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Methods for use inside a class only [duplicate]

I would like to have a function in my class, which I am going to use only inside methods of this class. I will not call it outside the implementations of these methods. In C++, I would use a method declared in the private section of the class. What is the best way to implement such a function in Python?
I am thinking of using a static decorator for this case. Can I use a function without any decorators and the self word?

Python doesn't have the concept of private methods or attributes. It's all about how you implement your class. But you can use pseudo-private variables (name mangling); any variable preceded by __(two underscores) becomes a pseudo-private variable.
From the documentation:
Since there is a valid use-case for class-private members (namely to
avoid name clashes of names with names defined by subclasses), there
is limited support for such a mechanism, called name mangling. Any
identifier of the form __spam (at least two leading underscores, at
most one trailing underscore) is textually replaced with
_classname__spam, where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard
to the syntactic position of the identifier, as long as it occurs
within the definition of a class.
class A:
def __private(self):
pass
So __private now actually becomes _A__private.
Example of a static method:
>>> class A:
... #staticmethod # Not required in Python 3.x
... def __private():
... print 'hello'
...
>>> A._A__private()
hello

Python doesn't have the concept of 'private' the way many other languages do. It is built on the consenting adult principle that says that users of your code will use it responsibly. By convention, attributes starting with a single or double leading underscore will be treated as part of the internal implementation, but they are not actually hidden from users. Double underscore will cause name mangling of the attribute name though.
Also, note that self is only special by convention, not by any feature of the language. Instance methods, when called as members of an instance, are implicitly passed the instance as a first argument, but in the implementation of the method itself, that argument can technically be named any arbitrary thing you want. self is just the convention for ease of understanding code. As a result, not including self in the signature of a method has no actual functional effect other than causing the implicit instance argument to be assigned to the next variable name in the signature.
This is of course different for class methods, which receive the instance of the class object itself as an implicit first argument, and static methods, which receive no implicit arguments at all.

Python just doesn't do private. If you like you can follow convention and precede the name with a single underscore, but it's up to other coders to respect that in a gentlemanly† fashion
† or gentlewomanly

There is plenty of great stuff here with obfuscation using leading underscores. Personally, I benefit greatly from the language design decision to make everything public as it reduces the time it takes to understand and use new modules.
However, if you're determined to implement private attributes/methods and you're willing to be unpythonic, you could do something along the lines of:
from pprint import pprint
# CamelCase because it 'acts' like a class
def SneakyCounter():
class SneakyCounterInternal(object):
def __init__(self):
self.counter = 0
def add_two(self):
self.increment()
self.increment()
def increment(self):
self.counter += 1
def reset(self):
print 'count prior to reset: {}'.format(self.counter)
self.counter = 0
sneaky_counter = SneakyCounterInternal()
class SneakyCounterExternal(object):
def add_two(self):
sneaky_counter.add_two()
def reset(self):
sneaky_counter.reset()
return SneakyCounterExternal()
# counter attribute is not accessible from out here
sneaky_counter = SneakyCounter()
sneaky_counter.add_two()
sneaky_counter.add_two()
sneaky_counter.reset()
# `increment` and `counter` not exposed (AFAIK)
pprint(dir(sneaky_counter))
It is hard to imagine a case where you'd want to do this, but it is possible.

You just don't do it:
The Pythonic way is to not document those methods/members using docstrings, only with "real" code comments. And the convention is to append a single or a double underscore to them;
Then you can use double underscores in front of your member, so they are made local to the class (it's mostly name mangling, i.e., the real name of the member outside of the class becomes: instance.__classname_membername). It's useful to avoid conflicts when using inheritance, or create a "private space" between children of a class.
As far as I can tell, it is possible to "hide" variables using metaclasses, but that violates the whole philosophy of Python, so I won't go into details about that.

Inheritance and Private attributes in Python

class Parent:
def __init__(self):
self._lst = []
class Child(Parent):
def __init__(self):
super().__init__()
Considering the code above, say I wanted to implement a method in the Child class, am I allowed to used self._lst (which is a private attribute initialized in the Parent Class) for this method? In other words, am I allowed to access private attributes that are initialized in the Parent Class through Subclasses?

