Inherit multiple methods with same name from multiple classes - python

I have two parent classes that share method names. I'd like to subclass them and reassign their methods under different names to the subclass. This is made complicated because these methods use other methods with shared names as well. Contrived but minimal example:
class Foo1():
def __init__(self):
super().__init__()
def print(self):
print(self.get())
def get(self):
return 1
class Foo2():
def __init__(self):
super().__init__()
def print(self):
print(self.get())
def get(self):
return 2
class Foo(Foo1, Foo2):
def __init__(self):
super().__init__()
self.print1 = super().print
self.print2 = super(Foo1, self).print
foo = Foo()
foo.print1()
>>> 1
foo.print2()
>>> 1
In the example above, foo.print1() prints 1 as expected, but foo.print2() prints 1 instead of 2. I would like to structure Foo such that it calls Foo2's print and get methods and thus prints 2.
I understand this is because the name "self" actually points to Foo(), and so the methods "print" and "get" are pointing to Foo1's methods due to Foo1 being first in the MRO.
For my use case, it doesn't even make sense for Foo() to have "print" and "get" methods, only "print1" and "print2". This makes me suspect that I'm going about it all wrong. Is there a better way to construct a subclass that correctly inherits these methods from Foo1 and Foo2?


Have your class wrap Foo1 and Foo2 instances rather than using multiple inheritance.
class Foo:
def __init__(self):
foo1 = Foo1()
foo2 = Foo2()
self.print1 = foo1.print
self.print2 = foo2.print
self.get1 = foo1.get
self.get2 = foo2.get


One option is to use composition, where your new class instance is just a wrapper around two instances of the original classes. Your methods will delegate to methods of the appropriate instances.
class Foo:
def __init__(self):
self.foo1 = Foo1()
self.foo2 = Foo2()
def print1(self):
return self.foo1.print()
def print2(self):
return self.foo2.print()
# etc
You might also be able to perform the composition at the class level, rather than the instance level. This will only work if the function signatures of Foo1 and Foo2 methods are compatible with the desired signature for the Foo methods.
class Foo:
print1 = Foo1.print
print2 = Foo2.print
get1 = Foo1.get
get2 = Foo2.get
In either case, you'll have to define Foo.__init__ to accept any necessary arguments for Foo1.__init__ and Foo2.__init__, and ensure they are passed to the correct one.

Related

Enumerate instance methods in Python

I would like to enumerate some instance methods inside a class. The operate function needs to use foo1, foo2,.. as Foo.FOO1, Foo.FOO2,.. .
class Machine:
def __init__(self):
self.operate()
def foo1(self):
pass
def foo2(self):
pass
..
class Foo(Enum):
FOO1 = Machine.foo1 # Machine is not defined
FOO2 = Machine.foo2 # Machine is not defined
..
def operate(self):
# use self.Foo.FOO1, self.Foo.FOO2,..
I do not know how to define the enum class.

