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Why is binding a class instance method different from binding a class method?

I was reading the python docs and stumbled upon the following lines:
It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class.
Please, someone explain what does that mean in plain english.
I'm going to introduce some shorthand notation:
let 'user-defined functions' be denoted by f,
let 'class instance' be denoted by ci while class denoted simply by c. Obviously(?), ci = c(), with some abuse of notation.
Also, allow membership statements to be recast in simple set notation eg 'user-defined functions which are attributes of a class instance' in shorthand is 'vf: fεa(ci)', where v: 'for all' and where 'a' is the shorthand for (set of) attributes (eg of a class or class instance) and 'ε' denotes the set membership function.
Also, the process of binding a function is described in shorthand by ci.f(*args) or c.f(*args) => f(ci, *args) or f(c, *args) (the former referring to an instance method call while the later referring to a class method call)
Using the newly introduced shorthand notation, does the quote from the docs imply that
vf: fεa(c), c.f(*args) => f(c, *args) is a true statement
while
vf: fεa(ci), ci.f(*args) => f(ci, *args) is false?

Setting a User Defined Method to be an Attribute of Class, The Wrong Way
Consider the following example class A and function f:
class A:
pass
def f(self):
print("I\'m in user-defined function")
a = A()
The function f is defined separately and not inside the class.
Let's say you want to add function f to be an instance method for a object.
Adding it, by setting f as a attribute, won't work:
import types
class A:
pass
def f(self):
print("I\'m in user-defined function")
a = A()
a.f = f
# <function f at 0x000002D81F0DED30>
print(a.f)
# TypeError: f() missing 1 required positional argument: 'self'
# a.f()
Because function f is not bound to the object a.
That is why when calling a.f() it shall raise an error regarding the missing argument (if f has been bounded to a, that object a was the missing argument self).
This part is what the docs referred at:
It is also important to note that user-defined functions which are attributes of a class instance are not converted to bound methods.
Of course, all this has not to happen if function f has been defined inside class A, that's what the following part from the docs states:
...this only happens when the function is an attribute of the class.
Setting a User Defined Method to be an Attribute of Class, The Right Way
To add function f to object a you should use:
import types
class A:
pass
def f(self):
print("I\'m in user-defined function")
a = A()
a.f = types.MethodType( f, a )
# <bound method f of <__main__.A object at 0x000001EDE4768E20>>
print(a.f)
# Works! I'm in user-defined function
a.f()
Which bounds the user-defined method f to instance a.

When you create a method the usual way, it will be a bound method: it receives the instance as first argument (which we usually assign to 'self'):
class A:
def meth(*args):
print(args)
a = A()
a.meth()
# (<__main__.A object at 0x7f56a137fd60>,)
If you take an ordinary function and add it to the class attributes, it will work the same way:
def f(*args):
print(args)
A.f = f
a = A()
a.f()
# (<__main__.A object at 0x7f56a137f700>,)
The instance gets passed as first argument, it is a bound method.
If, on the other side, you make the function an attribute of an instance of the class, it won't be a bound method = it won't be passed the instance as first argument when called:
a = A()
a.f = f
a.f()
# ()

I don't think the fancy-schmancy formal logic notation is helping here.
However, to answer the question: what does "user-defined functions which are attributes of a class instance are not converted to bound methods; this only happens when the function is an attribute of the class" mean?
A bound method is one which is dependent on the instance of the class as the first argument. It passes the instance as the first argument which is used to access the variables and functions. In Python 3 and newer versions of python, all functions in the class are by default bound methods.
So, if you create a user-defined function as an attribute of a class instance, it is not automatically converted to a bound method. 'Class instance' is just a Python way of saying what 'object' or 'object instance' means in other languages.
For example:
class HelloClass:
greeting = 'Hello'
def greet(self, name):
print(f'{greeting} {name}')
hc = HelloClass()
hc.greet('John')
Here HelloClass is the class, while hc is the class instance. greet is a bound method, expecting at least a single parameter (called self by convention) which is automatically assigned the class instance when called - i.e. the value of self before printing hello John is the hc class instance.
Now, if you try this:
def greet_with_hi(self, name):
print(f'Hi {name}')
class HiClass:
greet = greet_with_hi
hc = HiClass()
hc.greet('John')
That works (although your IDE may object), but this doesn't work at all:
def greet_with_hi(self, name):
print(f'Hi {name}')
class HiClass:
def __init__(self):
self.greet = greet_with_hi
hc = HiClass()
hc.greet('John')
It causes TypeError: greet_with_hi() missing 1 required positional argument: 'name'. And it should, because .greet on an instance of HiClass is not a bound method and the self greet_with_hi expects won't be filled automatically.

