如何从 Python 中的子类调用父类的方法？

Question

When creating a simple object hierarchy in Python, I'd like to be able to invoke methods of the parent class from a derived class. In Perl and Java, there is a keyword for this ( super ). In Perl, I might do this:

package Foo;

sub frotz {
    return "Bamf";
}

package Bar;
@ISA = qw(Foo);

sub frotz {
   my $str = SUPER::frotz();
   return uc($str);
}

In Python, it appears that I have to name the parent class explicitly from the child. In the example above, I'd have to do something like Foo::frotz() .

This doesn't seem right since this behavior makes it hard to make deep hierarchies. If children need to know what class defined an inherited method, then all sorts of information pain is created.

Is this an actual limitation in python, a gap in my understanding or both?

Answer 1

Use the super() function:

class Foo(Bar):
    def baz(self, arg):
        return super().baz(arg)

For Python < 3, you must explicitly opt in to using new-style classes and use:

class Foo(Bar):
    def baz(self, arg):
        return super(Foo, self).baz(arg)

Answer 2

Python also has super as well:

super (type[, object-or-type])

Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by getattr() except that the type itself is skipped.

Example:

class A(object):     # deriving from 'object' declares A as a 'new-style-class'
    def foo(self):
        print "foo"

class B(A):
    def foo(self):
        super(B, self).foo()   # calls 'A.foo()'

myB = B()
myB.foo()

Answer 3

ImmediateParentClass.frotz(self)

will be just fine, whether the immediate parent class defined frotz itself or inherited it. super is only needed for proper support of multiple inheritance (and then it only works if every class uses it properly). In general, AnyClass.whatever is going to look up whatever in AnyClass 's ancestors if AnyClass doesn't define/override it, and this holds true for "child class calling parent's method" as for any other occurrence!

Answer 4

Python 3 has a different and simpler syntax for calling parent method.

If Foo class inherits from Bar , then from Bar.__init__ can be invoked from Foo via super().__init__() :

class Foo(Bar):

    def __init__(self, *args, **kwargs):
        # invoke Bar.__init__
        super().__init__(*args, **kwargs)

Answer 5

Many answers have explained how to call a method from the parent which has been overridden in the child.

However

"how do you call a parent class's method from child class?"

could also just mean:

"how do you call inherited methods?"

You can call methods inherited from a parent class just as if they were methods of the child class, as long as they haven't been overwritten.

eg in python 3:

class A():
  def bar(self, string):
    print("Hi, I'm bar, inherited from A"+string)

class B(A):
  def baz(self):
    self.bar(" - called by baz in B")

B().baz() # prints out "Hi, I'm bar, inherited from A - called by baz in B"

yes, this may be fairly obvious, but I feel that without pointing this out people may leave this thread with the impression you have to jump through ridiculous hoops just to access inherited methods in python. Especially as this question rates highly in searches for "how to access a parent class's method in Python", and the OP is written from the perspective of someone new to python.

I found: https://docs.python.org/3/tutorial/classes.html#inheritance to be useful in understanding how you access inherited methods.

Answer 6

Here is an example of using super() :

#New-style classes inherit from object, or from another new-style class
class Dog(object):

    name = ''
    moves = []

    def __init__(self, name):
        self.name = name

    def moves_setup(self):
        self.moves.append('walk')
        self.moves.append('run')

    def get_moves(self):
        return self.moves

class Superdog(Dog):

    #Let's try to append new fly ability to our Superdog
    def moves_setup(self):
        #Set default moves by calling method of parent class
        super(Superdog, self).moves_setup()
        self.moves.append('fly')

dog = Superdog('Freddy')
print dog.name # Freddy
dog.moves_setup()
print dog.get_moves() # ['walk', 'run', 'fly']. 
#As you can see our Superdog has all moves defined in the base Dog class

