
                return super(cls, self).get_default(name) # cooperative superclass

        cls.get_default = get_default
        return super(DefaultsClass, cls).__init__(name, bases, dct)

class A(object):
    defaults = {'a': 1}
    __metaclass__ = DefaultsClass

    def __getattr__(self, name):
        return self.get_default(name)



class B(A):
    defaults = {'b': 2}

class C(B):
    defaults = {'c': 3}


c = C()
print('c.a =', c.a)
print('c.b =', c.b)
print('c.c =', c.c)

:

A.get_default(c) - <class '__main__.C'>
{'c': 3}
('c.c =', 3)
A.get_default(b) - <class '__main__.C'>
{'c': 3}
A.get_default(b) - <class '__main__.B'>
{'b': 2}
('c.b =', 2)
A.get_default(a) - <class '__main__.C'>
{'c': 3}
A.get_default(a) - <class '__main__.B'>
{'b': 2}
A.get_default(a) - <class '__main__.A'>
{'a': 1}
('c.a =', 1)

, Python , , , , .

+2

:

class A(object):
    def __init__(self,base=None):
        self.a=1
        if base is not None:
            self.set_base(base)
        super(A,self).__init__() 
    def set_base(self,base):
        for key in ('a b c'.split()):
            setattr(self,key,getattr(base,key))
class B(A): 
    def __init__(self,base=None):
        self.b=2
        super(B,self).__init__(base)        
class C(B): 
    def __init__(self,base=None):
        self.c=3
        super(C,self).__init__(base)

c1=C()
c1.b=55
print(c1.a)
print(c1.b)
print(c1.c)
# 1
# 55
# 3

c2=C(c1)
c2.c=99
print(c2.a)
print(c2.b)
print(c2.c)
# 1
# 55
# 99

c1.set_base(c2)
print(c1.a)
print(c1.b)
print(c1.c)
# 1
# 55
# 99
+1

"" "" .

>>> class A(object):
...     a = 1
... 
>>> class B(A):
...     b = 2
... 
>>> class C(B):
...     c = 3
... 
>>> a = A()
>>> b = B()
>>> c = C()
>>> 
>>> b.b = 23
>>> b.a
1
>>> b.b
23
>>> c.a
1
>>> c.b
2
>>> c.c
3
>>> c.c = 45
>>> c.c
45

. . usecase , , , , .;)

+1

name mangling.

defaults __defaults

, .

0

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