class Trout(Fish):
def __init__(self, water = "freshwater"):
self.water = water
super().__init__(self)
In this line super().__init__(self) how is the self parameter passed to the function?
Say you have the code
f = Trout()
Behind the scenes, this is roughly equivalent to
f = Trout.__new__()
Trout.__init__(f)
Inside Trout.__init__, super() returns a proxy object that represents the appropriate class in the method resolution order of Trout; you don't actually have to pass self as an explicit argument; super().__init__ defaults to a bound method with self already present.
The object calling the method is implicitly (automatically) the first argument to the method. This is an agreement the language makes with itself.
In practical terms, assume you have an object obj of some type that has a method stuff that takes a simple Boolean argument. You invoke the method as
obj.stuff(True)
If this were a "normal" function, you would invoke it as
stuff(obj, True)
... but that's not how the syntax of classes and objects works. In any case, the function/method header looks the same:
def stuff(self, flag):
The difference here is that the each object class can have a method stuff, and we don't have to give them different names. When we use class methods, the compiler knows which one to use by looking at the class of the invoking object.
In the specific case of the __init__ method, the object is created immediately upon entry and assigned to self. This is another automatic agreement the language makes with itself: this specially-named function includes an invisible creation of a default object when you enter.
Related
This question already has answers here:
How does `super` interacts with a class's `__mro__` attribute in multiple inheritance?
(2 answers)
Closed 4 years ago.
From Python3's documentation super() "returns a proxy object that delegates method calls to a parent or sibling class of type." What does that mean?
Suppose I have the following code:
class SuperClass():
def __init__(self):
print("__init__ from SuperClass.")
print("self object id from SuperClass: " + str(id(self)))
class SubClass(SuperClass):
def __init__(self):
print("__init__ from SubClass.")
print("self object id from SubClass: " + str(id(self)))
super().__init__()
sc = SubClass()
The output I get from this is:
__init__ from SubClass.
self object id from SubClass: 140690611849200
__init__ from SuperClass.
self object id from SuperClass: 140690611849200
This means that in the line super().__init__(), super() is returning the current object which is then implicitly passed to the superclass' __init__() method. Is this accurate or am I missing something here?
To put it simply, I want to understand the following:
When super().__init__() is run,
What exactly is being passed to __init__() and how? We are calling it on super() so whatever this is returning should be getting passed to the __init__() method from what I understand about Python so far.
Why don't we have to pass in self to super().__init__()?
returns a proxy object that delegates method calls to a parent or
sibling class of type.
This proxy is an object that acts as the method-calling portion of the parent class. It is not the class itself; rather, it's just enough information so that you can use it to call the parent class methods.
If you call __init__(), you get your own, local, sub-class __init__ function. When you call super(), you get that proxy object, which will redirect you to the parent-class methods. Thus, when you call super().__init__(), that proxy redirects the call to the parent-class __init__ method.
Similarly, if you were to call super().foo, you would get the foo method from the parent class -- again, re-routed by that proxy.
Is that clear to you?
Responses to OP comments
But that must mean that this proxy object is being passed to
__init__() when running super().__init__() right?
Wrong. The proxy object is like a package name, such as calling math.sqrt(). You're not passing math to sqrt, you're using it to denote which sqrt you're using. If you wanted to pass the proxy to __init__, the call would be __init__(super()). That call would be semantically ridiculous, of course.
When we have to actually pass in self which is the sc object in my example.
No, you are not passing in sc; that is the result of the object creation call (internal method __new__), which includes an invocation of init. For __init__, the self object is a new item created for you by the Python run-time system. For most class methods, that first argument (called self out of convention, this in other languages) is the object that invoked the method.
This means that in the line super().__init__(), super() is returning the current object which is then implicitly passed to the superclass' __init__() method. Is this accurate or am I missing something here?
