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I have a class with a private constant _BAR = object().
In a child class, outside of a method (no access to self), I want to refer to _BAR.
Here is a contrived example:
class Foo:
_BAR = object()
def __init__(self, bar: object = _BAR):
...
class DFoo(Foo):
"""Child class where I want to access private class variable from parent."""
def __init__(self, baz: object = super()._BAR):
super().__init__(baz)
Unfortunately, this doesn't work. One gets an error: RuntimeError: super(): no arguments
Is there a way to use super outside of a method to get a parent class attribute?
The workaround is to use Foo._BAR, I am wondering though if one can use super to solve this problem.
Inside of DFoo, you cannot refer to Foo._BAR without referring to Foo. Python variables are searched in the local, enclosing, global and built-in scopes (and in this order, it is the so called LEGB rule) and _BAR is not present in any of them.
Let's ignore an explicit Foo._BAR.
Further, it gets inherited: DFoo._BAR will be looked up first in DFoo, and when not found, in Foo.
What other means are there to get the Foo reference? Foo is a base class of DFoo. Can we use this relationship? Yes and no. Yes at execution time and no at definition time.
The problem is when the DFoo is being defined, it does not exist yet. We have no start point to start following the inheritance chain. This rules out an indirect reference (DFoo -> Foo) in a def method(self, ....): line and in a class attribute _DBAR = _BAR.
It is possible to work around this limitation using a class decorator. Define the class and then modify it:
def deco(cls):
cls._BAR = cls.__mro__[1]._BAR * 2 # __mro__[0] is the class itself
return cls
class Foo:
_BAR = 10
#deco
class DFoo(Foo):
pass
print(Foo._BAR, DFoo._BAR) # 10 20
Similar effect can be achieved with a metaclass.
The last option to get a reference to Foo is at execution time. We have the object self, its type is DFoo, and its parent type is Foo and there exists the _BAR. The well known super() is a shortcut to get the parent.
I have assumed only one base class for simplicity. If there were several base classes, super() returns only one of them. The example class decorator does the same. To understand how several bases are sorted to a sequence, see how the MRO works (Method Resolution Order).
My final thought is that I could not think up a use-case where such access as in the question would be required.
Short answer: you can't !
I'm not going into much details about super class itself here. (I've written a pure Python implementation in this gist if you like to read.)
But now let's see how we can call super:
1- Without arguments:
From PEP 3135:
This PEP proposes syntactic sugar for use of the super type to
automatically construct instances of the super type binding to the
class that a method was defined in, and the instance (or class object
for classmethods) that the method is currently acting upon.
The new syntax:
super()
is equivalent to:
super(__class__, <firstarg>)
...and <firstarg> is the first parameter of the method
So this is not an option because you don't have access to the "instance".
(Body of the function/methods is not executed unless it gets called, so no problem if DFoo doesn't exist yet inside the method definition)
2- super(type, instance)
From documentation:
The zero argument form only works inside a class definition, as the
compiler fills in the necessary details to correctly retrieve the
class being defined, as well as accessing the current instance for
ordinary methods.
What were those necessary details mentioned above? A "type" and A "instance":
We can't pass neither "instance" nor "type" which is DFoo here. The first one is because it's not inside the method so we don't have access to instance(self). Second one is DFoo itself. By the time the body of the DFoo class is being executed there is no reference to DFoo, it doesn't exist yet. The body of the class is executed inside a namespace which is a dictionary. After that a new instance of type type which is here named DFoo is created using that populated dictionary and added to the global namespaces. That's what class keyword roughly does in its simple form.
3- super(type, type):
If the second argument is a type, issubclass(type2, type) must be
true
Same reason mentioned in above about accessing the DFoo.
4- super(type):
If the second argument is omitted, the super object returned is
unbound.
If you have an unbound super object you can't do lookup(unless for the super object's attributes itself). Remember super() object is a descriptor. You can turn an unbound object to a bound object by calling __get__ and passing the instance:
class A:
a = 1
class B(A):
pass
class C(B):
sup = super(B)
try:
sup.a
except AttributeError as e:
print(e) # 'super' object has no attribute 'a'
obj = C()
print(obj.sup.a) # 1
obj.sup automatically calls the __get__.
And again same reason about accessing DFoo type mentioned above, nothing changed. Just added for records. These are the ways how we can call super.
This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Why do we use __init__ in Python classes?
