pythonの下線

Difference between _, __ and __xx__ in Python
多くの人がpythonを勉強している間に下線の意味を本当に理解していないので、以下に詳細な説明をします.
One underline in the beginning
Python doesn't have real private methods, so one underline in the beginning of a method or attribute means you shouldn't access this method, because it's not part of the API. It's very common when using properties:
PythonにはC++のようなプライベートドメインはありません.下線の先頭のメソッドまたはプロパティを使用すると、オブジェクト名またはクラス名でメソッドまたはプロパティにアクセスできません.つまり、メソッドまたはプロパティはprivateで、外部には表示されません.
propertiesにも適用されます.

class BaseForm(StrAndUnicode): ... def _get_errors(self): "Returns an ErrorDict for the data provided for the form" if self._errors is None: self.full_clean() return self._errors errors = property(_get_errors) 

This snippet was taken from django source code (django/forms/forms.py). This means  errors  is a property, and it's part of the API, but the method this property calls,  _get_errors , is "private", so you shouldn't access it.
Two underlines in the beginning
This one causes a lot of confusion. It should not be used to mark a method as private, the goal here is to avoid your method to be overridden by a subclass. Let's see an example:
二重下線の先頭の方法は、サブクラスがこの方法を書き換えることを防止するためです.

class A(object): def __method(self): print "I'm a method in A" def method(self): self.__method() a = A() a.method() 

出力は次のとおりです.

$ python example.py 
I'm a method in A

出力は大丈夫ですが、今見てもサブクラスには同じようなものがあります.methodの場合.


class B(A): def __method(self): print "I'm a method in B" b = B() b.method() 

出力は何だと思う?

$ python example.py
I'm a method in A

as you can see,  A.method()  didn't call  B.__method()  as we could expect. Actually this is the correct behavior for  __ . So when you create a method starting with  __  you're saying that you don't want anybody to override it, it will be accessible just from inside the own class.
How python does it? Simple, it just renames the method. Take a look:

a = A() a._A__method() # never use this!! please! 

$ python example.py
I'm a method in A

If you try to access  a.__method()  it won't work either, as I said,  __method  is just accessible inside the class itself.
Two underlines in the beginning and in the end
When you see a method like  __this__ , the rule is simple: don't call it. Why? Because it means it's a method python calls, not you. Take a look:

>>> name = "igor" >>> name.__len__() 4 >>> len(name) 4 >>> number = 10 >>> number.__add__(20) 30 >>> number + 20 30 

There is always an operator or native function that calls these magic methods. The idea here is to give you the ability to override operators in your own classes. Sometimes it's just a hook python calls in specific situations.  __init__() , for example, is called when the object is created so you can initialize it.  __new__()  is called to build the instance, and so on...
Here's an example:

class CrazyNumber(object): def __init__(self, n): self.n = n def __add__(self, other): return self.n - other def __sub__(self, other): return self.n + other def __str__(self): return str(self.n)
 num = CrazyNumber(10) print num # 10 print num + 5 # 5 print num - 20 # 30 

Another example:

class Room(object): def __init__(self): self.people = [] def add(self, person): self.people.append(person) def __len__(self): return len(self.people) room = Room() room.add("Igor") print len(room) # 1 

The  documentation  covers all these special methods.
Conclusion
Use  _one_underline  to mark you methods as not part of the API. Use __two_underlines__  when you're creating objects to look like native python objects or you wan't to customize behavior in specific situations. And don't use __just_to_underlines , unless you really know what you're doing!