>>> help(super)
super() -> same as super(__class__, <first argument>)
super call returns a proxy/wrapper object which remembers:
The instance invoking super()
The class of the calling object
The class that's invoking super()
This is perfectly sound. super always fetches the attribute of the next class in the hierarchy ( really the MRO) that has the attribute that you're looking for. So it's not returning the current object, but rather and more accurately, it returns an object that remembers enough information to search for attributes higher in the class hierarchy.
What exactly is being passed to __init__() and how? We are calling it on super() so whatever this is returning should be getting passed to the __init__() method from what I understand about Python so far.
You're almost right. But super loves to play tricks on us. super class defines __getattribute__, this method is responsible for attribute search. When you do something like: super().y(), super.__getattribute__ gets called searching for y. Once it finds y it passes the instance that's invoking the super call to y. Also, super has __get__ method, which makes it a descriptor, I'll omit the details of descriptors here, refer to the documentation to know more. This answers your second question as well, as to why self isn't passed explicitly.
*Note: super is a little bit different and relies on some magic. Almost for all other classes, the behavior is the same. That is:
a = A() # A is a class
a.y() # same as A.y(a), self is a
But super is different:
class A:
def y(self):
return self
class B(A):
def y(self)
return super().y() # equivalent to: A.y(self)
b = B()
b.y() is b # True: returns b not super(), self is b not super()
I wrote a simple test to investigate what CPython does for super:
class A:
pass
class B(A):
def f(self):
return super()
#classmethod
def g(cls):
return super()
def h(selfish):
selfish = B()
return super()
class C(B):
pass
c = C()
for method in 'fgh':
super_object = getattr(c, method)()
print(super_object, super_object.__self__, super_object.__self_class__, super_object.__thisclass__) # (These methods were found using dir.)
The zero-argument super call returns an object that stores three things:
__self__ stores the object whose name matches the first parameter of the method—even if that name has been reassigned.
__self_class__ stores its type, or itself in the case of a class method.
__thisclass__ stores the class in which the method is defined.
(It is unfortunate that __thisclass__ was implemented this way rather than fetching an attribute on the method because it makes it impossible to use the zero-argument form of super with meta-programming.)
The object returned by super implements getattribute, which forwards method calls to the type found in the __mro__ of __self_class__ one step after __thisclass__.
This question is in relation to posts at What does 'super' do in Python? , How do I initialize the base (super) class? , and Python: How do I make a subclass from a superclass? which describes two ways to initialize a SuperClass from within a SubClass as
class SuperClass:
def __init__(self):
return
def superMethod(self):
return
## One version of Initiation
class SubClass(SuperClass):
def __init__(self):
SuperClass.__init__(self)
def subMethod(self):
return
or
class SuperClass:
def __init__(self):
return
def superMethod(self):
return
## Another version of Initiation
class SubClass(SuperClass):
def __init__(self):
super(SubClass, self).__init__()
def subMethod(self):
return
So I'm a little confused about needing to explicitly pass self as a parameter in
SuperClass.__init__(self)
and
super(SubClass, self).__init__().
(In fact if I call SuperClass.__init__() I get the error
TypeError: __init__() missing 1 required positional argument: 'self'
). But when calling constructors or any other class method (ie :
## Calling class constructor / initiation
c = SuperClass()
k = SubClass()
## Calling class methods
c.superMethod()
k.superMethod()
k.subMethod()
), The self parameter is passed implicitly .
My understanding of the self keyword is it is not unlike the this pointer in C++, whereas it provides a reference to the class instance. Is this correct?
If there would always be a current instance (in this case SubClass), then why does self need to be explicitly included in the call to SuperClass.__init__(self)?
Thanks
This is simply method binding, and has very little to do with super. When you can x.method(*args), Python checks the type of x for a method named method. If it finds one, it "binds" the function to x, so that when you call it, x will be passed as the first parameter, before the rest of the arguments.
When you call a (normal) method via its class, no such binding occurs. If the method expects its first argument to be an instance (e.g. self), you need to pass it in yourself.