I think the meaning is best clarified by way of example.
Suppose that we have a class instance containing various attributes that are user-defined functions.
add1 was added by defining it as part of the class definition.
add2 was added by monkey-patching the class before instantiation.
add3 was added by monkey-patching the class after instantiation
add4 was added by monkey-patching the instance after instantiation
class Number:
def __init__(self, x):
self.x = x
def add1(self):
return self.x + 1
def add2(self):
return self.x + 2
def add3(self):
return self.x + 3
def add4(self):
return self.x + 4
setattr(Number, 'add2', add2)
two = Number(2)
setattr(Number, 'add3', add3)
setattr(two, 'add4', add4)
print(two.add1()) # prints 3
print(two.add2()) # prints 4
print(two.add3()) # prints 5
print(two.add4()) # TypeError: add4() missing 1 required positional argument: 'self'
We try calling these.
The first three all work (in the case of add3 it doesn't even matter that it wasn't an attribute of the class at the time of instantiation).
Note that when we call these, we do not explicitly pass anything corresponding to the first positional argument inside the function (i.e. self) -- it is added automatically for us. This is what is meant by it being a bound method. It is declared with one positional argument and we are explicitly passing none at all.
But in the case of add4 it is complaining about missing positional argument - this is because the instance is not automatically added as a first argument. (It would work if you used explicitly two.add4(two).)

Bound methods are based on descriptors which are documented here. There is even a section "Functions and Methods".
It is a known fact, that descriptors work only in the classes, not in instances. That is documented here.
Putting those two pieces of information together explains the difference quoted in the question:
user-defined functions which are
attributes of a class instance are not converted to bound methods;
this only happens when the function is an attribute of the class.

Bound methods python
a bound-method is the one which is dependent on the instance of the class as the first argument. It passes the instance as the first argument which is used to access the variables and functions. In Python 3 and newer versions of python, all functions in the class are by default bound methods.
Let’s understand this concept with an example:
# Python code to demonstrate
# use of bound methods
class A:
def func(self, arg):
self.arg = arg
print("Value of arg = ", arg)
# Creating an instance
obj = A()
# bound method
print(obj.func)
Output:
< bound method A.func of <__main__.A object at 0x7fb81c5a09e8>>
Here,
obj.func(arg) is translated by python as A.func(obj, arg).
The instance obj is automatically passed as the first argument to the function called and hence the first parameter of the function will be used to access the variables/functions of the object.
Let’s see another example of the Bound method.
# Python code to demonstrate
# use of bound methods
class Car:
# Car class created
gears = 5
# a class method to change the number of gears
#classmethod
def change_gears(cls, gears):
cls.gears = gears
# instance of class Car created
Car1 = Car()
print("Car1 gears before calling change_gears() = ", Car1.gears)
Car1.change_gears(6)
print("Gears after calling change_gears() = ", Car1.gears)
# bound method
print(Car1.change_gears)
Output:
Car1 gears before calling change_gears() = 5
Gears after calling change_gears() = 6
<bound method Car.change_gears of <class '__main__.Car'>>
The above code is an example of a class method. A class method is like a bound method except that the class of the instance is passed as an argument rather than the instance itself. Here in the above example when we call Car1.change_gears(6), the class ‘Car’ is passed as the first argument.
Need for these bound methods
The methods inside the classes would take at least one argument. To make them zero-argument methods, ‘decorators‘ has to be used. Different instances of a class have different values associated with them.
For example, if there is a class “Fruits”, and instances like apple, orange, mango are possible. Each instance may have a different size, color, taste, and nutrients in it. Thus to alter any value for a specific instance, the method must have ‘self’ as an argument that allows it to alter only its property.
Example:
class sample(object):
# Static variable for object number
objectNo = 0
def __init__(self, name1):
# variable to hold name
self.name = name1
# Increment static variable for each object
sample.objectNo = sample.objectNo + 1
# each object's unique number that can be
# considered as ID
self.objNumber = sample.objectNo
def myFunc(self):
print("My name is ", self.name,
"from object ", self.objNumber)
def alterIt(self, newName):
self.name = newName
def myFunc2():
print("I am not a bound method !!!")
# creating first instance of class sample
samp1 = sample("A")
samp1.myFunc()
# unhide the line below to see the error
# samp1.myFunc2() #----------> error line
# creating second instance of class sample
samp2 = sample("B")
samp2.myFunc()
samp2.alterIt("C")
samp2.myFunc()
samp1.myFunc()
Output:
My name is A from object 1
My name is B from object 2
My name is C from object 2
My name is A from object 1
In the above example two instances namely samp1 and samp2 are created. Note that when the function alterIt() is applied to the second instance, only that particular instance’s value is changed. The line samp1.myFunc() will be expanded as sample.myFunc(samp1). For this method, no explicit argument is required to be passed. The instance samp1 will be passed as an argument to the myFunc(). The line samp1.myFunc2() will generate the error :
Traceback (most recent call last):
File "/home/4f130d34a1a72402e0d26bab554c2cf6.py", line 26, in
samp1.myFunc2() #----------> error line
TypeError: myFunc2() takes 0 positional arguments but 1 was given
It means that this method is unbound. It does not accept any instance as an argument. These functions are unbound functions.
Sources:
Geeks For Geeks: Bound Methods Python
Geeks For Geeks: Bound, unbound and static methods in Python

