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Why is that __init__ function of python doesn't have a return statement even though its a function

This may be a silly question but i am curious to know the answer.
As per official documentation, __init__ doesn't need return statement. Any particular reason why is it that way.
>>> class Complex:
... def __init__(self, realpart, imagpart):
... self.r = realpart
... self.i = imagpart
...
>>> x = Complex(3.0, -4.5)
>>> x.r, x.i
(3.0, -4.5)

__init__() is not a normal function. It is a special method Python uses to customize an instance of a class. It is part of Python's data model:
Called after the instance has been created (by __new__()), but before it is returned to the caller[...].
As you can see from above, when you create a new instance of a class, Python first calls __new_() - which is also a special method - to create a new instance of the class. Then __init__() is called to customize the new instance.
It wouldn't make sense to return anything from __init__(), since the class instance is already created. In fact, Python goes as far as raising an error to prevent this:
>>> class A:
... def __init__(self):
... return 'foo'
...
>>> A()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __init__() should return None, not 'str'
>>>
If you want to know what exactly is going on behind the scenes, #eryksun provides a nice explanation:
To completely explain this story, you have to step back to the metaclass __call__ method. In particular the default type.__call__ in CPython calls __new__ and __init__ via their C slot functions, and it's slot_tp_init (defined in Objects/typeobject.c) that enforces the return value to be None. If you use a custom metaclass that overrides type.__call__, it can manually call the __new__ and __init__ methods of the class with no restriction on what __init__ can return -- as silly as that would be.

__init__ is called when you create a new instance of a class.
It's main use is initializing the instance variables, and it can be called only with an instance - so you can't call it before you create an instance anyways (what triggers it automatically).
For these reasons, __init__s have no reason to be able to return any value - it's simply not their use case.

How come an object that implements __iter__ is not recognized as iterable?

Let's say you work with a wrapper object:
class IterOrNotIter:
def __init__(self):
self.f = open('/tmp/toto.txt')
def __getattr__(self, item):
try:
return self.__getattribute__(item)
except AttributeError:
return self.f.__getattribute__(item)
This object implements __iter__, because it passes any call to it to its member f, which implements it. Case in point:
>>> x = IterOrNotIter()
>>> x.__iter__().__next__()
'Whatever was in /tmp/toto.txt\n'
According to the documentation (https://docs.python.org/3/library/stdtypes.html#iterator-types), IterOrNotIter should thus be iterable.
However, the Python interpreter does not recognize an IterOrNotIter object as actually being iterable:
>>> x = IterOrNotIter()
>>> for l in x:
... print(l)
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'IterOrNotIter' object is not iterable
Whereas this works:
>>> x = IterOrNotIter()
>>> for l in x.f:
... print(l)
...
Whatever was in /tmp/toto.txt
I don't understand why.

Basically because your class just doesn't have a real __iter__ method:
>>> hasattr(IterOrNotIter, '__iter__')
False
So it doesn't qualify as iterator because the actual check for __iter__ checks for the existence instead of assuming it's implemented. So workarounds with __getattr__ or __getattribute__ (unfortunatly) don't work.
This is actually mentioned in the documentation for __getattribute__:
Note
This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or built-in functions. See Special method lookup.
The latter section also explains the why:
Bypassing the __getattribute__() machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method must be set on the class object itself in order to be consistently invoked by the interpreter).
Emphasis mine.

Setting attributes on a method after class creation raises "'instancemethod' object has no attribute" but the attribiute is clearly there

In Python (2 and 3) we can assign attributes to function:
>>> class A(object):
... def foo(self):
... """ This is obviously just an example """
... return "FOO{}!!".format(self.foo.bar)
... foo.bar = 123
...
>>> a = A()
>>> a.foo()
'FOO123!!'
And that's cool.
But why cannot we change foo.bar at a later time? For example, in the constructor, like so:
>>> class A(object):
... def __init__(self, *args, **kwargs):
... super(A, self).__init__(*args, **kwargs)
... print(self.foo.bar)
... self.foo.bar = 456 # KABOOM!
... def foo(self):
... """ This is obviously just an example """
... return "FOO{}!!".format(self.foo.bar)
... foo.bar = 123
...
>>> a = A()
123
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in __init__
AttributeError: 'instancemethod' object has no attribute 'bar'
Python claims there is no bar even though it printed it fine on just the previous line.
Same error happens if we try to change it directly on the class:
>>> A.foo.bar
123
>>> A.foo.bar = 345 # KABOOM!
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'instancemethod' object has no attribute 'bar'
What's happening here, i.e. why are we seeing this behaviour?
Is there a way to set attributes on a function after class creation?
(I'm aware of multiple alternatives, but I'm explicitly wondering about attributes on methods here, or possibly a broader issue.)
Motivation: Django makes use of the possibility to set attributes on methods, e.g:
class MyModelAdmin(ModelAdmin):
...
def custom_admin_column(self, obj):
return obj.something()
custom_admin_column.admin_order_field ='relation__field__span'
custom_admin_column.allow_tags = True