The actual implementation of this binding behavior is pretty neat. Python objects are "descriptors" if they have a __get__ method (and/or __set__ or __delete__ methods, but those don't matter for methods). When you look up an attribute like a.b, Python checks the class of a to see if it has a attribute b that is a descriptor. If it does, it translates a.b into type(a).b.__get__(a, type(a)). If b is a function, it will have a __get__ method that implements the binding behavior I described above. Other kinds of descriptors can have different behaviors. For instance, the classmethod decorator replaces a method with a special descriptor that binds the function the class, rather than the instance.
Python's super creates special objects that handle attribute lookups differently than normal objects, but the details don't matter too much for this issue. The binding behavior of methods called through super is just like what I described in the first paragraph, so self gets passed automatically to the bound method when it is called. The only thing special about super is that it may bind a different function than you'd get lookup up the same method name on self (that's the whole point of using it).
The following example might elucidate things:
class Example:
def method(self):
pass
>>> print(Example.method)
<unbound method Example.method>
>>> print(Example().method)
<bound method Example.method of <__main__.Example instance at 0x01EDCDF0>>
When a method is bound, the instance is passed implicitly. When a method is unbound, the instance needs to be passed explicitly.
The other answers will definitely offer some more detail on the binding process, but I think it's worth showing the above snippet.
The answer is non-trivial and would probably warrant a good article. A very good explanation of how super() works is brilliantly given by Raymond Hettinger in a Pycon 2015 talk, available here and a related article.
I will attempt a short answer and if it is not sufficient I (and hopefully the community) will expand on it.
The answer has two key pieces:
Python's super() needs to have an object on which the method being overridden is called, so it is explicitly passed with self. This is not the only possible implementation and in fact, in Python 3, it is no longer required that you pass the self instance.
Python super() is not like Java, or other compiled languages, super. Python's implementation is designed to support the multiple collaborative inheritance paradigm, as explained in Hettinger's talk.
This has an interesting consequence in Python: the method resolution in super() depends not only on the parent class, but on the children classes as well (consequence of multiple inheritance). Note that Hettinger is using Python 3.
The official Python 2.7 documentation on super is also a good source of information (better understood after watching the talk, in my opinion).
Because in SuperClass.__init__(self), you're calling the method on the class, not the instance, so it cannot be passed implicitly. Similarly you cannot just call SubClass.subMethod(), but you can call SubClass.subMethod(k) and it'll be equivalent to k.subMethod(). Similarly if self refers to a SubClass then self.__init__() means SubClass.__init__(self), so if you want to call SuperClass.__init you have to call it directly.
New to Python, trying to understand exactly what the self in the __init_(self) function is referring to.
A few tutorials I'm working with describe self as
referring to the instance whose method was called.
Which is not exactly a trivial statement for someone new to OOP.
I've been reading a lot about the whole backstory as to why you have to actually include an explicit self in Python, but need a simple explanation as to what it means to say that self is used to refer to the instance object ——> Does that mean that self is actually referring to the object that is the class itself you've just created? In other words, self somehow "boots up" the class in memory as an object?
Your second-last sentence is correct, but the last sentence is not. It has nothing to do with "booting up" or creating the object at all - the object already exists by that point.
I think you are missing the fact that self is used in all methods, not just __init__, to refer to the specific object that the method belongs to.
For instance, if you had a simple object with a name property, and a method called print_name, it might look like this:
def print_name(self):
print(self.name)
So here the method is using self to refer to the properties of the object it has been called on.
When objects are instantiated, the object itself is passed into the self parameter.
Because of this, the object’s data is bound to the object. Below is an example of how you might like to visualize what each object’s data might look. Notice how ‘self’ is replaced with the objects name. I'm not saying this example diagram below is wholly accurate but it hopefully with serve a purpose in visualizing the use of self.
EDIT (due to further question: Could you explain why exactly when objects are instantiated, the object itself is passed into the self parameter?)
The Object is passed into the self parameter so that the object can keep hold of its own data.
