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single and double underscore of class member name in Python

Single underscore

_single_leading_underscore: weak “internal use” indicator. E.g. from M import * does not import objects whose name starts with an underscore. However, nothing special is done with the name itself.

_foo: this is just a convention, a way for the programmer to indicate that the variable is private.

Double underscore

Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped.

__foo: this has real meaning, the interpreter replaces this name with _classname__foo as a way to ensure that the name will not overlap with a similar name in another class.

__foo__: this is just a convention, a way for the Python system to use names that won’t conflict with user names.


>>> 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!'}


  • _var: 下划线开头的变量或方法只在内部使用,与Java不同,Python在“私有”和“公共”变量之间并没有很强的区别。
  • var_: 某个变量最合适的名称已被Python语言中的关键字占用,可以追加一个下划线来绕过命名冲突。
  • __var: 也称为名称改写(name mangling),双下划线前缀会让Python解释器重写属性名称,以避免子类中的命名冲突。
  • __var__: 双下划线方法通常被称为魔法方法,参考下表。
  • _: 循环中不需要访问运行的索引可以使用_来表示临时值;还能表示REPL会话中上一个表达式的结果。

Other magic methods in Python

Magic MethodWhen it gets invoked (example)Explanation
__new__(cls [,...])instance = MyClass(arg1, arg2)__new__ is called on instance creation
__init__(self [,...])instance = MyClass(arg1, arg2)__init__ is called on instance creation
__cmp__(self, other)self == other, self > other, etc.Called for any comparison
__pos__(self)+selfUnary plus sign
__neg__(self)-selfUnary minus sign
__invert__(self)~selfBitwise inversion
__index__(self)x[self]Conversion when object is used as index
__nonzero__(self)bool(self)Boolean value of the object
__getattr__(self, name) # name doesn't existAccessing nonexistent attribute
__setattr__(self, name, val) = valAssigning to an attribute
__delattr__(self, name)del self.nameDeleting an attribute
__getattribute__(self, name)self.nameAccessing any attribute
__getitem__(self, key)self[key]Accessing an item using an index
__setitem__(self, key, val)self[key] = valAssigning to an item using an index
__delitem__(self, key)del self[key]Deleting an item using an index
__iter__(self)for x in selfIteration
__contains__(self, value)value in self, value not in selfMembership tests using in
__call__(self [,...])self(args)“Calling” an instance
__enter__(self)with self as x:with statement context managers
__exit__(self, exc, val, trace)with self as x:with statement context managers
__getstate__(self)pickle.dump(pkl_file, self)Pickling
__setstate__(self)data = pickle.load(pkl_file)Pickling