Underscores in Python

Underscores in Python

Single and double underscores have a meaning in Python variable and method names. Some of that meaning is merely by convention and intended as a hint to the programmer — and some of it is enforced by the Python interpreter.

If you're wondering "What's the meaning of single and double underscores in Python variable and method names?" I'll do my best to get you the answer here.

In this article I’ll discuss the following five underscore patterns and naming conventions and how they affect the behavior of your Python programs:

  • Single Leading Underscore: _var
  • Single Trailing Underscore: var_
  • Double Leading Underscore: __var
  • Double Leading and Trailing Underscore: __var__
  • Single Underscore: _

Let’s dive right in!

Single Leading Underscore: _var

The underscore prefix is meant as a hint to another programmer that a variable or method starting with a single underscore is intended for internal use. This convention is defined in PEP 8.

Example:

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

What's going to happen if you instantiate this class and try to access the foo and _bar attributes defined in its __init__ constructor? Let’s find out:

>>> t = Test()
>>> t.foo
11
>>> t._bar
23

You just saw that the leading single underscore in _bar did not prevent us from "reaching into" the class and accessing the value of that variable. That's because the single underscore prefix in Python is merely an agreed upon convention — at least when it comes to variable and method names.

Example:

my_module.py
# This is my_module.py:

def external_func():
    return 23

def _internal_func():
    return 42

Now if you use a python wildcard import to import all names from the module, Python will not import names with a leading underscore (unless the module defines an __all__ list that overrides this behavior):

>>> from my_module import *
>>> external_func()
23
>>> _internal_func()
NameError: "name '_internal_func' is not defined"

But generic import statement works OK.

>>> import my_module
>>> my_module.external_func()
23
>>> my_module._internal_func()
42

I know this might be a little confusing at this point. If you stick to the PEP 8 recommendation that wildcard imports should be avoided, then really all you need to remember is this:

Single Trailing Underscore: var_

Sometimes the most fitting name for a variable is already taken by a keyword. Therefore names like class or def cannot be used as variable names in Python. In this case you can append a single underscore to break the naming conflict:

>>> def make_object(name, class):
SyntaxError: "invalid syntax"

>>> def make_object(name, class_):
...     pass

Double Leading Underscore: __var

The naming patterns we covered so far received their meaning from agreed upon conventions only. With Python class attributes (variables and methods) that start with double underscores, things are a little different.

I know this sounds rather abstract. This is why I put together this little code example we can use for experimentation:

class Test:
    def __init__(self):
        self.foo = 11
        self._bar = 23
        self.__baz = 23

Let’s take a look at the attributes on this object using the built-in dir() function:

>>> t = Test()
>>> dir(t)
['_Test__baz', '__class__', '__delattr__', '__dict__', '__dir__',
 '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
 '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__',
 '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
 '__weakref__', '_bar', 'foo']

This gives us a list with the object's attributes. Let's take this list and look for our original variable names foo, _bar, and __baz. I promise you'll notice some interesting changes.

  • The self.foo variable appears unmodified as foo in the attribute list.
  • self._bar behaves the same way. It shows up on the class as _bar. Like I said before, the leading underscore is just a convention in this case. A hint for the programmer.
  • However with self.__baz, things look a little different. When you search for __baz in that list you'll see that there is no variable with that name.

So what happened to __baz?

If you look closely you'll see there's an attribute called _Test__baz on this object. This is the name mangling that the Python interpreter applies. It does this to protect the variable from getting overridden in subclasses.