Although this may not be wholly accurate, think of the process of instantiating an object like this: When an object is made it uses the class as a template for its own data and methods. Without passing it's own name into the self parameter, the attributes and methods in the class would remain as a general template and would not be referenced to (belong to) the object. So by passing the object's name into the self parameter it means that if 100 objects are instantiated from the one class, they can all keep track of their own data and methods.
See the illustration below:
Every member function of a class, including the constructor (__init__) is invoked for a certain instance (object) of that class. Member functions have to be able to access the object for which they are called.
So e.g. in a.f(), f() has to have acces to a. In f, defined as f (this), this refers to a.
The special thing for a constructor is that there is no object "before the dot" yet, because precisely that object is being constructed. So this refers to the object "just being constructed" in that case.
When you write myClass(), python first creates an instance of your class, then immediately calls __init__() passing this object as the argument. self is a defined object in memory by the time you call __init__().
Behind the scenes, object construction is actually quite complicated.
Classes are objects too, and the type of a class is type (or a subclass, if using metaclasses). type has a __call__ method that is responsible for constructing instances. It works something like:
class type:
def __call__(cls, *args, **kwargs):
self = cls.__new__(cls, *args, **kwargs)
if isinstance(self, cls):
cls.__init__(self, *args, **kwargs)
Note, the above is for demonstrative purposes only.
Remember that, if a function is not defined on a class itself, it is looked up on its parent (as controlled by the mro), and usually.
Ultimately, __new__ must either call object.__new__(cls) to allocate a new instance of a class cls, or else return an existing object. If the existing object is of a different class, __init__ will not be called. Note that if it returns an existing object of the right class (or a subclass), __init__ will be called more than once. For such classes, all of the work is usually done in __new__.
Chances are you'll never use any of this, but it might help you understand what's going on behind the scenes.
Simply, it means you are referring to a method or variable that is local to the object.
You can look at 'self' as referrer or a pointer to class internals which with that you can invoke methods or add/remove/update/delete attributes . Class is somehow an isolated object which has its own representation of data given to it . So basically , self is only explicitly defined as an argument, which with using that you can get access to class internals . Some programming languages does not explicitly include the keyword self. or some uses this ( like C ++ ) . take a look here:
a = 1
b = 2
class test(object):
def __init__(self,a,b):
self.a = a + 1
self.b = b + 1
def show_internals(self):
print self.a, '\t', self.b
def change_internals(self,a,b):
self.a = a
self.b = b
_my_class = test(3,4)
print a , b
_my_class.show_internals()
_my_class.change_internals(5,6)
_my_class.show_internals()
print a , b
the result is :
1 2
4 5
5 6
1 2
As you can see, with using self you can manipulate the data within the object itself. Otherwise you would end up editing global variables.
I am confused with built-in method in Python.
For instance, what is the some_string.lower() and str.lower(some_string) and how are they different?
str is the name of the class of all strings in Python. str.lower is one of its methods.
If you call lower on one of its instances (e.g. 'ABC'.lower()), you call a bound method, which automatically sends the called object as the first argument (usually called self).
If you call lower on the class itself (i.e. you use str.lower()), then you call an unbound method, which doesn't provide the self argument automatically. Therefore, you have to specify the object to act upon by yourself.
If all of this seems hard to understand, it will be easier when you consider how methods are defined in the classes. Let's say we create our own very simple class, which represents a point (X,Y coordinate in space). And has a show() method to print the point.
class Point:
"""This is the constructor of Point"""
def __init__(self, x, y):
# Save arguments as field of our class instance (self)
self.x = x
self.y = y
def show(self):
print self.x, self.y
# We now create an instance of Point:
p = Point(1.0, 2.0)
# We now show p by calling a bound method
p.show()
Note that we didn't have to specify the self argument (so p.show() was called with no arguments). In reality, the previous call was more or less equivalent to this:
Point.show(p)
They're not entirely equivalent, but that's a more advanced topic. One of the simplest cases when they will not be equivalent is if you change the value of p.show after creating the object, for instance:
p.show = 4
Now, p.show() won't even compile, since p.show is not a function anymore, but an integer! However, Point.show(p) would still be unchanged, since we only modified the show attribute in the class instance (p) and not in the class itself (Point).