In python, truly private attributes / methods don't exist. There are only naming conventions. What this means is if an attribute / method has its name beginning with one underscore, it can still be accessed from anywhere, just like regular attributes / methods. The only thing that this does is serve as a reminder to yourself and let other developers know that this attribute was not meant to be accessed from the outside.
To answer your question, yes, you can use _lst in the function. Even in languages that do have real access modifiers, there is frequently a different keyword to distinguish attributes not accessible from anywhere vs those that are not accessible anywhere but derived classes. In python, this is generally signified with a double underscore (__) vs a single underscore (_). Double underscores are not meant to be accessed from anywhere, while single underscores can be accessed by derived classes. See here for more information.

How python resolves private(double underscore) methods inside a class?

Consider the following class
class Foo(object):
#staticmethod
def __is():
print('__is')
def m(self):
Foo.__is() # executes
Foo.__is() # fails because of mangling
print(Foo.__dict__.keys())
Foo.__is() when it is run after the class is defined, it fails because of name mangaling. How is python interpreter able to resolve Foo.__is() inside methods but not outside the class?

Name mangling for names starting with __ inside a class, is implemented by rewriting this names to the mangled form, inside the class ONLY. So your Foo.__is inside the class gets replaced by _Foo__is that is now present in the class __dict__. That attribute is accessible either inside or outside the class, so no private protection. But after replacement the __is name does not exist anywhere (I think), that's why it does not work from outside.
From the Python help:
"__*"
Class-private names. Names in this category, when used within the
context of a class definition, are re-written to use a mangled form
to help avoid name clashes between “private” attributes of base and
derived classes. See section Identifiers (Names).
Also see my comment to errors in your code snippet.

You can think of classes as of namespaces with some whistles.
One of whistles is ability to resolve __methods inside themselves without _class__method. This is no different from "magic" of automatically passing instances as first argument to methods (your regular self)
To illustrate "namespace" idea, you can do some weird things like
class A:
def insane():
return 42
for _ in range(5):
print(insane()

Python: class with double underscore

I'm following this link and trying to make a singleton class using Metaclass. But, I want to make some internal tweaks to this singleton class and want the users to use another class (let's call it MySingleton(__Singleton)). So I decided to make it private but it gives the following error.
My sole purpose is to prevent __Singleton from being used outside. How can I achieve this?
On a separate note, is it a good practice to use double underscore with classes?

Inside the class, the identifier __Singleton is getting mangled. You end up having problems because name mangling only happens inside classes (not outside). So __Singleton as a class name means something different than __Singleton when you are inside a class suite.
Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.
Note that the primary reason for mangling is because it
... is helpful for letting subclasses override methods without breaking intraclass method calls.
Also:
... to avoid name clashes of names with names defined by subclasses
As such, there really isn't any reason to have a class with leading double underscores in the name (there is no chance of intraclass method calls having conflicts with class names). A single leading underscore is a good enough signal to users that they shouldn't use that class:
... a name prefixed with an underscore (e.g. _spam) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an implementation detail and subject to change without notice.
I wouldn't advise it, but if you really want it to work, you can probably use globals to look up the class:
class __Foo(object):
def __init__(self):
super(globals()['__Foo'], self).__init__()
f = __Foo()
print f

Every name inside a class definition with two leading underscores is mangled, so __Singleton becomes _Singleton__Singleton. To make clear, that some class is not supposed to be used publicly use one underscore.

Python does not have private variables; they are all accessible externally.
“Private” instance variables that cannot be accessed except from inside an object don’t exist in Python. However, there is a convention that is followed by most Python code: a name prefixed with an underscore (e.g. _spam) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an implementation detail and subject to change without notice.
The Python Cookbook provides a Singleton class that can be inherited by other classes to become Singletons.

Inheritance of private and protected methods in Python

I know, there are no 'real' private/protected methods in Python. This approach isn't meant to hide anything; I just want to understand what Python does.
class Parent(object):
def _protected(self):
pass
def __private(self):
pass
class Child(Parent):
def foo(self):
self._protected() # This works
def bar(self):
self.__private() # This doesn't work, I get a AttributeError:
# 'Child' object has no attribute '_Child__private'
So, does this behaviour mean, that 'protected' methods will be inherited but 'private' won't at all?
Or did I miss anything?