Setting foo.bar inside the class body works because foo is the actual foo function. However, when you do
self.foo.bar = 456
self.foo isn't that function. self.foo is an instance method object, created on demand when you access it. You can't set attributes on it for several reasons:
If those attributes are stored on the foo function, then assigning to a.foo.bar has an unexpected effect on b.foo.bar, contrary to all the usual expectations about attribute assignment.
If those attributes are stored on the self.foo instance method object, they won't show up the next time you access self.foo, because you'll get a new instance method object next time.
If those attributes are stored on the self.foo instance method object and you change the rules so self.foo is always the same object, then that massively bloats every object in Python to store a bunch of instance method objects you almost never need.
If those attributes are stored in self.__dict__, what about objects that don't have a __dict__? Also, you'd need to come up with some sort of name mangling rule, or store non-string keys in self.__dict__, both of which have their own problems.
If you want to set attributes on the foo function after the class definition is done, you can do that with A.__dict__['foo'].bar = 456. (I've used A.__dict__ to bypass the issue of whether A.foo is the function or an unbound method object, which depends on your Python version. If A inherits foo, you'll have to either deal with that issue or access the dict of the class it inherits foo from.)

Why isn't self being automatically passed to a method that is set on an object after its instantiation?

class Person():
pass;
def say_hi(self):
print 'hii'
me=Person()
me.say_hi=say_hi
me.say_hi()
Isn't the self argument automatically passed in python ? why why is calling me.say_hi() is giving a stack trace ?
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: say_hi() takes exactly 1 argument (0 given)

It's not passed in the way that you are doing it.
You would have to do.
import types
me.say_hi = types.MethodType(say_hi, me, Person)
for it to work.
When python instantiates a class, it essentially carries out the above procedure for each of the class methods. When you 'monkey-patch' a method onto an object in the way that you were trying to do it, it's not a bound method and just exists as a function in instance.__dict__. Calling it is no different than calling any other function. If you want to stick a method on an instance, you have to manually make it a method as shown above.
If you were to do
class Person(object):
pass
def say_hi(self):
print 'hii'
Person.say_hi = say_hi
me = Person()
me.say_hi()
then it would work because Python will create the method for you.
Chris Morgan put up an answer that shows this one in action. It's good stuff.

(This can act as some demonstration for aaronasterling's answer.)
Here are the definitions:
>>> class Person(object):
... def bound(self):
... print "Hi, I'm bound."
...
>>> def unbound(self):
... print "Hi, I'm unbound."
...
Note the types of these methods and functions.
>>> type(Person.bound)
<type 'instancemethod'>
>>> type(Person().bound)
<type 'instancemethod'>
>>> type(unbound)
<type 'function'>
>>> Person.unbound = unbound
When it gets set on the Person before instantiation, it gets bound.
>>> Person().bound()
Hi, I'm bound.
>>> Person().unbound()
Hi, I'm unbound.
However, if it's set after instantiation, it's still of type 'function'.
>>> me = Person()
>>> me.rebound = unbound
>>> type(me.rebound)
<type 'function'>
>>> type(me.unbound)
<type 'instancemethod'>
>>> me.rebound
<function unbound at 0x7fa05efac9b0>
>>> me.rebound()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unbound() takes exactly 1 argument (0 given)
The 'instancemethod' type can be used to bind a 'function' to an object. It's in the types module as MethodType.
>>> import types
>>> me.rebound = types.MethodType(unbound, me, Person)
Now it's bound properly.
>>> type(me.rebound)
<type 'instancemethod'>
>>> me.rebound()
Hi, I'm unbound.
>>> # Not true any more!

