Single Underscore
In a class, names with a leading underscore indicate to other programmers that the attribute or method is intended to be be used inside that class. However, privacy is not enforced in any way. Using leading underscores for functions in a module indicates it should not be imported from somewhere else.
From the PEP-8 style guide:
_single_leading_underscore: weak "internal use" indicator. E.g.from M import *does not import objects whose name starts with an underscore.
Double Underscore (Name Mangling)
From the Python docs:
Any identifier of the form
__spam(at least two leading underscores, at most one trailing underscore) is textually replaced with_classname__spam, whereclassnameis the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, so it can be used to define class-private instance and class variables, methods, variables stored in globals, and even variables stored in instances. private to this class on instances of other classes.
And a warning from the same page:
Name mangling is intended to give classes an easy way to define “private” instance variables and methods, without having to worry about instance variables defined by derived classes, or mucking with instance variables by code outside the class. Note that the mangling rules are designed mostly to avoid accidents; it still is possible for a determined soul to access or modify a variable that is considered private.
Example
>>> class MyClass():
... def __init__(self):
... self.__superprivate = "Hello"
... self._semiprivate = ", world!"
...
>>> mc = MyClass()
>>> print mc.__superprivate
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: myClass instance has no attribute '__superprivate'
>>> print mc._semiprivate
, world!
>>> print mc.__dict__
{'_MyClass__superprivate': 'Hello', '_semiprivate': ', world!'}
Single Underscore
In a class, names with a leading underscore indicate to other programmers that the attribute or method is intended to be be used inside that class. However, privacy is not enforced in any way. Using leading underscores for functions in a module indicates it should not be imported from somewhere else.
From the PEP-8 style guide:
_single_leading_underscore: weak "internal use" indicator. E.g.from M import *does not import objects whose name starts with an underscore.
Double Underscore (Name Mangling)
From the Python docs:
Any identifier of the form
__spam(at least two leading underscores, at most one trailing underscore) is textually replaced with_classname__spam, whereclassnameis the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, so it can be used to define class-private instance and class variables, methods, variables stored in globals, and even variables stored in instances. private to this class on instances of other classes.
And a warning from the same page:
Name mangling is intended to give classes an easy way to define “private” instance variables and methods, without having to worry about instance variables defined by derived classes, or mucking with instance variables by code outside the class. Note that the mangling rules are designed mostly to avoid accidents; it still is possible for a determined soul to access or modify a variable that is considered private.
Example
>>> class MyClass():
... def __init__(self):
... self.__superprivate = "Hello"
... self._semiprivate = ", world!"
...
>>> mc = MyClass()
>>> print mc.__superprivate
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: myClass instance has no attribute '__superprivate'
>>> print mc._semiprivate
, world!
>>> print mc.__dict__
{'_MyClass__superprivate': 'Hello', '_semiprivate': ', world!'}
_foo: Only a convention. A way for the programmer to indicate that the variable is private (whatever that means in Python).__foo: This has real meaning. The interpreter replaces this name with_classname__fooas a way to ensure that the name will not overlap with a similar name in another class.__foo__: Only a convention. A way for the Python system to use names that won't conflict with user names.
No other form of underscores have meaning in the Python world. Also, there's no difference between class, variable, global, etc in these conventions.
python - Finding a list of all double-underscore variables - Stack Overflow
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What’s the Meaning of Single and Double Underscores In Python?
I need help understanding the underscore in setters and getters
Videos
Have you ever been curious about the several meanings of underscores in Python? A little break-down?
- you can find detailed explanations and code snippets here
1️⃣ single leading underscore ("_var"): indicates that the variable is meant for internal use. This is not enforced by the interpreter and is rather a hint to the programmer.
2️⃣ single trailing underscore ("var_"): it's used to avoid conflicts with Python reserved keywords ("class_", "def_", etc.)
3️⃣ double leading underscores ("__var"): Triggers name mangling when used in a class context and is enforced by the Python interpreter.
What this means is that it should be used to avoid your method is being overridden by a subclass or accessed accidentally.
4️⃣ double leading and trailing underscores ("__var__"): used for special methods defined in the Python language (ex. __init__, __len__, __call__, etc.). They should be avoided to use for your own attributes.
5️⃣ single underscore ("_"): Generally used as a temporary or unused variable. (If you don't use the running index of a for-loop, you can replace it with "_").
If you want to see magic names whether documented or not, go to the Lib directory and run:
egrep -oh '__[A-Za-z_][A-Za-z_0-9]*__' *.py | sort | uniq
That produces:
'__all__'
'__args__'
'__author__'
'__bases__'
'__builtin__'
'__builtins__'
'__cached__'
'__call__'
'__class__'
'__copy__'
'__credits__'
'__date__'
'__decimal_context__'
'__deepcopy__'
'__dict__'
'__doc__'
'__exception__'
'__file__'
'__flags__'
'__ge__'
'__getinitargs__'
'__getstate__'
'__gt__'
'__import__'
'__importer__'
'__init__'
'__ispkg__'
'__iter__'
'__le__'
'__len__'
'__loader__'
'__lt__'
'__main__'
'__module__'
'__mro__'
'__name__'
'__package__'
'__path__'
'__pkgdir__'
'__return__'
'__safe_for_unpickling__'
'__setstate__'
'__slots__'
'__temp__'
'__test__'
'__version__'
The complete list used by Python is given in the Python Language Reference section 3, "Data model". Every other one is non-standard or used by third-party modules and is documented separately.