I was just trying something with multiple inheritance in python. I come up with this
class ParentOne:
def foo(self):
print("ParentOne foo is called")
class ParentTwo:
def foo(self):
print("ParentTwo foo is called")
class Child(ParentOne, ParentTwo):
# how is this working
def call_parent_two_foo(self):
super(ParentOne, self).foo()
# This does not work
def call_parent_foo(self):
super(ParentTwo, self).foo()
def call_super_foo(self):
super(Child, self).foo()
def foo(self):
print("Child foo is called")
if __name__ == "__main__":
child = Child()
child.foo()
child.call_super_foo()
child.call_parent_two_foo()
# child.call_parent_foo() #This gives the below error
# super(ParentTwo, self).foo()
# AttributeError: 'super' object has no attribute 'foo'
and it gives the following output
Child foo is called
ParentOne foo is called
ParentTwo foo is called
I am getting confused as to how calling of super(ParentOne, self).foo() is evaluated in this case. As per my understanding ParentOne class does not have any idea of the methods and attributes of ParentTwo class. How does super works in case of multiple inheritance
Python constructs a method resolution order (MRO) when it builds a class. The MRO is always linear. If python cannot create a linear MRO, then a ValueError will be raised. In this case, your MRO probably looks like:
Child -> ParentOne -> ParentTwo -> object
Now when python see's a super(cls, self), it basically looks at self and figures out the MRO. It then uses cls to determine where we are currently at in the MRO and finally it returns an object which delegates to the next class in the MRO. So, in this case, a super(Child, self) call would return an object that delegates to ParentOne. A super(ParentOne, self) class would return an object that delegates to ParentTwo. Finally a super(ParentTwo, self) call would delegate to object. In other words, you can think of super as a fancier version of the following code:
def kinda_super(cls, self):
mro = inspect.getmro(type(self))
idx = mro.index(cls)
return Delegate(mro[idx + 1]) # for a suitably defined `Delegate`
Note that since super(ParentTwo, self) returns a "Delegate" to object, we can see why you're getting an AttributeError when you try super(ParentTwo, self).foo() -- Specifically the reason is because object has no foo method.
class X1:
def run(self):
print("x1")
class X2:
def run(self):
print("x2")
class X3:
def run(self):
print("x3")
class X2:
def run(self):
print("x2")
class Y(X1, X2, X3):
def run(self):
print("y")
Given an instance:
y = Y()
To call base class function:
super(Y,y).run()
super(X1,y).run()
super(X2,y).run()
y.run()
Output
x1
x2
x3
y
Similarity,
super(Y, y).run()
for cls in y.__class__.__bases__:
if(cls != X3):
super(cls,y).run()
y.run()
Output
x1
x2
x3
y
You may understand Child(ParentOne, ParentTwo) as two separate inheritances within a chain: Child(ParentOne(ParentTwo)). Actually, ParentOne doesn't inherit ParentTwo, they are two separate classes, but the method super works like there's a chain of inheritances (in case of multiple inheritance only). I like this example to understand better what's going on (for Python 3.x):
class P:
def m(self):
print("P")
class A(P):
def m(self):
super().m() # -> B, if we inherit like C(A, B)
print("A")
class B(P):
def m(self):
super().m() # -> P, if we inherit like C(A, B)
print("B")
class C(A, B):
def m(self):
super().m() # -> A
print("C")
A.m(self)
B.m(self)
c = C()
c.m()
It also considers a case if two parents inherit one base class. The script above prints:
P
B
A
C
P
B
A
P
B
Related
Here is a simplified code of my main code illustrating the behaviour I obtain.
Suppose I have a main class (MAIN) and two classes (A,B) inheriting from it. This main class has a method which is overwriten by A but not by B, which means that B inherits the method from main.