In this case say_hi is not method of your class. It is just reference to a function. This is why the self argument is not passed automatically.
Or just use:
class Person():
def say_hi(self):
print 'hii'
me=Person() me.say_hi=say_hi me.say_hi()

The self argument of a method is passed automatically. You don't have a method, but a function that is an attribute of an object. If you did Person.say_hi = say_hi, then Person().say_hi() would work as expected. A method is a function that's attribute of a class, not an instance, and self is passed only for methods.
Class attributes define how the instances should work, while instance attributes are just normal you access. This means that class attributes are modified when they are accessed from an instance (e.g. functions are turned into methods), while instance attributes are left unchanged.
>>> class A(object): pass
...
>>> def f(self): print self
...
>>> ob = A()
>>> A.f = f
>>> ob.g = f
>>> print ob.f
<bound method A.f of <__main__.A object at 0xb74204ec>>
>>> print ob.g
<function f at 0xb7412ae4>
>>> ob.f()
<__main__.A object at 0xb74204ec>
>>> ob.g('test')
test
Since A is a class, f, A().f and A.f are different things. Since ob is an object, f and ob.g are the same thing.

because the function say_hi() wasn't defined inside of the person class it doesn't know what self is, and when you call it it isn't passing self to the method. This would be like calling a static method.
you could do this though
me=Person()
me.say_hi=say_hi
me.say_hi(me)

No, self is not automatically passed to the object, because it has not been defined within the class block. Instead, you have defined a function, say_hi, in the wrong block. When you run it, 'self' in this context, is actually the first parameter of a function which is outside the class block, and therefore not a part of the class, hence the error.

i think you wanted to do this
say_hi(me)
but the usual way to program OO is this:
class Person:
def say_hi(self):
print 'hii'
me = Person()
me.say_hi()

maybe this works?
class Person():
def say_hi(self):
print 'hii'
me=Person()
me.say_hi()
I put the function inside the class because I feel thats what you wanted. Then you can call it later from the class object me.

Class method differences in Python: bound, unbound and static

What is the difference between the following class methods?
Is it that one is static and the other is not?
class Test(object):
def method_one(self):
print "Called method_one"
def method_two():
print "Called method_two"
a_test = Test()
a_test.method_one()
a_test.method_two()

In Python, there is a distinction between bound and unbound methods.
Basically, a call to a member function (like method_one), a bound function
a_test.method_one()
is translated to
Test.method_one(a_test)
i.e. a call to an unbound method. Because of that, a call to your version of method_two will fail with a TypeError
>>> a_test = Test()
>>> a_test.method_two()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: method_two() takes no arguments (1 given)
You can change the behavior of a method using a decorator
class Test(object):
def method_one(self):
print "Called method_one"
#staticmethod
def method_two():
print "Called method two"
The decorator tells the built-in default metaclass type (the class of a class, cf. this question) to not create bound methods for method_two.
Now, you can invoke static method both on an instance or on the class directly:
>>> a_test = Test()
>>> a_test.method_one()
Called method_one
>>> a_test.method_two()
Called method_two
>>> Test.method_two()
Called method_two

Methods in Python are a very, very simple thing once you understood the basics of the descriptor system. Imagine the following class:
class C(object):
def foo(self):
pass
Now let's have a look at that class in the shell:
>>> C.foo
<unbound method C.foo>
>>> C.__dict__['foo']
<function foo at 0x17d05b0>
As you can see if you access the foo attribute on the class you get back an unbound method, however inside the class storage (the dict) there is a function. Why's that? The reason for this is that the class of your class implements a __getattribute__ that resolves descriptors. Sounds complex, but is not. C.foo is roughly equivalent to this code in that special case:
>>> C.__dict__['foo'].__get__(None, C)
<unbound method C.foo>
That's because functions have a __get__ method which makes them descriptors. If you have an instance of a class it's nearly the same, just that None is the class instance:
>>> c = C()
>>> C.__dict__['foo'].__get__(c, C)
<bound method C.foo of <__main__.C object at 0x17bd4d0>>
Now why does Python do that? Because the method object binds the first parameter of a function to the instance of the class. That's where self comes from. Now sometimes you don't want your class to make a function a method, that's where staticmethod comes into play:
class C(object):
#staticmethod
def foo():
pass
The staticmethod decorator wraps your class and implements a dummy __get__ that returns the wrapped function as function and not as a method:
>>> C.__dict__['foo'].__get__(None, C)
<function foo at 0x17d0c30>
Hope that explains it.