The first is a bound method call and the second is an unbound method call.
Think about how you'd write a method like this:
class str:
...
def lower(self):
# And so on
The first argument is self. If you use this method from an instance (e.g. some_string.lower(), the instance automatically gets passed as the first argument to the method (as self).
However, if you call it from the class (as an unbound method), e..g str.lower(some_string), there is no instance to automatically pass as the first argument. So, instead, some_string gets passed as self and whatever would've been done with the instance in the first case gets done with some_string.
It's never really necessary to use the unbound version, however, as any string will have a lower() method that you can call. The preferred style is to use some_string.lower().
From what I read/understand, the 'self' parameter is similiar to 'this'.
Is that true?
If its optional, what would you do if self wasnt' passed into the method?
Yes, it's used in similar ways. Note that it's a positional parameter and you can call it what you want; however there is a strong convention to call it self (not this or anything else). Some positional parameter must be there for a usable instance method; it is not optional.
The joy of Python
That is true to some extend. Methods are bound to the object instance they are a part of. When you see
def some_func(self, foo, bar)
The passing of self is sometimes implicit when you call, for example:
obj.some_func(foo_val, bar_val)
Which is equal (presuming obj is of class MyClass) to
MyClass.some_func(obj, foo_val, bar_val)
Because the method is bound to obj, the self argument gets populated. This is part of Python being explicit with what it means. In other languages, this just pops into scope, with Python there is some exposure of how this happens.
You can also pass methods around, and manually pass them self when not calling from a bound context.
The Python docs do a good Job:
xf = x.f
while True:
print xf()
will continue to print hello world until the end of time.
What exactly happens when a method is called? You may have noticed that x.f() was called >without an argument above, even though the function definition for f() specified an >argument. What happened to the argument? Surely Python raises an exception when a function >that requires an argument is called without any — even if the argument isn’t actually >used...
Actually, you may have guessed the answer: the special thing about methods is that the >object is passed as the first argument of the function. In our example, the call x.f() is >exactly equivalent to MyClass.f(x). In general, calling a method with a list of n arguments >is equivalent to calling the corresponding function with an argument list that is created >by inserting the method’s object before the first argument.
self is this, just you have to explicitly pass it and explicitly use it to refer to class methods/properties.
It isn't optional in class methods. You will get a TypeError if you try to define a classmethod without at least one argument (i.e., the self parameter).
However, you can call it something other than self, but I have never seen otherwise.
self refers to the object on which the method was called, much like this in C++. But it is important that self is merely a convention, you can name it as you like and pass instances of subclasses.
In classes a self variable (or cls for classmethods) is required. What you want to call it is your decision though. If you prefer you could call it this instead.
A classmethod is a method that gets the class as a first argument instead of a instance. It can be called without passing an instance.
i.e. with a classmethod you can do:
SomeObject.some_class_method()
while a normal method would require you to do
SomeObject().some_normal_method()
or
SomeObject.some_normal_method(instance)
self is definitely similar to this, however, in Python, the name self is just a convention, and could be named anything else. The variable is named after whatever you call it in the function's prototype (def function(whatever, params...):).
For instance methods, self IS actually required. For class or static methods, you need to specify that they should be treated as such, and then self is not required. For example:
def type_to_display(type):
"""Converts a pass type to the full written pass type."""
return list((pair[1] for pair in Pass.TYPE_CHOICES if pair[0] ==
type[0:1].upper()))[0]
type_to_display = staticmethod(type_to_display)
You will never be able to use an instance method in such a way that self is not passed in. For example, if I have an instance my_car of a Car class, and I use the Car class's drive instance method, the my_car instance will be implicitly passed into the drive method as the first parameter (self).
class Car:
def drive(self):
self.do_some_stuff()
my_car = Car()
my_car.drive() # actually calls Car.drive(my_car)