Different way to create an instance method object in Python

The Python 3.x language reference described two ways to create a method object:
User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object or a class method object.
When an instance method object is created by retrieving a user-defined function object from a class via one of its instances, its self attribute is the instance, and the method object is said to be bound. The new method’s func attribute is the original function object.
When a user-defined method object is created by retrieving another method object from a class or instance, the behaviour is the same as for a function object, except that the func attribute of the new instance is not the original method object but its func attribute.
When an instance method object is called, the underlying function (func) is called, inserting the class instance (self) in front of the argument list. For instance, when C is a class which contains a definition for a function f(), and x is an instance of C, calling x.f(1) is equivalent to calling C.f(x, 1).
When an instance method object is derived from a class method object, the “class instance” stored in self will actually be the class itself, so that calling either x.f(1) or C.f(1) is equivalent to calling f(C,1) where f is the underlying function.
In different ways, they both have different __func__ and __self__ values, but I'm not very aware of these two different ways, could someone explain it to me?
Python Language Reference | The Standard Type Hierarchy:
https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy

I'm not 100% sure that I completely understand your question, but maybe looking at an example will be helpful. To start, lets create a class which has a function in the definition:
>>> class Foo(object):
... def method(self):
... pass
...
>>> f = Foo()
User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object or a class method object.
Ok, so we can create a method object by just accessing the attribute on an instance (if the attribute is a function). In our setup, f is an instance of the class Foo:
>>> type(f.method)
<class 'method'>
Compare that with accessing the method attribute on the class:
>>> type(Foo.method)
<class 'function'>
When an instance method object is created by retrieving a user-defined function object from a class via one of its instances, its __self__ attribute is the instance, and the method object is said to be bound. The new method’s __func__ attribute is the original function object.
This is just telling us what attributes exist on instance methods. Let's check it out:
>>> instance_method = f.method
>>> instance_method.__func__ is Foo.method
True
>>> instance_method.__self__ is f
True
So we see that the method object has a __func__ attribute which is just a reference to the actual Foo.method function. It also has a __self__ attribute that is a reference to the instance.
When an instance method object is called, the underlying function (func) is called, inserting the class instance (self) in front of the argument list. For instance, when C is a class which contains a definition for a function f(), and x is an instance of C, calling x.f(1) is equivalent to calling C.f(x, 1).
Basically, in reference to our example above, this is just saying that If:
instance_method = f.method
Then:
instance_method(arg1, arg2)
executes the following:
instance_method.__func__(instance_method.__self__, arg1, arg2)

For completeness and as an addendum to the excellent answer provided by #mgilson, I wanted to explain the remaining 2 paragraphs referenced in the original question.
First let's create a class with a classmethod:
>>> class Foo(object):
... #classmethod
... def cmethod(cls):
... pass
...
>>> f = Foo()
Now for the 3rd paragraph:
When a user-defined method object is created by retrieving another method object from a class or instance, the behaviour is the same as for a function object, except that the func attribute of the new instance is not the original method object but its func attribute.
This means:
>>> class_method = f.cmethod
>>> class_method.__func__ is Foo.cmethod.__func__
True
>>> class_method.__self__ is Foo
True
Note that __self__ is a reference to the Foo class. Finally, the last paragraph:
When an instance method object is derived from a class method object, the “class instance” stored in self will actually be the class itself, so that calling either x.f(1) or C.f(1) is equivalent to calling f(C,1) where f is the underlying function.
This just says that all of the following are equivalent:
>>> f.cmethod(arg1, arg2)
>>> Foo.cmethod(arg1, arg2)
>>> f.cmethod.__func__(Foo, arg1, arg2)
>>> Foo.cmethod.__func__(Foo, arg1, arg2)
>>> f.cmethod.__func__(f.cmethod.__self__, arg1, arg2)
>>> Foo.cmethod.__func__(Foo.cmethod.__self__, arg1, arg2)

Python 3: When is a call to an instance attribute resolved as if it is an instance method?

class C:
def func(self, a):
print(a)
c = C()
print(c.__dict__) # {}
c.func = c.func
# c.func is now an instance attribute, which hides instance method
print(c.__dict__) # {'func' : (bound method C.f of (__main.C object at ...))}
# how come it still works as if it's an instance method:
c.func(1) # prints 1
# with a regular function assigned to c.func, this won't happen:
def newfunc(self, a):
print(a)
c.func = newfunc
c.func(1) # TypeError
What are the conditions under which a call c.func(1) magically adds the instance c as the first parameter? I thought it only happens when func was defined as an instance method in the class. But it seems it sometimes works when func is just an instance attribute.