The solution proposed by #giannisl9 is bugged, although it apparently works at first sight, a closer inspection reveals the Enum is broken:
from enum import Enum
class Machine:
def __init__(self):
class Foo(Enum):
FOO1 = self.foo1
self.foo = Foo
self.operate()
def foo1(self):
pass
def operate(self):
# breaks Enum contract, breaks syntax, breaks functionality...
self.foo.FOO1() # Enum member is NOT available! Method of class Machine bound in its place.
print(type(self.foo)) # {type}<class'enum.EnumMeta'> - Enum 'Foo'
print(type(self.foo.FOO1)) # {type} <class 'method'> - should be Enum member
print(type(self.foo.FOO1.name)) # {AttributeError}'function'object has no attribute 'name'
print(type(self.foo.FOO1.value)) # {AttributeError}'function'object has no attribute 'value'
Building on the answer by #Epic Programmer -since the original question only stated as requirement defining an Enum to run instance methods- given the application, organizing procedures in the __init__ or other methods, could suffice:
from inspect import ismethod
from inspect import isbuiltin
class Machine(object):
def operate(self):
for method in self.__dir__():
if ismethod(getattr(self, method)) \
and not isbuiltin(getattr(self, method)) \
and '__' not in method \
and 'operate' != method: # delete this to see a recursion
self.__getattribute__(method)() # after much filtering runs the method
def __init__(self):
self.operate()
def foo1(self):
print("drinks at bar1")
However, as I understand the question, it makes perfect sense the Enum should be internal to the class, since ontologically it pertains to encode/abbreviate a set of states proper to all instances of the class. That makes lots of sense!
It doesn't make much sense declaring it inside the __init__ as a self instance constant. Instead, it should be used as a symbolic class constant allowing to encode everything that in common may pertain to the instances.
from enum import Enum
class Machine:
class Foo(Enum):
# you could comma separate any combination for a given state
FOO1 = "foo1"
FOO2 = "foo2"
def __init__(self, arg_foo):
self.foo = arg_foo
self.operate()
self.all_operations()
def foo1(self):
print('drinks at bar1')
def foo2(self):
print('drinks at bar2')
def all_operations(self):
for one_member in Machine.Foo:
self.__getattribute__(one_member.value)()
def operate(self):
self.__getattribute__(str(self.foo.value))()
go_bar1 = Machine(Machine.Foo.FOO1)
go_bar2 = Machine(Machine.Foo.FOO2)
go_bar1.all_operations() # bar crawl
Or perhaps this is, approximately, what you're looking for:
from enum import Enum
class Machine:
def __init__(self, receive: Enum):
for one in receive.value:
if one is not None:
one(self) # Zen of Python
def foo1(self):
print('drinks at bar1')
def foo2(self):
print('drinks at bar2')
class Runner(Enum):
FOO1 = getattr(Machine, 'foo1'), getattr(Machine, 'foo2')
FOO2 = getattr(Machine, 'foo2'), None
first = Machine(Runner.FOO1)
second = Machine(Runner.FOO2)
I hope this helps.

Provided all methods in the Foo class that do not start with _ are methods you want to use, just iterate over the contents of the Foo class and get the attributes of the methods that match:
class Machine:
def operate(self):
for attribute in dir(self.Foo):
if attribute[0] != "_":
getattr(self.Foo, attribute)()

Following How to use class name in class scope?
and what made the most sense for my case, defining the enum inside the init method seems the way to go.
class Machine:
def __init__(self):
class Foo(Enum):
FOO1 = self.foo1
FOO2 = self.foo2
..
self.Foo = Foo
self.operate()
def foo1(self):
pass
def foo2(self):
pass
..
def operate(self):
#self.Foo.FOO1(), self.Foo.FOO2(),.. availabe
#self.Foo holds the enumeration

Is there a way to share def's between different classes?

I have an application with quite a number of classes. The classes have a number of functions as per def's that are common and obviously some that are not.
Is there a way to define the functions that have common function in one place and be available to a number of classes, or do I have to have at least the def's in every class?

Start by defining the func, which is defined outside.
Then to refer that function in different classes, use the same func inside the classes
def func():
print('func')
class B:
def funcB(self):
print('funcB')
def func(self):
func()
class C:
def funcC(self):
print('funcC')
def func(self):
func()
These can be called now as follows.
b = B()
b.func()
#funcA
b.funcB()
#funcB
c = C()
c.func()
#funcA
c.funcC()
#funcC

As pointed out by Amadan in the comments, class inheritance is a good and consistent way of sharing class objects. Below is an example:
class animals:
def __init__(self):
pass
def has_legs(self, type):
if type == "snake":
print(False)
else:
print(True)
class dog(animals):
def __init__(self):
# This is where the magic happens
animals.__init__(self)
pass
def dog_has_legs(self):
self.has_legs("dog")
bofur = dog()
bofur.dog_has_legs()
bofur.has_legs("snake")
Result:
>>> bofur.dog_has_legs()
True
>>> bofur.has_legs("snake")
False
As you can see, the class dog inherits from animals, and so it can call functions and other objects from the animals class as if they belonged to the dog class.