(9 answers)
Closed 6 months ago.
I'm learning the Python programming language and I've came across something I don't fully understand.
In a method like:
def method(self, blah):
def __init__(?):
....
....
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
I think they might be OOP constructs, but I don't know very much.
In this code:
class A(object):
def __init__(self):
self.x = 'Hello'
def method_a(self, foo):
print self.x + ' ' + foo
... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...
a = A() # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument
The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.
Yep, you are right, these are oop constructs.
__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).
class Point:
def __init__(self, x, y):
self._x = x
self._y = y
The __init__ method gets called after memory for the object is allocated:
x = Point(1,2)
It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:
class Point:
def __init__(self, x, y):
_x = x
_y = y
Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).
N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).
A brief illustrative example
In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:
class MyClass(object):
i = 123
def __init__(self):
self.i = 345
a = MyClass()
print(a.i)
print(MyClass.i)
Output:
345
123
In short:
self as it suggests, refers to itself- the object which has called the method. That is, if you have N objects calling the method, then self.a will refer to a separate instance of the variable for each of the N objects. Imagine N copies of the variable a for each object
__init__ is what is called as a constructor in other OOP languages such as C++/Java. The basic idea is that it is a special method which is automatically called when an object of that Class is created
Class objects support two kinds of operations: attribute references and instantiation
Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class".
Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example:
x = MyClass()
The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__(), like this:
def __init__(self):
self.data = []
When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:
x = MyClass()
Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i
Taken from official documentation which helped me the most in the end.
Here is my example
class Bill():
def __init__(self,apples,figs,dates):
self.apples = apples
self.figs = figs
self.dates = dates
self.bill = apples + figs + dates
print ("Buy",self.apples,"apples", self.figs,"figs
and",self.dates,"dates.
Total fruitty bill is",self.bill," pieces of fruit :)")
When you create instance of class Bill:
purchase = Bill(5,6,7)
You get:
> Buy 5 apples 6 figs and 7 dates. Total fruitty bill is 18 pieces of
> fruit :)
__init__ does act like a constructor. You'll need to pass "self" to any class functions as the first argument if you want them to behave as non-static methods. "self" are instance variables for your class.
Try out this code. Hope it helps many C programmers like me to Learn Py.
#! /usr/bin/python2
class Person:
'''Doc - Inside Class '''
def __init__(self, name):
'''Doc - __init__ Constructor'''
self.n_name = name
def show(self, n1, n2):
'''Doc - Inside Show'''
print self.n_name
print 'Sum = ', (n1 + n2)
def __del__(self):
print 'Destructor Deleting object - ', self.n_name
p=Person('Jay')
p.show(2, 3)
print p.__doc__
print p.__init__.__doc__
print p.show.__doc__
Output:
Jay
Sum = 5
Doc - Inside Class
Doc - __init__ Constructor
Doc - Inside Show
Destructor Deleting object - Jay
Had trouble undestanding this myself. Even after reading the answers here.
To properly understand the __init__ method you need to understand self.
The self Parameter
The arguments accepted by the __init__ method are :
def __init__(self, arg1, arg2):
But we only actually pass it two arguments :
instance = OurClass('arg1', 'arg2')
Where has the extra argument come from ?
When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.
In order to access object attributes from within the __init__ method we need a reference to the object.
Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.
This means in the __init__ method we can do :
self.arg1 = arg1
self.arg2 = arg2
Here we are setting attributes on the object. You can verify this by doing the following :
instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1
values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.
source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method
note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:
class A(object):
def __init__(foo):
foo.x = 'Hello'
def method_a(bar, foo):
print bar.x + ' ' + foo
and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.
What does self do? What is it meant to be? Is it mandatory?
The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.
Python doesn't force you on using "self". You can give it any name you want. But remember the first argument in a method definition is a reference to the object. Python adds the self argument to the list for you; you do not need to include it when you call the methods.
if you didn't provide self in init method then you will get an error
TypeError: __init___() takes no arguments (1 given)
What does the init method do? Why is it necessary? (etc.)
init is short for initialization. It is a constructor which gets called when you make an instance of the class and it is not necessary. But usually it our practice to write init method for setting default state of the object. If you are not willing to set any state of the object initially then you don't need to write this method.
__init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
self represents that object which will inherit those properties.
Basically, you need to use the 'self' keyword when using a variable in multiple functions within the same class. As for init, it's used to setup default values incase no other functions from within that class are called.
