What is the meaning of a single and a double underscore before an object name?

Question

Can someone please explain the exact meaning of having leading underscores before an object's name in Python? Also, explain the difference between a single and a double leading underscore. Also, does that meaning stay the same whether the object in question is a variable, a function, a method, etc.?

Answer 1

Single leading underscore ( _foo )

----------------------------------------------------------------------

This naming convention is intended to tell the programmer that the attribute, function, or method is private or used for internal use. This is a convention that is not enforced by the python interpreter but rather a way to help organize the code for the programmer and communicate where the object should be used:

class Foo:

def __init__(self):

self.foo = 1

self._foo = 2 #<-- PRIVATE ATTRIBUTE

The leading underscore tells the programmer that self._foo should only be referenced within the class. However, there is nothing stopping the programmer from accessing the attribute outside of the class, for example:

obj1 = Foo()

obj1.foo

obj1._foo #still accessible outside of the class

Double leading underscores ( __foo ):

-----------------------------------------------------------------------

Double underscores in front of an attribute has meaning to a Python interpreter. The interpreter enforces "name mangling", which is a way for the interpreter to rename the attribute by adding the class name to the front of a double leading underscore attribute. This can be observed by looking at the object's attributes:

class Foo:

def __init__(self):

self.foo = 1

self.__foo = 2

obj1 = Foo()

dir(obj1)

['_Foo__foo', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'foo']

The "dir()" function returns a list of the object's attributes. As demonstrated from the list, the attribute "foo" is in the list but "__foo" is not. "__foo" got renamed to '_Foo_foo' which is at the beginning of the list. This naming convention also affects the programmer's access to the attribute. Unlike attributes with a single leading underscore, double leading underscore attributes cannot be accessed outside of the class.

f.__foo

Traceback (most recent call last):

File "<pyshell#19>", line 1, in <module>

f.__foo

AttributeError: 'Foo' object has no attribute '__foo'

The attribute can still be accessed externally by using the renamed variable name "_Foo__foo", however, this is generally not advised. The program can cause errors if a class gets renamed. If needed, the safest way to access the attribute would be through a class method or accessed internally. (Name mangling also applies to methods but does not apply to variables or functions that are just within a module).

f._Foo__foo

2