Python has no privacy model, there are no access modifiers like in C++, C# or Java. There are no truly 'protected' or 'private' attributes.
Names with a leading double underscore and no trailing double underscore are mangled to protect them from clashes when inherited. Subclasses can define their own __private() method and these will not interfere with the same name on the parent class. Such names are considered class private; they are still accessible from outside the class but are far less likely to accidentally clash.
Mangling is done by prepending any such name with an extra underscore and the class name (regardless of how the name is used or if it exists), effectively giving them a namespace. In the Parent class, any __private identifier is replaced (at compilation time) by the name _Parent__private, while in the Child class the identifier is replaced by _Child__private, everywhere in the class definition.
The following will work:
class Child(Parent):
def foo(self):
self._protected()
def bar(self):
self._Parent__private()
See Reserved classes of identifiers in the lexical analysis documentation:
__*
Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between “private” attributes of base and derived classes.
and the referenced documentation on names:
Private name mangling: When an identifier that textually occurs in a class definition begins with two or more underscore characters and does not end in two or more underscores, it is considered a private name of that class. Private names are transformed to a longer form before code is generated for them. The transformation inserts the class name, with leading underscores removed and a single underscore inserted, in front of the name. For example, the identifier __spam occurring in a class named Ham will be transformed to _Ham__spam. This transformation is independent of the syntactical context in which the identifier is used.
Don't use class-private names unless you specifically want to avoid having to tell developers that want to subclass your class that they can't use certain names or risk breaking your class. Outside of published frameworks and libraries, there is little use for this feature.
The PEP 8 Python Style Guide has this to say about private name mangling:
If your class is intended to be subclassed, and you have attributes
that you do not want subclasses to use, consider naming them with
double leading underscores and no trailing underscores. This invokes
Python's name mangling algorithm, where the name of the class is
mangled into the attribute name. This helps avoid attribute name
collisions should subclasses inadvertently contain attributes with the
same name.
Note 1: Note that only the simple class name is used in the mangled
name, so if a subclass chooses both the same class name and attribute
name, you can still get name collisions.
Note 2: Name mangling can make certain uses, such as debugging and
__getattr__(), less convenient. However the name mangling algorithm
is well documented and easy to perform manually.
Note 3: Not everyone likes name mangling. Try to balance the need to
avoid accidental name clashes with potential use by advanced callers.

The double __ attribute is changed to _ClassName__method_name which makes it more private than the semantic privacy implied by _method_name.
You can technically still get at it if you'd really like to, but presumably no one is going to do that, so for maintenance of code abstraction reasons, the method might as well be private at that point.
class Parent(object):
def _protected(self):
pass
def __private(self):
print("Is it really private?")
class Child(Parent):
def foo(self):
self._protected()
def bar(self):
self.__private()
c = Child()
c._Parent__private()
This has the additional upside (or some would say primary upside) of allowing a method to not collide with child class method names.

By declaring your data member private :
__private()
you simply can't access it from outside the class
Python supports a technique called name mangling.
This feature turns class member prefixed with two underscores into:
_className.memberName
if you want to access it from Child() you can use: self._Parent__private()

Also PEP8 says
Use one leading underscore only for non-public methods and instance
variables.
To avoid name clashes with subclasses, use two leading underscores to
invoke Python's name mangling rules.
Python mangles these names with the class name: if class Foo has an
attribute named __a, it cannot be accessed by Foo.__a. (An insistent
user could still gain access by calling Foo._Foo__a.) Generally,
double leading underscores should be used only to avoid name conflicts
with attributes in classes designed to be subclassed.
You should stay away from _such_methods too, by convention. I mean you should treat them as private

Although this is an old question, I encountered it and found a nice workaround.
In the case you name mangled on the parent class because you wanted to mimic a protected function, but still wanted to access the function in an easy manner on the child class.
parent_class_private_func_list = [func for func in dir(Child) if func.startswith ('_Parent__')]
for parent_private_func in parent_class_private_func_list:
setattr(self, parent_private_func.replace("_Parent__", "_Child"), getattr(self, parent_private_func))
The idea is manually replacing the parents function name into one fitting to the current namespace.
After adding this in the init function of the child class, you can call the function in an easy manner.
self.__private()

AFAIK, in the second case Python perform "name mangling", so the name of the __private method of the parent class is really:
_Parent__private
And you cannot use it in child in this form neither