Then I have a class D which inherits from A and from B, and has a method which calls the aforementioned method. From what I have understood in the way multiple inheritance work, if I define D as class D(A,B) then if A and B have a shared method, calling D.method() will call A.method, and vice-versa (i.e if class D(B,A) then B.method is called. The following code exemplifies this text.
class MAIN(object):
def __init__(self):
pass
def print(self):
print('HELLO MAIN')
class A(MAIN):
def __init__(self):
pass
def print(self):
print('HELLO A')
class B(MAIN):
def __init__(self):
pass
class C(A,B):
def __init__(self):
pass
def Cprint(self):
self.print()
c = C()
c.Cprint()
class C(B,A):
def __init__(self):
pass
def Cprint(self):
self.print()
c = C()
c.Cprint()
However this code always print 'HELLO A', i.e even in the case class C(B,A) I don't get a HELLO MAIN as I would expect. What is happening here? Thanks so much in advance
The mro is (C, A, B, MAIN) with class C(A, B) and (C, B, A, MAIN) with class C(B, A). In both cases, A is before MAIN. B doesn't define .print, so it doesn't matter.
The method uplooks works like this: (pseudo code)
def find_attribute(obj, name):
if name in obj.__dict__:
return obj.__dict__[name]
mro = type(obj).__mro__
for cls in mro:
if name in cls.__dict__:
return cls.__dict__[name] # (Here a bit more magic for descriptors happens)
raise AttributeError(name)
For the classes this is what their __dict__ look like:
MAIN.__dict__ = {"print": <method MAIN.print>}
A.__dict__ = {"print": <method A.print>}
B.__dict__ = {}
C.__dict__ = {"Cprint": <method C.Cprint>}
As you can see, B does not have a print defined, so in mro=(C, B, A, MAIN) the first print that does get found is in A.
You are inheriting the Class A everywhere and class A overrrides Main functions print() thats why you dont get the "HELLO MAIN"
class C(B):
def __init__(self):
pass
def Cprint(self):
self.print()
inherit only B class which does not overrides Main class print function then you will get the HELLO MAIN output
(How) Is it possible in Python to treat an instance of class B exactly as an instance of class A, where A is a parent of B (like up-casting in compiled languages)?
Say we have the following:
class A:
def __init__(self, prop=None):
self.prop = prop
def f(self):
return 'A.f'
def g(self):
return 'A.g', self.f()
class B(A):
def f(self):
return 'B.f'
def g(self):
return 'B.g', self.f()
Calling A().g() produces ('A.g', 'A.f'), whereas B().g() produces ('B.g', 'B.f'). Calling super(B, B()).g() produces ('A.g', 'B.f'), which is different from compiled languages, so I cannot use super. Instead, I need a function that changes the type from which an instance's methods are resolved, but preserves (a reference to) the original state. In short, I'm looking for a way to do this:
b = B(object())
a = upcast(b, A)
a.prop = object()
assert isinstance(a, A)
assert a.g() == ('A.g', 'A.f')
assert a.prop is b.prop
The closest I could get is
a = copy.copy(b)
a.__class__ = A
a.__dict__ = b.__dict__
(assuming A/B are "nice" "heap" classes), but this makes unnecessary copies of all objects in the __dict__ before I discard them. Is there a better way to do this?
I am working through the O Reilly Python Cookbook and have been struggling with the below code. It is to with calling a method on a parent class using super():
class Proxy:
def __init__(self, obj):
self._obj = obj
# Delegate attribute lookup to internal obj
def __getattr__(self, name):
return getattr(self._obj, name)
# Delegate attribute assignment
def __setattr__(self, name, value):
if name.startswith('_'):
super().__setattr__(name, value) # Call original __setattr__
else:
setattr(self._obj, name, value)
if __name__ == '__main__':
class A:
def __init__(self, x):
self.x = x
def spam(self):
print('A.spam')
a = A(42)
p = Proxy(a)
print(p.x)
print(p.spam())
p.x = 37
print('Should be 37:', p.x)
print('Should be 37:', a.x)
The book states:
In this code the implementation of __setatrr__() includes a name
check. If the name starts with an underscore it invokes the original
implementation of __setattr__() using super(). Otherwise, it delegates
to the internally held object self._obj.