The 'self' is a reference to the class instance
class foo:
def bar(self):
print "hi"
Now we can create an instance of foo and call the method on it, the self parameter is added by Python in this case:
f = foo()
f.bar()
But it can be passed in as well if the method call isn't in the context of an instance of the class, the code below does the same thing
f = foo()
foo.bar(f)
Interestingly the variable name 'self' is just a convention. The below definition will work exactly the same.. Having said that it is very strong convention which should be followed always, but it does say something about flexible nature of the language
class foo:
def bar(s):
print "hi"
Just a demo for the question.
class MyClass:
def __init__(self):
print('__init__ is the constructor for a class')
def __del__(self):
print('__del__ is the destructor for a class')
def __enter__(self):
print('__enter__ is for context manager')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('__exit__ is for context manager')
def greeting(self):
print('hello python')
if __name__ == '__main__':
with MyClass() as mycls:
mycls.greeting()
$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class
In this code:
class Cat:
def __init__(self, name):
self.name = name
def info(self):
print 'I am a cat and I am called', self.name
Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.
c = Cat('Kitty')
c.info()
The result of the above statements will be as follows:
I am a cat and I am called Kitty
# Source: Class and Instance Variables
# https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables
class MyClass(object):
# class variable
my_CLS_var = 10
# sets "init'ial" state to objects/instances, use self argument
def __init__(self):
# self usage => instance variable (per object)
self.my_OBJ_var = 15
# also possible, class name is used => init class variable
MyClass.my_CLS_var = 20
def run_example_func():
# PRINTS 10 (class variable)
print MyClass.my_CLS_var
# executes __init__ for obj1 instance
# NOTE: __init__ changes class variable above
obj1 = MyClass()
# PRINTS 15 (instance variable)
print obj1.my_OBJ_var
# PRINTS 20 (class variable, changed value)
print MyClass.my_CLS_var
run_example_func()
Here, the guy has written pretty well and simple: https://www.jeffknupp.com/blog/2014/06/18/improve-your-python-python-classes-and-object-oriented-programming/
Read above link as a reference to this:
self? So what's with that self parameter to all of the Customer
methods? What is it? Why, it's the instance, of course! Put another
way, a method like withdraw defines the instructions for withdrawing
money from some abstract customer's account. Calling
jeff.withdraw(100.0) puts those instructions to use on the jeff
instance.
So when we say def withdraw(self, amount):, we're saying, "here's how
you withdraw money from a Customer object (which we'll call self) and
a dollar figure (which we'll call amount). self is the instance of the
Customer that withdraw is being called on. That's not me making
analogies, either. jeff.withdraw(100.0) is just shorthand for
Customer.withdraw(jeff, 100.0), which is perfectly valid (if not often
seen) code.
init self may make sense for other methods, but what about init? When we call init, we're in the process of creating an object, so how can there already be a self? Python allows us to extend
the self pattern to when objects are constructed as well, even though
it doesn't exactly fit. Just imagine that jeff = Customer('Jeff
Knupp', 1000.0) is the same as calling jeff = Customer(jeff, 'Jeff
Knupp', 1000.0); the jeff that's passed in is also made the result.
This is why when we call init, we initialize objects by saying
things like self.name = name. Remember, since self is the instance,
this is equivalent to saying jeff.name = name, which is the same as
jeff.name = 'Jeff Knupp. Similarly, self.balance = balance is the same
as jeff.balance = 1000.0. After these two lines, we consider the
Customer object "initialized" and ready for use.
Be careful what you __init__
After init has finished, the caller can rightly assume that the
object is ready to use. That is, after jeff = Customer('Jeff Knupp',
1000.0), we can start making deposit and withdraw calls on jeff; jeff is a fully-initialized object.
Python __init__ and self what do they do?
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
The example given is not correct, so let me create a correct example based on it:
class SomeObject(object):
def __init__(self, blah):
self.blah = blah
def method(self):
return self.blah
When we create an instance of the object, the __init__ is called to customize the object after it has been created. That is, when we call SomeObject with 'blah' below (which could be anything), it gets passed to the __init__ function as the argument, blah:
an_object = SomeObject('blah')
The self argument is the instance of SomeObject that will be assigned to an_object.
