What is the meaning of single and double leading underscore in Python

Learn how the number of underscores affect the accessibility of variables in Python.

Python does not have access identifiers like public, private and protected. Single and Double underscores are used as alternatives for pseudo access restriction.

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In this article, we will understand the usage of single and double underscores in variable names.

1. Single underscore¶

Variables declared in class with single leading underscore are to be treated as private by convention, it works as a weak indicator to signify internal use only.

Outside a class, such variables do not have any special significance, they are treated as a public variable.

When importing objects from a file, if from module import * is used Python does not import objects whose name start with a single leading underscore.

Example:

class Sample:
    def __init__(self):
        self.foo = "lorem"
        self._bar = "ipsum"

S = Sample()
print(s.foo, s._bar)

Output:

lorem ipsum

Through this example, we can see that there is no access restriction imposed by python on _bar however an IDE such as PyCharm would generate a warning about usage of this pseudo private variable outside class Sample.

2. Double underscore¶

Inside a class, when a variable name has two leading underscores, it is renamed to _classname__variable, this process is called as Name Mangling and it helps Python distinguish between the same variable names from different classes.

Here too, if from module import * is used Python does not import objects whose names start with double leading underscore.

Example:

class Sample:
    def __init__(self):
        self.foo = "hello"
        self.__bar = "world"

s = Sample()
print(dir(s))

Output:

['_Sample__bar',...,'foo']

We can see that an instance of class Sample does not have a reference to __bar.

That's why variables with two leading underscores are sometimes thought of as "real private" attributes since they cannot be accessed from outside the class. However, they can still be accessed via the new given name:

class Sample:
    def __init__(self):
        self.foo = "hello"
        self.__bar = "world"

s = Sample()
s.foo           # ok
s.__bar         # not ok
s._Sample__bar  # ok

3.Double leading and trailing underscores¶

Names with double leading and trailing underscores are reserved for special use in Python. They are called Magic Methods/Attributes or Special Methods/Attributes.

Examples are:

__init__
__name__
__new__
__str__
__repr__
__del__

A full list can be found here.