Error is:
Equilateral object has no attribute angle1.
please suggest how to fix this error and also please explain how self works. I am confused where to use self and where to not
class Triangle(object):
number_of_sides=3
def __init__(self,angle1,angle2,angle3):
self.angle1=angle1
self.angle2=angle2
self.angle3=angle3
def check_angles(self):
if self.angle1+self.angle2+self.angle3==180:
return True
else:
return False
class Equilateral(Triangle): //inheritance
angle=60
def __init__(self):
self.angle=self.angle1
self.angle=self.angle2
self.angle=self.angle3
man=Equilateral()
man.check_angles()
You have it the wrong way around
self.angle1= self.angle
etc
Self refers to the instantiated object, much like 'this' in java. You attach attributes to the object using this keyword.
When defining variables on an object, attributes at the beginning of your class definition do not need self- they are class attributes which all instances of the object will create on instantiation, whereas variables you change or set using self are instance variables and not found on all instances of the object.
Different from other languages, Python does not call __init__() of the super class. You have to call it yourself:
class Equilateral(Triangle):
angle=60
def __init__(self, ...):
super().__init__(...)
self.angle=self.angle1
More details
You have to call __init__ from the super class:
class Triangle(object):
number_of_sides=3
def __init__(self,angle1,angle2,angle3):
self.angle1=angle1
self.angle2=angle2
self.angle3=angle3
def check_angles(self):
return self.angle1+self.angle2+self.angle3==180:
class Equilateral(Triangle):
angle=60
def __init__(self):
Triangle.__init__(self, self.angle, self.angle, self.angle)
man=Equilateral()
man.check_angles()
Related
I am trying to learn how to define classes in Python, starting by creating a very basic example:
class CustomClass:
def print_class(self):
print(self)
CustomClass = 'Hello World'
CustomClass.print_class()
Yet when I run this, I get the error:
AttributeError: 'str' object has no attribute 'print_class'
I don't want to try to access or define some attribute of this class. I want to run the function defined when the class is defined. Could someone point out what I am doing wrong?
what do you wanted to do with this statement: CustomClass = 'Hello World'
if you want to instantiate an object of that class you need to do something like this:
name_of_object = CustomClass() # create an object
name_of_object.print_class() # this will call the method in that class
There is no self in a stand-alone function. If you define a class and instantiate it, self will refer to that object. You’re trying to use an attribute of a class object in the context of no object.
Example:
class Test(object):
def __init__(self, text):
self.text = text
def print_me(self):
print(self.text)
o = Test(‘Hello world’)
o.print_me()
Try that.
Fixed Code:
class CustomClass:
def print_class(self):
print(self)
myClass = CustomClass()
myClass.print_class()
In most Python examples, when super is used to call a parent class's constructors, it appears at the top.
Is it bad form to have it at the bottom of an init method?
In the examples below, super is at the bottom of A's constructor, but at the top of B's constructor.
class A:
def __init__(self):
# Do some stuff
b = result_of_complex_operation()
super(A, self).__init__(b)
class B:
def __init__(self):
super(A, self).__init__(b)
# Do some stuff
This totally depends on the use case. Consider this.
class Foo():
def __init__(self):
print(self.name)
#property
def name(self):
return self.__class__.__name__
class Bar(Foo):
def __init__(self, name):
self.name = name
super().__init__()
#property
def name(self):
return self.__name
#name.setter
def name(self, name):
self.__name = name
If you'd invoke super() before setting self.name within Bar.__init__ you'd get an AttributeError because the required name has not yet been set.
Is it bad form to have it at the bottom of an init method?
You're asking the wrong question. Regardless of whether it's bad from or not, there are valid use cases for moving the superclass initialization to the bottom of a sub-class's constructor. Where to put the call to the superclass's constructor entirely depends on the implementation of the superclass's constructor.
For example, suppose you have a superclass. When constructing the superclass, you want to give an attribute a certain value depending on an attribute of the subclasses:
class Superclass:
def __init__(self):
if self.subclass_attr:
self.attr = 1
else:
self.attr = 2
As you can see from above, we expect the subclasses to have the attribute subclass_attr. So what does this mean? We can't initialize Supperclass until we've given the subclasses the subclass_attr attribute.
Thus, we have to defer calling the superclass's constructor until we initialize subclass_attr. In other words, the call to super will have to be put at the bottom of a subclasses constructor:
class Subclass(Superclass):
def __init__(self):
self.subclass_attr = True
super(Superclass, self).__init__()
In the end, the choice of where to put super should not be based upon some style, but on what's necessary.
I'm trying to define self.data inside a class inheriting from a class
class Object():
def __init__(self):
self.data="1234"
class New_Object(Object):
# Code changing self.data here
But I ran into an issue.
class Object():
def __init__(self):
self.data="1234"
So I have the beginning class here, which is imported from elsewhere, and let's say that the class is a universal one so I can't modify the original at all.
In the original, the instance is referred to as "self" inside the class, and it is defined as self inside the definition __init__.
class New_Object(Object):
# Code changing self.data here
So if I wanted to inherit from the class Object, but define self.data inside New_Object, I thought I would have to define __init__ in New_Object, but this overrides the __init__ from New_Object
Is there any way I could do this without copypasting the __init__ from Object?
