Understanding Python Underscore Naming Conventions and Name Mangling

1. Single Leading Underscore _var

A single leading underscore is a convention that signals an internal‑use variable or method; Python does not enforce any restriction, but it does affect "from module import *" by excluding such names unless they appear in __all__ .

<code>class Test:
    def __init__(self):
        self.foo = 11
        self._bar = 23

>>> t = Test()
>>> t.foo
11
>>> t._bar
23</code>

2. Single Trailing Underscore var_

When a desired identifier collides with a Python keyword, appending a trailing underscore resolves the conflict while keeping the name readable.

<code>def make_object(name, class_):
    pass</code>

3. Double Leading Underscore __var

Names that start with two underscores trigger name mangling: the interpreter rewrites the attribute name to _ClassName__var to avoid accidental overriding in subclasses.

<code>class Test:
    def __init__(self):
        self.__baz = 23

>>> t = Test()
>>> dir(t)
['_Test__baz', '__class__', '__delattr__', ...]
</code>

When a subclass defines its own __baz , the original attribute remains mangled as _Test__baz while the subclass gets _SubClass__baz .

<code>class ExtendedTest(Test):
    def __init__(self):
        super().__init__()
        self.__baz = 'overridden'

>>> et = ExtendedTest()
>>> et._ExtendedTest__baz
'overridden'
>>> et._Test__baz
23</code>

4. Double Leading and Trailing Underscore __var__

Identifiers that both start and end with double underscores are reserved for special Python methods ("dunder" methods) such as __init__ or __call__ . They are not subject to name mangling.

<code>class PrefixPostfixTest:
    def __init__(self):
        self.__bam_ = 42

>>> PrefixPostfixTest().__bam_
42</code>

5. Single Underscore _

A solitary underscore is often used as a throw‑away variable in loops, unpacking, or as the REPL’s placeholder for the last evaluated expression.

<code>for _ in range(3):
    print('Hello')

car = ('red', 'auto', 12, 3812.4)
color, _, _, mileage = car

>>> _
12
</code>

The article concludes with a quick reference table summarizing the five underscore patterns and their typical meanings.