I am confused. How does super() work then if there is no explicit base class listed?
What exactly then is super() referring to?
There is always a base class; with none explicitly mentioned, Proxy inherits directly from object.
Each class defines a method-resolution order, determined recursively by its base class(es) and its ancestors. When super() gets called, it resolves to a "proxy" of the next class in the MRO of self, whether or not that class appears in the MRO of the class you are currently defining.
Consider the following classes:
class A:
def foo(self):
print("A.foo")
class B(A):
def foo(self):
super().foo()
print("B.foo")
class C(A):
def foo(self):
super().foo()
print("C.foo")
class D(C):
def foo(self):
super().foo()
print("D.foo")
class E(B,D):
def foo(self):
super().foo()
print("E.foo")
e = E()
The MRO of E is [E, B, D, C, A, object]. When you call e.foo(), you start a chain of calls in MRO order. In particular, the call to super in B.foo does not invoke A.foo, but D.foo, a method in a class B knows nothing about, as D is not an ancestor of B. But both B and D are ancestors of E, which is what matters.
I have 2 classes: A and B, both of them instantiated by __new__ with different set of arguments (like a for A, and foo, bar for B). Now I want to implement class C, inherited from A and B, and instantiate it with 3 args: a, foo, bar, passing correspondent arguments to super classes __new__, but things goes wrong from here.
If we have no arguments I just call super(C, cls).__new__() and object of class C successfully created (it calls both A.__new__() and B.__new__() and combines it somehow). But how to do it 'by hands'? So I want to pass a to A.__new__, foo, bar to B.__new__ and combine somehow returned instances (is this right way to get object of class C at the end?).
Anyway I can't do both.
Fist - calling A.__new__ raises incorrect number of arguments exception in o = super(A, cls).__new__(cls) in A.__new__() (but A can be instantiated as standalone)
Second - I have no idea how to combine even successfully instantiated object of classes A and B into object of class C.
So could please someone to explain what is going on here?
class A(object):
def __new__(cls, a):
o = super(A, cls).__new__(cls)
setattr(o, 'a', a)
return o
class B(object):
def __new__(cls, foo, bar):
o = super(B, cls).__new__(cls)
setattr(o, 'foo', foo)
setattr(o, 'bar', bar)
return o
print A(1) # ok, <__main__.A object at 0x00000000022F1630>
print B(2,3) # ok, <__main__.B object at 0x00000000022F1630>
class C(A,B):
def __new__(cls, a, foo, bar):
o1 = A.__new__(cls, a) #fail - exception while calling super.new in A
o2 = B.__new__(cls, foo, bar) #fail - exception while calling super.new in A
# return # What? How to combine o1 o2 even if they are created succesfuly?
# # return super(C, cls).__new__(cls, ?????)
print C(1,2,3)
The method __new__ is what creates your instance, you should not call super(...).__new__ multiple times as it would create multiple instances.
What you want to do it use __init__ which initializes your already created instance.
class A(object):
def __init__(self, a):
self.a = a
class B(object):
def __init__(self, foo, bar):
self.foo = foo
self.bar = bar
class C(A, B):
def __init__(self, a, foo, bar):
A.__init__(self, a)
B.__init__(self, foo, bar)
In particular, I want to point out that it is not true that on multiple inheritance Python will call both A.__new__ andB.__new__ and "combine somehow". Have a look at this code
class A(object):
def __new__(*args):
print('A.__new__ was called')
return type(*args) # This is what ultimately creates every object in Python
class B(object):
def __new__(*args):
print('B.__new__ was called')
return type(*args)
# As expected the following is printed when instances are created
a = A() # Prints 'A.__new__ was called'
b = B() # Prints 'B.__new__ was called'
class C(A, B):
pass
c = C() # Prints 'A.__new__ was called'
So we observe that B.__new__ was never called. On multiple inheritance, Python will inherit the method from the left-most class that has this method. In this case, C inherited A.__new__.