Later, we might want to call a method on this object:
an_object.method()
Doing the dotted lookup, that is, an_object.method, binds the instance to an instance of the function, and the method (as called above) is now a "bound" method - which means we do not need to explicitly pass the instance to the method call.
The method call gets the instance because it was bound on the dotted lookup, and when called, then executes whatever code it was programmed to perform.
The implicitly passed self argument is called self by convention. We could use any other legal Python name, but you will likely get tarred and feathered by other Python programmers if you change it to something else.
__init__ is a special method, documented in the Python datamodel documentation. It is called immediately after the instance is created (usually via __new__ - although __new__ is not required unless you are subclassing an immutable datatype).
In Python, within a class, can a staticmethod call on another local function/method defined within the same class?
I tried the following code and obtained an error message saying foo1() is not defined.
class trialOne(object):
#staticmethod
def foo1():
a = 3.1
return a
#staticmethod
def foo():
a = foo1()
return a
obj = trialOne()
b = obj.foo()
class Tester:
def local(self):
print "I'm a local!"
#staticmethod
def another_stat():
print "I'm a static!"
#staticmethod
def stat(inst):
inst.local()
Tester.another_stat()
t = Tester()
Tester.stat(t)
# Out:
# I'm a local!
# I'm a static!
Yes, you can! By definition, instance methods need an instance to associate themselves with, but as long as you have that instance, you can call local methods just as you normally would.
To go into this in a little more depth, there's nothing special about the word self. That's a variable just like any other. Any instance method of a class MUST take in an instance of that class as its first parameter, and it's convention to call that parameter self, but you could just as easily use any other name.
If it helps you understand the distinction, these two statements are semantically equivalent:
t.local()
Tester.local(t)
The first is just syntactic sugar for the second. The second is using the class name to reference a method of the Tester class, then passes in the instance as the first parameter. The first simply pretends that local is a field of t and calls it, but that call is transformed into Tester.local(t) by the Python interpreter.
Thus, calling a static method is the same syntax as Tester.local(t), except the first parameter does not have to be an instance of that class.
So classmethods and staticmethods are called in the same way, but the difference is that a class method "knows" what class it's coming from. The first parameter of a class method is always a variable that contains the class that it's being invoked from. That way if the method is inherited, it knows which method it's coming from, where a staticmethod would not know. In your comment, you said this:
#classmethod
def stat(cls):
cls.another_stat()
In this example, cls is a variable that contains the class that the method is being called from, not an instance of the class that it is being called from. That is why you can call static methods with cls - because it is equivalent to Tester
I was searching for the meaning of default parameters object,self that are present as default class and function parameters, so moving away from it, if we are calling an attribute of a class should we use Foo (class reference) or should we use Foo() (instance of the class).
If you are reading a normal attribute then it doesn't matter. If you are binding a normal attribute then you must use the correct one in order for the code to work. If you are accessing a descriptor then you must use an instance.
The details of python's class semantics are quite well documented in the data model. Especially the __get__ semantics are at work here. Instances basically stack their namespace on top of their class' namespace and add some boilerplate for calling methods.
There are some large "it depends on what you are doing" gotchas at work here. The most important question: do you want to access class or instance attributes? Second, do you want attribute or methods?
Let's take this example:
class Foo(object):
bar = 1
baz = 2
def __init__(self, foobar="barfoo", baz=3):
self.foobar = foobar
self.baz = baz
def meth(self, param):
print self, param
#classmethod
def clsmeth(cls, param):
print cls, param
#staticmethod
def stcmeth(param):
print param
Here, bar is a class attribute, so you can get it via Foo.bar. Since instances have implicit access to their class namespace, you can also get it as Foo().bar. foobar is an instance attribute, since it is never bound to the class (only instances, i.e. selfs) - you can only get it as Foo().foobar. Last, baz is both a class and an instance attribute. By default, Foo.baz == 2 and Foo().baz == 3, since the class attribute is hidden by the instance attribute set in __init__.
Similarly, in an assignment there are slight differences whether you work on the class or an instance. Foo.bar=2 will set the class attribute (also for all instances) while Foo().bar=2 will create an instance attribute that shadows the class attribute for this specific instance.
For methods, it is somewhat similar. However, here you get the implicit self parameter for instance method (what a function is if defined for a class). Basically, the call Foo().meth(param=x) is silently translated to Foo.meth(self=Foo(), param=x). This is why it is usually not valid to call Foo.meth(param=x) - meth is not "bound" to an instance and thus lacks the self parameter.