The binding to the instance happens when you access c.func, not when you call it. When you do c.func = c.func, the attribute access on the right-hand side is handled by the class C and evalutes to a bound method. Reassigning it to c.func assigns it to the instance, but it's already a bound method, so this doesn't change the behavior.
In your newfunc example, you just defined a function and assigned it, so it never got made into an instance method. Since you assign it directly to an instance, the class doesn't get a chance to intercept the access and wrap it in an instancemethod.
You can assign a bound method anywhere you like and it remains bound to the instance via which you got it. See these examples:
>>> class C(object):
... def __init__(self, name):
... self.name = name
... def func(self, a):
... print("Called {0} with {1}".format(self.name, a))
>>> one = C("one")
>>> two = C("two")
>>> one.func(1)
Called one with 1
>>> two.func(1)
Called two with 1
>>> one.func2 = two.func
>>> one.func2(1)
Called two with 1
Note that once I assigned one.func2 = two.func, calling one.func2 calls an instance method bound to two, not one. This is because when I accessed two.func, I got a method bound to that instance. It doesn't matter where I assign it later. It remains bound to the instance via which it was accessed.
If you access the unbound method directly and try to assign it elsewhere, then you get the TypeError because the method was never bound:
>>> one.func3 = C.func
>>> one.func3(1)
Traceback (most recent call last):
File "<pyshell#16>", line 1, in <module>
one.func3(1)
TypeError: unbound method func() must be called with C instance as first argument (got int instance instead)
Also note that if you add the method to the class, it will work when called on an instance, even if the instance was created before you added the method to the class. Keeping the same one object from before, I can do:
>>> def newFunc(self, a):
... print("Newfunc of {0} called with {1}".format(self.name, a))
>>> C.newFunc = newFunc
>>> one.newFunc(1)
Newfunc of one called with 1
To answer your question succinctly, c.func is bound to c if c is an instance of a class that has a method func, and c does not itself have an instance attribute func. (If c did have an instance attribute of the same name, that overrides the class one, so the class doesn't get to do its wrapping to bind the method to the instance.)

c.func(1) will magically add the instance c as the first parameter when the method c.func is bound to the object c.
Functions that are attributes of the class are automatically bound to instances of that class. So immediately after c = C(), c.func is different from C.func in that it accepts one fewer argument and c magically becomes the first argument. This is why c.func = c.func doesn't change anything, and everything works normally.
If you want to patch an instance of a class with a new instance method, you need to bind the function to the instance. Here are a few options:
Using types.MethodType:
import types
c.func = types.MethodType(newfunc, c)
Using functools.partial:
import functools
c.func = functools.partial(newfunc, c)
Note that the second method mimics the behavior, but is slightly different because type(c.func) will be <type 'functools.partial'> instead of <type 'instancemethod'>, so the first method is probably preferable.

Python newbie having a problem using classes

Im just beginning to mess around a bit with classes; however, I am running across a problem.
class MyClass(object):
def f(self):
return 'hello world'
print MyClass.f
The previous script is returning <unbound method MyClass.f> instead of the intended value. How do I fix this?

MyClass.f refers to the function object f which is a property of MyClass. In your case, f is an instance method (has a self parameter) so its called on a particular instance. Its "unbound" because you're referring to f without specifying a specific class, kind of like referring to a steering wheel without a car.
You can create an instance of MyClass and call f from it like so:
x = MyClass()
x.f()
(This specifies which instance to call f from, so you can refer to instance variables and the like.)
You're using f as a static method. These methods aren't bound to a particular class, and can only reference their parameters.
A static method would be created and used like so:
class MyClass(object):
def f(): #no self parameter
return 'hello world'
print MyClass.f()

Create an instance of your class: m = MyClass()
then use m.f() to call the function
Now you may wonder why you don't have to pass a parameter to the function (the 'self' param). It is because the instance on which you call the function is actually passed as the first parameter.
That is, MyClass.f(m) equals m.f(), where m is an instance object of class MyClass.
Good luck!

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!