I have a class that is a super-class to many other classes. I would like to know (in the __init__() of my super-class) if the subclass has overridden a specific method.
I tried to accomplish this with a class method, but the results were wrong:
class Super:
def __init__(self):
if self.method == Super.method:
print 'same'
else:
print 'different'
#classmethod
def method(cls):
pass
class Sub1(Super):
def method(self):
print 'hi'
class Sub2(Super):
pass
Super() # should be same
Sub1() # should be different
Sub2() # should be same
>>> same
>>> different
>>> different
Is there any way for a super-class to know if a sub-class has overridden a method?
It seems simplest and sufficient to do this by comparing the common subset of the dictionaries of an instance and the base class itself, e.g.:
def detect_overridden(cls, obj):
common = cls.__dict__.keys() & obj.__class__.__dict__.keys()
diff = [m for m in common if cls.__dict__[m] != obj.__class__.__dict__[m]]
print(diff)
def f1(self):
pass
class Foo:
def __init__(self):
detect_overridden(Foo, self)
def method1(self):
print("Hello foo")
method2=f1
class Bar(Foo):
def method1(self):
print("Hello bar")
method2=f1 # This is pointless but not an override
# def method2(self):
# pass
b=Bar()
f=Foo()
Runs and gives:
['method1']
[]
If you want to check for an overridden instance method in Python 3, you can do this using the type of self:
class Base:
def __init__(self):
if type(self).method == Base.method:
print('same')
else:
print('different')
def method(self):
print('Hello from Base')
class Sub1(Base):
def method(self):
print('Hello from Sub1')
class Sub2(Base):
pass
Now Base() and Sub2() should both print "same" while Sub1() prints "different". The classmethod decorator causes the first parameter to be bound to the type of self, and since the type of a subclass is by definition different to its base class, the two class methods will compare as not equal. By making the method an instance method and using the type of self, you're comparing a plain function against another plain function, and assuming functions (or unbound methods in this case if you're using Python 2) compare equal to themselves (which they do in the C Python implementation), the desired behavior will be produced.
You can use your own decorator. But this is a trick and will only work on classes where you control the implementation.
def override(method):
method.is_overridden = True
return method
class Super:
def __init__(self):
if hasattr(self.method, 'is_overridden'):
print 'different'
else:
print 'same'
#classmethod
def method(cls):
pass
class Sub1(Super):
#override
def method(self):
print 'hi'
class Sub2(Super):
pass
Super() # should be same
Sub1() # should be different
Sub2() # should be same
>>> same
>>> different
>>> same
In reply to answer https://stackoverflow.com/a/9437273/1258307, since I don't have enough credits yet to comment on it, it will not work under python 3 unless you replace im_func with __func__ and will also not work in python 3.4(and most likely onward) since functions no longer have the __func__ attribute, only bound methods.
EDIT: Here's the solution to the original question(which worked on 2.7 and 3.4, and I assume all other version in between):
class Super:
def __init__(self):
if self.method.__code__ is Super.method.__code__:
print('same')
else:
print('different')
#classmethod
def method(cls):
pass
class Sub1(Super):
def method(self):
print('hi')
class Sub2(Super):
pass
Super() # should be same
Sub1() # should be different
Sub2() # should be same
And here's the output:
same
different
same
You can compare whatever is in the class's __dict__ with the function inside the method
you can retrieve from the object -
the "detect_overriden" functionbellow does that - the trick is to pass
the "parent class" for its name, just as one does in a call to "super" -
else it is not easy to retrieve attributes from the parentclass itself
instead of those of the subclass:
# -*- coding: utf-8 -*-
from types import FunctionType
def detect_overriden(cls, obj):
res = []
for key, value in cls.__dict__.items():
if isinstance(value, classmethod):
value = getattr(cls, key).im_func
if isinstance(value, (FunctionType, classmethod)):
meth = getattr(obj, key)
if not meth.im_func is value:
res.append(key)
return res
# Test and example
class A(object):
def __init__(self):
print detect_overriden(A, self)
def a(self): pass
#classmethod
def b(self): pass
def c(self): pass
class B(A):
def a(self): pass
##classmethod
def b(self): pass
edit changed code to work fine with classmethods as well:
if it detects a classmethod on the parent class, extracts the underlying function before proceeding.