Now, sometimes you do not need any instance data in a method - for example, you have strict string transformation that is an implementation detail of a larger parser class. This is where #classmethod and #staticmethod come into play. A classmethod's first parameter is always the class, as opposed to the instance for regular methods. Foo().clsmeth(param=x) and Foo.clsmeth(param=x) result in a call of clsmethod(cls=Foo, param=x). Here, the two are equivalent. Going one step further, a staticmethod doesn't get any class or instance information - it is like a raw function bound to the classes namespace.
This question already has answers here:
What is the purpose of the `self` parameter? Why is it needed?
(26 answers)
Why do we use __init__ in Python classes?
(9 answers)
Closed 6 months ago.
I'm learning the Python programming language and I've came across something I don't fully understand.
In a method like:
def method(self, blah):
def __init__(?):
....
....
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
I think they might be OOP constructs, but I don't know very much.
In this code:
class A(object):
def __init__(self):
self.x = 'Hello'
def method_a(self, foo):
print self.x + ' ' + foo
... the self variable represents the instance of the object itself. Most object-oriented languages pass this as a hidden parameter to the methods defined on an object; Python does not. You have to declare it explicitly. When you create an instance of the A class and call its methods, it will be passed automatically, as in ...
a = A() # We do not pass any argument to the __init__ method
a.method_a('Sailor!') # We only pass a single argument
The __init__ method is roughly what represents a constructor in Python. When you call A() Python creates an object for you, and passes it as the first parameter to the __init__ method. Any additional parameters (e.g., A(24, 'Hello')) will also get passed as arguments--in this case causing an exception to be raised, since the constructor isn't expecting them.
Yep, you are right, these are oop constructs.
__init__ is the constructor for a class. The self parameter refers to the instance of the object (like this in C++).
class Point:
def __init__(self, x, y):
self._x = x
self._y = y
The __init__ method gets called after memory for the object is allocated:
x = Point(1,2)
It is important to use the self parameter inside an object's method if you want to persist the value with the object. If, for instance, you implement the __init__ method like this:
class Point:
def __init__(self, x, y):
_x = x
_y = y
Your x and y parameters would be stored in variables on the stack and would be discarded when the init method goes out of scope. Setting those variables as self._x and self._y sets those variables as members of the Point object (accessible for the lifetime of the object).
N.B. Some clarification of the use of the word "constructor" in this answer. Technically the responsibilities of a "constructor" are split over two methods in Python. Those methods are __new__ (responsible for allocating memory) and __init__ (as discussed here, responsible for initialising the newly created instance).
A brief illustrative example
In the hope it might help a little, here's a simple example I used to understand the difference between a variable declared inside a class, and a variable declared inside an __init__ function:
class MyClass(object):
i = 123
def __init__(self):
self.i = 345
a = MyClass()
print(a.i)
print(MyClass.i)
Output:
345
123
In short:
self as it suggests, refers to itself- the object which has called the method. That is, if you have N objects calling the method, then self.a will refer to a separate instance of the variable for each of the N objects. Imagine N copies of the variable a for each object
__init__ is what is called as a constructor in other OOP languages such as C++/Java. The basic idea is that it is a special method which is automatically called when an object of that Class is created
Class objects support two kinds of operations: attribute references and instantiation
Attribute references use the standard syntax used for all attribute references in Python: obj.name. Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:
class MyClass:
"""A simple example class"""
i = 12345
def f(self):
return 'hello world'
then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class".
Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example:
x = MyClass()
The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__(), like this:
def __init__(self):
self.data = []
When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance. So in this example, a new, initialized instance can be obtained by:
x = MyClass()
Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,
class Complex:
def __init__(self, realpart, imagpart):
self.r = realpart
self.i = imagpart
x = Complex(3.0, -4.5)
x.r, x.i
Taken from official documentation which helped me the most in the end.
Here is my example
class Bill():
def __init__(self,apples,figs,dates):
self.apples = apples
self.figs = figs
self.dates = dates
self.bill = apples + figs + dates
print ("Buy",self.apples,"apples", self.figs,"figs
and",self.dates,"dates.
Total fruitty bill is",self.bill," pieces of fruit :)")
When you create instance of class Bill:
purchase = Bill(5,6,7)
You get:
> Buy 5 apples 6 figs and 7 dates. Total fruitty bill is 18 pieces of
> fruit :)
__init__ does act like a constructor. You'll need to pass "self" to any class functions as the first argument if you want them to behave as non-static methods. "self" are instance variables for your class.