Using getattr to call a function in a separate class

I may be trying to do something that is outside of the realm of possibility here, but I figured I would ask first before abandoning hope. So here it goes...
I have 2 classes, A and B. Each class has an arbitrary number of functions. Class B will be instantiated somewhere in Class A and Class A will utilize one of Class B functions via that instantiation. A function in Class B will need to refer to one or more of Class A's functions using it's current instantiation data of Class A.
Class A
#!/usr/bin/python
from classB import classB
class classA(object):
def Apple(self):
print("Inside Apple")
b = classB()
b.Banana()
b.bar()
def foo(self):
print("foo inside apple")
a = classA()
a.Apple()
Class B:
#!/usr/bin/python
import inspect
class classB(object):
def Banana(self):
print("Inside banana")
def bar(self):
print("bar inside banana")
'''
The following lines just show I can get the names of the
calling class and methods.
'''
stack = inspect.stack()
the_class = stack[1][0].f_locals["self"].__class__
the_method = stack[1][0].f_code.co_name
print("Caller Class: {}".format(the_class))
print("Caller Method: {}".format(the_method))
function_name = 'foo'
if hasattr(the_class, function_name):
print("Class {} has method {}".format(the_class,
function_name))
getattr(the_class, function_name)()
I get the following error:
getattr(the_class, function_name)()
TypeError: unbound method foo() must be called with classA instance as first argument (got nothing instead)
Thanks!

As the error suggests, you must build an object of classA (i.e. the_class) before calling getattr on it.
objA = the_class()
But taking a step back, why don't you just pass class A to class B while initializing it?
b = classB(self)
That will allow you to access the exact method of class A that you need.
Else, if method 'foo' in class A is supposed to be a static method, make it so by using #staticmethod decorator.

Inheritance method overwrite in some conditions [duplicate]

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?

Use the super() function:
class Foo(Bar):
def baz(self, **kwargs):
return super().baz(**kwargs)
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)

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()

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!

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)

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.
e.g. 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.

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

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 e.g. in constructors:
class Foo(Bar):
def __init__(self):
super(Foo, self).__init__()
self.baz = 5

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()

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?)

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.

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

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()'

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()

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

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()

This is a more abstract method:
super(self.__class__,self).baz(arg)

Naming conventions for class method vs instance method

I have two methods, one for the individual Instance, and one for every Instance in that class:
class MasterMatches(models.Model):
#classmethod
def update_url_if_any_matches_has_one(cls):
# apply to all instances, call instance method.
def update_url_if_any_matches_has_one(self):
# do something
Should I name these the same? Or, what is a good naming convention here?

The question of using the same names can be clarified by understanding how decorators work.
#dec
def foo(x):
print(x)
translates to
def foo(x):
print(x)
foo = dec(foo)
In your example the decorator syntax can be expanded to
class MasterMatches(models.Model):
def update_url_if_any_matches_has_one(cls):
# apply to all instances, call instance method.
update_url_if_any_matches_has_one = classmethod(update_url_if_any_matches_has_one)
def update_url_if_any_matches_has_one(self):
# do something
The former implementation of update_url_if_any_matches_has_one will be overwritten by the latter.

Usually use self declaration style. #classmethod use only if method not works with class instance fields.
Function decorated as #classmethod takes the first argument is the class type, while normal method takes instance of object.
class A:
#classmethod
def a(cls):
print(cls)
def b(self):
print(self)
a = A()
a.a()
a.b()
# Output:
# <class '__main__.A'>
# <__main__.A object at 0x03FC5DF0>
It can be useful if you have a static class fields. The to access therm you don't need explicitly specify the class name. But you don't get access to instance fields. Example:
class A:
field = 1
#classmethod
def a(cls):
print(cls.field)
def b(self):
self.field = 2
print(self.field, A.field)
a = A()
a.a()
a.b()
# Outputs:
# 1
# 2 1

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