Answer 7

There's a super() in Python too. It's a bit wonky, because of Python's old- and new-style classes, but is quite commonly used eg in constructors:

class Foo(Bar):
    def __init__(self):
        super(Foo, self).__init__()
        self.baz = 5

Answer 8

I would recommend using CLASS.__bases__ something like this

class A:
   def __init__(self):
        print "I am Class %s"%self.__class__.__name__
        for parentClass in self.__class__.__bases__:
              print "   I am inherited from:",parentClass.__name__
              #parentClass.foo(self) <- call parents function with self as first param
class B(A):pass
class C(B):pass
a,b,c = A(),B(),C()

Answer 9

If you don't know how many arguments you might get, and want to pass them all through to the child as well:

class Foo(bar)
    def baz(self, arg, *args, **kwargs):
        # ... Do your thing
        return super(Foo, self).baz(arg, *args, **kwargs)

(From: Python - Cleanest way to override __init__ where an optional kwarg must be used after the super() call? )

Answer 10

There is a super() in python also.

Example for how a super class method is called from a sub class method

class Dog(object):
    name = ''
    moves = []

    def __init__(self, name):
        self.name = name

    def moves_setup(self,x):
        self.moves.append('walk')
        self.moves.append('run')
        self.moves.append(x)
    def get_moves(self):
        return self.moves

class Superdog(Dog):

    #Let's try to append new fly ability to our Superdog
    def moves_setup(self):
        #Set default moves by calling method of parent class
        super().moves_setup("hello world")
        self.moves.append('fly')
dog = Superdog('Freddy')
print (dog.name)
dog.moves_setup()
print (dog.get_moves()) 

This example is similar to the one explained above.However there is one difference that super doesn't have any arguments passed to it.This above code is executable in python 3.4 version.

Answer 11

In this example cafec_param is a base class (parent class) and abc is a child class. abc calls the AWC method in the base class.

class cafec_param:

    def __init__(self,precip,pe,awc,nmonths):

        self.precip = precip
        self.pe = pe
        self.awc = awc
        self.nmonths = nmonths

    def AWC(self):

        if self.awc<254:
            Ss = self.awc
            Su = 0
            self.Ss=Ss
        else:
            Ss = 254; Su = self.awc-254
            self.Ss=Ss + Su   
        AWC = Ss + Su
        return self.Ss


    def test(self):
        return self.Ss
        #return self.Ss*4

class abc(cafec_param):
    def rr(self):
        return self.AWC()


ee=cafec_param('re',34,56,2)
dd=abc('re',34,56,2)
print(dd.rr())
print(ee.AWC())
print(ee.test())

Output

56

56

56

Answer 12

In Python 2, I didn't have a lot luck with super(). I used the answer from jimifiki on this SO thread how to refer to a parent method in python? . Then, I added my own little twist to it, which I think is an improvement in usability (Especially if you have long class names).

Define the base class in one module:

 # myA.py

class A():     
    def foo( self ):
        print "foo"

Then import the class into another modules as parent :

# myB.py

from myA import A as parent

class B( parent ):
    def foo( self ):
        parent.foo( self )   # calls 'A.foo()'

Answer 13

class department:
    campus_name="attock"
    def printer(self):
        print(self.campus_name)

class CS_dept(department):
    def overr_CS(self):
        department.printer(self)
        print("i am child class1")

c=CS_dept()
c.overr_CS()

Answer 14

If you want to call the method of any class, you can simply call Class.method on any instance of the class. If your inheritance is relatively clean, this will work on instances of a child class too:

class Foo:
    def __init__(self, var):
        self.var = var
    
    def baz(self):
        return self.var

class Bar(Foo):
    pass

bar = Bar(1)
assert Foo.baz(bar) == 1

Answer 15

class a(object):
    def my_hello(self):
        print "hello ravi"

class b(a):
    def my_hello(self):
    super(b,self).my_hello()
    print "hi"

obj = b()
obj.my_hello()

Answer 16

这是一个更抽象的方法：

super(self.__class__,self).baz(arg)