Try out this code. Hope it helps many C programmers like me to Learn Py.
#! /usr/bin/python2
class Person:
'''Doc - Inside Class '''
def __init__(self, name):
'''Doc - __init__ Constructor'''
self.n_name = name
def show(self, n1, n2):
'''Doc - Inside Show'''
print self.n_name
print 'Sum = ', (n1 + n2)
def __del__(self):
print 'Destructor Deleting object - ', self.n_name
p=Person('Jay')
p.show(2, 3)
print p.__doc__
print p.__init__.__doc__
print p.show.__doc__
Output:
Jay
Sum = 5
Doc - Inside Class
Doc - __init__ Constructor
Doc - Inside Show
Destructor Deleting object - Jay
Had trouble undestanding this myself. Even after reading the answers here.
To properly understand the __init__ method you need to understand self.
The self Parameter
The arguments accepted by the __init__ method are :
def __init__(self, arg1, arg2):
But we only actually pass it two arguments :
instance = OurClass('arg1', 'arg2')
Where has the extra argument come from ?
When we access attributes of an object we do it by name (or by reference). Here instance is a reference to our new object. We access the printargs method of the instance object using instance.printargs.
In order to access object attributes from within the __init__ method we need a reference to the object.
Whenever a method is called, a reference to the main object is passed as the first argument. By convention you always call this first argument to your methods self.
This means in the __init__ method we can do :
self.arg1 = arg1
self.arg2 = arg2
Here we are setting attributes on the object. You can verify this by doing the following :
instance = OurClass('arg1', 'arg2')
print instance.arg1
arg1
values like this are known as object attributes. Here the __init__ method sets the arg1 and arg2 attributes of the instance.
source: http://www.voidspace.org.uk/python/articles/OOP.shtml#the-init-method
note that self could actually be any valid python identifier. For example, we could just as easily write, from Chris B's example:
class A(object):
def __init__(foo):
foo.x = 'Hello'
def method_a(bar, foo):
print bar.x + ' ' + foo
and it would work exactly the same. It is however recommended to use self because other pythoners will recognize it more easily.
What does self do? What is it meant to be? Is it mandatory?
The first argument of every class method, including init, is always a reference to the current instance of the class. By convention, this argument is always named self. In the init method, self refers to the newly created object; in other class methods, it refers to the instance whose method was called.
Python doesn't force you on using "self". You can give it any name you want. But remember the first argument in a method definition is a reference to the object. Python adds the self argument to the list for you; you do not need to include it when you call the methods.
if you didn't provide self in init method then you will get an error
TypeError: __init___() takes no arguments (1 given)
What does the init method do? Why is it necessary? (etc.)
init is short for initialization. It is a constructor which gets called when you make an instance of the class and it is not necessary. But usually it our practice to write init method for setting default state of the object. If you are not willing to set any state of the object initially then you don't need to write this method.
__init__ is basically a function which will "initialize"/"activate" the properties of the class for a specific object, once created and matched to the corresponding class..
self represents that object which will inherit those properties.
Basically, you need to use the 'self' keyword when using a variable in multiple functions within the same class. As for init, it's used to setup default values incase no other functions from within that class are called.
The 'self' is a reference to the class instance
class foo:
def bar(self):
print "hi"
Now we can create an instance of foo and call the method on it, the self parameter is added by Python in this case:
f = foo()
f.bar()
But it can be passed in as well if the method call isn't in the context of an instance of the class, the code below does the same thing
f = foo()
foo.bar(f)
Interestingly the variable name 'self' is just a convention. The below definition will work exactly the same.. Having said that it is very strong convention which should be followed always, but it does say something about flexible nature of the language
class foo:
def bar(s):
print "hi"
Just a demo for the question.
class MyClass:
def __init__(self):
print('__init__ is the constructor for a class')
def __del__(self):
print('__del__ is the destructor for a class')
def __enter__(self):
print('__enter__ is for context manager')
return self
def __exit__(self, exc_type, exc_value, traceback):
print('__exit__ is for context manager')
def greeting(self):
print('hello python')
if __name__ == '__main__':
with MyClass() as mycls:
mycls.greeting()
$ python3 class.objects_instantiation.py
__init__ is the constructor for a class
__enter__ is for context manager
hello python
__exit__ is for context manager
__del__ is the destructor for a class
In this code:
class Cat:
def __init__(self, name):
self.name = name
def info(self):
print 'I am a cat and I am called', self.name
Here __init__ acts as a constructor for the class and when an object is instantiated, this function is called. self represents the instantiating object.