When you call a class member, Python automatically uses a reference to the object as the first parameter. The variable self actually means nothing, it's just a coding convention. You could call it gargaloo if you wanted. That said, the call to method_two would raise a TypeError, because Python is automatically trying to pass a parameter (the reference to its parent object) to a method that was defined as having no parameters.
To actually make it work, you could append this to your class definition:
method_two = staticmethod(method_two)
or you could use the #staticmethod function decorator.

>>> class Class(object):
... def __init__(self):
... self.i = 0
... def instance_method(self):
... self.i += 1
... print self.i
... c = 0
... #classmethod
... def class_method(cls):
... cls.c += 1
... print cls.c
... #staticmethod
... def static_method(s):
... s += 1
... print s
...
>>> a = Class()
>>> a.class_method()
1
>>> Class.class_method() # The class shares this value across instances
2
>>> a.instance_method()
1
>>> Class.instance_method() # The class cannot use an instance method
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unbound method instance_method() must be called with Class instance as first argument (got nothing instead)
>>> Class.instance_method(a)
2
>>> b = 0
>>> a.static_method(b)
1
>>> a.static_method(a.c) # Static method does not have direct access to
>>> # class or instance properties.
3
>>> Class.c # a.c above was passed by value and not by reference.
2
>>> a.c
2
>>> a.c = 5 # The connection between the instance
>>> Class.c # and its class is weak as seen here.
2
>>> Class.class_method()
3
>>> a.c
5

method_two won't work because you're defining a member function but not telling it what the function is a member of. If you execute the last line you'll get:
>>> a_test.method_two()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: method_two() takes no arguments (1 given)
If you're defining member functions for a class the first argument must always be 'self'.

Accurate explanation from Armin Ronacher above, expanding on his answers so that beginners like me understand it well:
Difference in the methods defined in a class, whether static or instance method(there is yet another type - class method - not discussed here so skipping it), lay in the fact whether they are somehow bound to the class instance or not. For example, say whether the method receives a reference to the class instance during runtime
class C:
a = []
def foo(self):
pass
C # this is the class object
C.a # is a list object (class property object)
C.foo # is a function object (class property object)
c = C()
c # this is the class instance
The __dict__ dictionary property of the class object holds the reference to all the properties and methods of a class object and thus
>>> C.__dict__['foo']
<function foo at 0x17d05b0>
the method foo is accessible as above. An important point to note here is that everything in python is an object and so references in the dictionary above are themselves pointing to other objects. Let me call them Class Property Objects - or as CPO within the scope of my answer for brevity.
If a CPO is a descriptor, then python interpretor calls the __get__() method of the CPO to access the value it contains.
In order to determine if a CPO is a descriptor, python interpretor checks if it implements the descriptor protocol. To implement descriptor protocol is to implement 3 methods
def __get__(self, instance, owner)
def __set__(self, instance, value)
def __delete__(self, instance)
for e.g.
>>> C.__dict__['foo'].__get__(c, C)
where
self is the CPO (it could be an instance of list, str, function etc) and is supplied by the runtime
instance is the instance of the class where this CPO is defined (the object 'c' above) and needs to be explicity supplied by us
owner is the class where this CPO is defined(the class object 'C' above) and needs to be supplied by us. However this is because we are calling it on the CPO. when we call it on the instance, we dont need to supply this since the runtime can supply the instance or its class(polymorphism)
value is the intended value for the CPO and needs to be supplied by us
Not all CPO are descriptors. For example
>>> C.__dict__['foo'].__get__(None, C)
<function C.foo at 0x10a72f510>
>>> C.__dict__['a'].__get__(None, C)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute '__get__'
This is because the list class doesnt implement the descriptor protocol.
Thus the argument self in c.foo(self) is required because its method signature is actually this C.__dict__['foo'].__get__(c, C) (as explained above, C is not needed as it can be found out or polymorphed)
And this is also why you get a TypeError if you dont pass that required instance argument.
If you notice the method is still referenced via the class Object C and the binding with the class instance is achieved via passing a context in the form of the instance object into this function.
This is pretty awesome since if you chose to keep no context or no binding to the instance, all that was needed was to write a class to wrap the descriptor CPO and override its __get__() method to require no context.
This new class is what we call a decorator and is applied via the keyword #staticmethod
class C(object):
#staticmethod
def foo():
pass
The absence of context in the new wrapped CPO foo doesnt throw an error and can be verified as follows:
>>> C.__dict__['foo'].__get__(None, C)
<function foo at 0x17d0c30>
Use case of a static method is more of a namespacing and code maintainability one(taking it out of a class and making it available throughout the module etc).
It maybe better to write static methods rather than instance methods whenever possible, unless ofcourse you need to contexualise the methods(like access instance variables, class variables etc). One reason is to ease garbage collection by not keeping unwanted reference to objects.