Let's create another class that extends the Test class and attempts to override its existing attributes added in the constructor:

class ExtendedTest(Test):
    def __init__(self):
        super().__init__()
        self.foo = 'overridden'
        self._bar = 'overridden'
        self.__baz = 'overridden'

Now what do you think the values of foo, _bar, and __baz will be on instances of this ExtendedTest class? Let's take a look:

>>> t2 = ExtendedTest()
>>> t2.foo
'overridden'
>>> t2._bar
'overridden'
>>> t2.__baz
AttributeError: "'ExtendedTest' object has no attribute '__baz'"

Wait, why did we get that AttributeError when we tried to inspect the value of t2.__baz? Name mangling strikes again! It turns out this object doesn't even have a __baz attribute:

>>> dir(t2)
['_ExtendedTest__baz', '_Test__baz', '__class__', '__delattr__',
 '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
 '__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
 '__lt__', '__module__', '__ne__', '__new__', '__reduce__',
 '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
 '__subclasshook__', '__weakref__', '_bar', 'foo', 'get_vars']

As you can see __baz got turned into _ExtendedTest__baz to prevent accidental modification:

>>> t2._ExtendedTest__baz
'overridden'

But the original _Test__baz is also still around:

>>> t2._Test__baz
42

Double underscore name mangling is fully transparent to the programmer. Take a look at the following example that will confirm this:

class ManglingTest:
    def __init__(self):
        self.__mangled = 'hello'

    def get_mangled(self):
        return self.__mangled

>>> ManglingTest().get_mangled()
'hello'
>>> ManglingTest().__mangled
AttributeError: "'ManglingTest' object has no attribute '__mangled'"

Does name mangling also apply to method names? It sure does —name mangling affects all names that start with two underscore characters ("dunders") in a class context:

class MangledMethod:
    def __method(self):
        return 42

    def call_it(self):
        return self.__method()

>>> MangledMethod().__method()
AttributeError: "'MangledMethod' object has no attribute '__method'"
>>> MangledMethod().call_it()
42

Here’s another, perhaps surprising, example of name mangling in action:

_MangledGlobal__mangled = 23

class MangledGlobal:
    def test(self):
        return __mangled

>>> MangledGlobal().test()
23

In this example I declared a global variable called _MangledGlobal__mangled. Then I accessed the variable inside the context of a class named MangledGlobal. Because of name mangling I was able to reference the _MangledGlobal__mangled global variable as just __mangled inside the test() method on the class.

The Python interpreter automatically expanded the name __mangled to _MangledGlobal__mangled because it begins with two underscore characters. This demonstrated that name mangling isn't tied to class attributes specifically. It applies to any name starting with two underscore characters used in a class context.

Double Leading and Trailing Underscore: __var__

class PrefixPostfixTest:
    def __init__(self):
        self.__bam__ = 42

>>> PrefixPostfixTest().__bam__
42

Single Underscore: _

For example, in the following loop we don't need access to the running index and we can use _ to indicate that it is just a temporary value:

>>> for _ in range(32):
...     print('Hello, World.')

You can also use single underscores in unpacking expressions as a "don't care" variable to ignore particular values. Again, this meaning is per convention only and there's no special behavior triggered in the Python interpreter. The single underscore is simply a valid variable name that's sometimes used for this purpose.

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

>>> color
'red'
>>> mileage
3812.4
>>> _
12

Besides its use as a temporary variable, _ is a special variable in most Python REPLs that represents the result of the last expression evaluated by the interpreter.

This is handy if you're working in an interpreter session and you'd like to access the result of a previous calculation. Or if you're constructing objects on the fly and want to interact with them without assigning them a name first:

>>> 20 + 3
23
>>> _
23
>>> print(_)
23

>>> list()
[]
>>> _.append(1)
>>> _.append(2)
>>> _.append(3)
>>> _
[1, 2, 3]

Python Underscore Naming Patterns – Summary

Pattern Example Meaning
Single Leading Underscore _var Naming convention indicating a name is meant for internal use. Generally not enforced by the Python interpreter (except in wildcard imports) and meant as a hint to the programmer only.
Single Trailing Underscore var_ Used by convention to avoid naming conflicts with Python keywords.
Double Leading Underscore __var Triggers name mangling when used in a class context. Enforced by the Python interpreter.
Double Leading and Trailing Underscore __var__ Indicates special methods defined by the Python language. Avoid this naming scheme for your own attributes.
Single Underscore _ Sometimes used as a name for temporary or insignificant variables ("don't care"). Also: The result of the last expression in a Python REPL.