c = Cat('Kitty')
c.info()
The result of the above statements will be as follows:
I am a cat and I am called Kitty
# Source: Class and Instance Variables
# https://docs.python.org/2/tutorial/classes.html#class-and-instance-variables
class MyClass(object):
# class variable
my_CLS_var = 10
# sets "init'ial" state to objects/instances, use self argument
def __init__(self):
# self usage => instance variable (per object)
self.my_OBJ_var = 15
# also possible, class name is used => init class variable
MyClass.my_CLS_var = 20
def run_example_func():
# PRINTS 10 (class variable)
print MyClass.my_CLS_var
# executes __init__ for obj1 instance
# NOTE: __init__ changes class variable above
obj1 = MyClass()
# PRINTS 15 (instance variable)
print obj1.my_OBJ_var
# PRINTS 20 (class variable, changed value)
print MyClass.my_CLS_var
run_example_func()
Here, the guy has written pretty well and simple: https://www.jeffknupp.com/blog/2014/06/18/improve-your-python-python-classes-and-object-oriented-programming/
Read above link as a reference to this:
self? So what's with that self parameter to all of the Customer
methods? What is it? Why, it's the instance, of course! Put another
way, a method like withdraw defines the instructions for withdrawing
money from some abstract customer's account. Calling
jeff.withdraw(100.0) puts those instructions to use on the jeff
instance.
So when we say def withdraw(self, amount):, we're saying, "here's how
you withdraw money from a Customer object (which we'll call self) and
a dollar figure (which we'll call amount). self is the instance of the
Customer that withdraw is being called on. That's not me making
analogies, either. jeff.withdraw(100.0) is just shorthand for
Customer.withdraw(jeff, 100.0), which is perfectly valid (if not often
seen) code.
init self may make sense for other methods, but what about init? When we call init, we're in the process of creating an object, so how can there already be a self? Python allows us to extend
the self pattern to when objects are constructed as well, even though
it doesn't exactly fit. Just imagine that jeff = Customer('Jeff
Knupp', 1000.0) is the same as calling jeff = Customer(jeff, 'Jeff
Knupp', 1000.0); the jeff that's passed in is also made the result.
This is why when we call init, we initialize objects by saying
things like self.name = name. Remember, since self is the instance,
this is equivalent to saying jeff.name = name, which is the same as
jeff.name = 'Jeff Knupp. Similarly, self.balance = balance is the same
as jeff.balance = 1000.0. After these two lines, we consider the
Customer object "initialized" and ready for use.
Be careful what you __init__
After init has finished, the caller can rightly assume that the
object is ready to use. That is, after jeff = Customer('Jeff Knupp',
1000.0), we can start making deposit and withdraw calls on jeff; jeff is a fully-initialized object.
Python __init__ and self what do they do?
What does self do? What is it meant to be? Is it mandatory?
What does the __init__ method do? Why is it necessary? (etc.)
The example given is not correct, so let me create a correct example based on it:
class SomeObject(object):
def __init__(self, blah):
self.blah = blah
def method(self):
return self.blah
When we create an instance of the object, the __init__ is called to customize the object after it has been created. That is, when we call SomeObject with 'blah' below (which could be anything), it gets passed to the __init__ function as the argument, blah:
an_object = SomeObject('blah')
The self argument is the instance of SomeObject that will be assigned to an_object.
Later, we might want to call a method on this object:
an_object.method()
Doing the dotted lookup, that is, an_object.method, binds the instance to an instance of the function, and the method (as called above) is now a "bound" method - which means we do not need to explicitly pass the instance to the method call.
The method call gets the instance because it was bound on the dotted lookup, and when called, then executes whatever code it was programmed to perform.
The implicitly passed self argument is called self by convention. We could use any other legal Python name, but you will likely get tarred and feathered by other Python programmers if you change it to something else.
__init__ is a special method, documented in the Python datamodel documentation. It is called immediately after the instance is created (usually via __new__ - although __new__ is not required unless you are subclassing an immutable datatype).