You use super to call the original implementation.
class New_Object(Object):
def __init__(self):
super(NewObject, self).__init__()
self.info = 'whatever'
That's what super is for:
class NewObject(Object):
def __init__(self):
super(NewObject, self).__init__()
# self.data exists now, and you can modify it if necessary
You can use super().__init__() to call Object.__init__() from New_Object.__init__().
What you would do:
class Object:
def __init__(self):
print("Object init")
self.data = "1234"
class New_Object(Object):
def __init__(self):
print("calling super")
super().__init__()
print("data is now", self.data)
self.data = self.data.split("3")
o = New_Object()
# calling super
# Object init
# data is now 1234
Note that you do not have to give any arguments to super(), as long as you are using Python 3.
The answer is that you call the superclass's __init__ explicitly during the subclass's __init__. This can be done either of two ways:
Object.__init__(self) # requires you to name the superclass explicitly
or
super(NewObject, self).__init__() # requires you to name the subclass explicitly
The latter also requires you to ensure that you're using "new-style" classes: in Python 3 that's always the case, but in Python 2 you must be sure to inherit from the builtin object class. In Python 3 it can actually be expressed even more simply:
super().__init__()
Personally, in most of my code the "disadvantage" of having to name the superclass explicitly is no disadvantage at all, and Object.__init__() lends transparency since it makes it absolutely clear what is being called. This is because most of my code is single-inheritance only. The super route comes into its own when you have multiple inheritance. See What does 'super' do in Python?
Python 2 example:
class Object(object):
def __init__(self):
self.data = "1234"
class NewObject:
def __init__(self):
# subclass-specific stuff
super(NewObject, self).__init__()
# more subclass-specific stuff
I am confused even after checking many questions asked in SO. I have 2 different class (2 different script) & I want to inherit super class's __init__ method's parameters.
script1.py
class MainClass():
def __init__(self,params):
self.one=params['ONE']
self.two=params['TWO']
self.three=params['THREE']
self.four=params['FOUR']
self.five=params['FIVE']
def a():
#---------
#somecode
#Initializing other class's object to access it's method.
s=SubClass() #HERE I WANT TO PASS 'PARAMS' (WHICH IS A DICTIONARY)
s.method1(....)
script2.py
class SubClass(SuperClass):
def __init__(self,params):
#Here I want all the parameters inside the 'param' in super class.
#(one,two,three...., etc).
#By checking some SO questions, I changed class SubClass() -->
#class Subclass(SuperClass) & below line:
MainClass.__init__(self,params) #But technically I don't have anything
#in param in subclass.
def method1():
#some code...
Since sub class's param doesn't have anything, It gives me an error:
self.one=params['ONE']
TypeError: 'int' object has no attribute '__getitem__'
I am not getting:
How can I access all the parameters of super class to sub class in a simplest way? I don't want to pass individual arguments (like self.one, self.two..) to the sub class.
If I am calling third class inside SubClass -> method1 --> Call 3rd class same as passing 'params'. Is it possible?
Is this what you need?
script1.py
class MainClass():
def __init__(self,params):
# Save params for use by a
self.params = params
self.one=params['ONE']
self.two=params['TWO']
...
self.five=params['FIVE']
def a():
s=SubClass(self.params)
s.method1(...)
script2.py
class SubClass(SuperClass):
def __init__(self,params):
MainClass.__init__(self,params)
def method1():
#some code...
You can pass any and all the non-keyword arguments from the subclass's __init__()to the superclass's like this:
class SubClass(SuperClass):
def __init__(self, *params):
MainClass.__init__(self, *params)
...
This same idea will work for other methods, too.
I know this question might be pointless but there is a reason why I am looking to do it this way. I want to call something exactly opposite to super()
class A(object):
def use_attack(self, damage, passive, spells):
#do stuff with passed parameters
#return something
def use_spell(self, name , enemy_hp):
#other code
if name == 'Enrage':
#call child method use_attack right here
class B(A):
def use_attack(self):
#bunch of code here
return super(B, self).use_attack(damage, passive, spells)
def use_spell(self, name , enemy_hp):
return super(B , self).use_attack(name ,enemy_hp)
b = B()
b.use_spell('Enrage', 100)
I have a bunch of code in class B's use_attack() method that I would not like to replicate in the parent method of use_spell() .
I would like to call the child method use_attack() in the line indicated.
I have a bunch of code in class B's use_attack() method that I would not like to replicate in the parent method of use_spell() .
Then factor that code out into a method on the parent class. This is exactly what inheritance is for. Children inherit code from parents, not the other way around.
From the python docs: "The mro attribute of the type lists the method resolution search order used by both getattr() and super()"
https://docs.python.org/3/library/functions.html#super
This should help shed some light on Inheritance and Method Resolution Order (mro).
class Foo(object):
def __init__(self):
print('Foo init called')
def call_child_method(self):
self.child_method()
class Bar(Foo):
def __init__(self):
print('Bar init called')
super().__init__()
def child_method(self):
print('Child method called')
bar = Bar()
bar.call_child_method()