that is an error.
first of all, first line should be like this (be careful of capitals)
class Test(object):
Whenever you call a method of a class, it gets itself as the first argument (hence the name self) and method_two gives this error
>>> a.method_two()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: method_two() takes no arguments (1 given)

The second one won't work because when you call it like that python internally tries to call it with the a_test instance as the first argument, but your method_two doesn't accept any arguments, so it wont work, you'll get a runtime error.
If you want the equivalent of a static method you can use a class method.
There's much less need for class methods in Python than static methods in languages like Java or C#. Most often the best solution is to use a method in the module, outside a class definition, those work more efficiently than class methods.

The call to method_two will throw an exception for not accepting the self parameter the Python runtime will automatically pass it.
If you want to create a static method in a Python class, decorate it with the staticmethod decorator.
Class Test(Object):
#staticmethod
def method_two():
print "Called method_two"
Test.method_two()

Please read this docs from the Guido First Class everything Clearly explained how Unbound, Bound methods are born.

Bound method = instance method
Unbound method = static method.

The definition of method_two is invalid. When you call method_two, you'll get TypeError: method_two() takes 0 positional arguments but 1 was given from the interpreter.
An instance method is a bounded function when you call it like a_test.method_two(). It automatically accepts self, which points to an instance of Test, as its first parameter. Through the self parameter, an instance method can freely access attributes and modify them on the same object.

Unbound Methods
Unbound methods are methods that are not bound to any particular class instance yet.
Bound Methods
Bound methods are the ones which are bound to a specific instance of a class.
As its documented here, self can refer to different things depending on the function is bound, unbound or static.
Take a look at the following example:
class MyClass:
def some_method(self):
return self # For the sake of the example
>>> MyClass().some_method()
<__main__.MyClass object at 0x10e8e43a0># This can also be written as:>>> obj = MyClass()
>>> obj.some_method()
<__main__.MyClass object at 0x10ea12bb0>
# Bound method call:
>>> obj.some_method(10)
TypeError: some_method() takes 1 positional argument but 2 were given
# WHY IT DIDN'T WORK?
# obj.some_method(10) bound call translated as
# MyClass.some_method(obj, 10) unbound method and it takes 2
# arguments now instead of 1
# ----- USING THE UNBOUND METHOD ------
>>> MyClass.some_method(10)
10
Since we did not use the class instance — obj — on the last call, we can kinda say it looks like a static method.
If so, what is the difference between MyClass.some_method(10) call and a call to a static function decorated with a #staticmethod decorator?
By using the decorator, we explicitly make it clear that the method will be used without creating an instance for it first. Normally one would not expect the class member methods to be used without the instance and accesing them can cause possible errors depending on the structure of the method.
Also, by adding the #staticmethod decorator, we are making it possible to be reached through an object as well.
class MyClass:
def some_method(self):
return self
#staticmethod
def some_static_method(number):
return number
>>> MyClass.some_static_method(10) # without an instance
10
>>> MyClass().some_static_method(10) # Calling through an instance
10
You can’t do the above example with the instance methods. You may survive the first one (as we did before) but the second one will be translated into an unbound call MyClass.some_method(obj, 10) which will raise a TypeError since the instance method takes one argument and you unintentionally tried to pass two.
Then, you might say, “if I can call static methods through both an instance and a class, MyClass.some_static_method and MyClass().some_static_method should be the same methods.” Yes!