--
Another way of doing this, without having to hard code the class name, would be to follow the instance's class ( self.__class__) method resolution order (given by the __mro__ attribute) and search for duplicates of the methods and attributes defined in each class along the inheritance chain.
I'm using the following method to determine if a given bound method is overridden or originates from the parent class
class A():
def bla(self):
print("Original")
class B(A):
def bla(self):
print("Overridden")
class C(A):
pass
def isOverriddenFunc(func):
obj = func.__self__
prntM = getattr(super(type(obj), obj), func.__name__)
return func.__func__ != prntM.__func__
b = B()
c = C()
b.bla()
c.bla()
print(isOverriddenFunc(b.bla))
print(isOverriddenFunc(c.bla))
Result:
Overridden
Original
True
False
Of course, for this to work, the method must be defined in the base class.
You can also check if something is overridden from its parents, without knowing any of the classes involved using super:
class A:
def fuzz(self):
pass
class B(A):
def fuzz(self):
super().fuzz()
class C(A):
pass
>>> b = B(); c = C()
>>> b.__class__.fuzz is super(b.__class__, b).fuzz.__func__
False
>>> c.__class__.fuzz is super(c.__class__, c).fuzz.__func__
True
See this question for some more nuggets of information.
A general function:
def overrides(instance, function_name):
return getattr(instance.__class__, function_name) is not getattr(super(instance.__class__, instance), function_name).__func__
>>> overrides(b, "fuzz")
True
>>> overrides(c, "fuzz")
False
You can check to see if the function has been overridden by seeing if the function handle points to the Super class function or not. The function handler in the subclass object points either to the Super class function or to an overridden function in the Subclass. For example:
class Test:
def myfunc1(self):
pass
def myfunc2(self):
pass
class TestSub(Test):
def myfunc1(self):
print('Hello World')
>>> test = TestSub()
>>> test.myfunc1.__func__ is Test.myfunc1
False
>>> test.myfunc2.__func__ is Test.myfunc2
True
If the function handle does not point to the function in the Super class, then it has been overridden.
Not sure if this is what you're looking for but it helped me when I was looking for a similar solution.
class A:
def fuzz(self):
pass
class B(A):
def fuzz(self):
super().fuzz()
assert 'super' in B.__dict__['fuzz'].__code__.co_names
The top-trending answer and several others use some form of Sub.method == Base.method. However, this comparison can return a false negative if Sub and Base do not share the same import syntax. For example, see discussion here explaining a scenario where issubclass(Sub, Base) -> False.
This subtlety is not apparent when running many of the minimal examples here, but can show up in a more complex code base. The more reliable approach is to compare the method defined in the Sub.__bases__ entry corresponding to Base because __bases__ is guaranteed to use the same import path as Sub
import inspect
def method_overridden(cls, base, method):
"""Determine if class overriddes the implementation of specific base class method
:param type cls: Subclass inheriting (and potentially overriding) the method
:param type base: Base class where the method is inherited from
:param str method: Name of the inherited method
:return bool: Whether ``cls.method != base.method`` regardless of import
syntax used to create the two classes
:raises NameError: If ``base`` is not in the MRO of ``cls``
:raises AttributeError: If ``base.method`` is undefined
"""
# Figure out which base class from the MRO to compare against
base_cls = None
for parent in inspect.getmro(cls):
if parent.__name__ == base.__name__:
base_cls = parent
break
if base_cls is None:
raise NameError(f'{base.__name__} is not in the MRO for {cls}')
# Compare the method implementations
return getattr(cls, method) != getattr(base_cls, method)