Difference between _, __ and __xx__ in Python

When learning Python many people don't really understand why so much underlines in the beginning of the methods, sometimes even in the end like __this__! I've already had to explain it so many times, it's time to document it.

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:
class BaseForm(StrAndUnicode):
    def _get_errors(self):
        "Returns an ErrorDict for the data provided for the form"
        if self._errors is None:
        return self._errors
    errors = property(_get_errors)
This snippet was taken from django source code (django/forms/forms.py). This meanserrors 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:
class A(object):
    def __method(self):
        print "I'm a method in A"
    def method(self):
a = A()
The output here is
$ python example.py 
I'm a method in A
Fine, as we expected. Now let's subclass A and customize __method
class B(A):
    def __method(self):
        print "I'm a method in B"

b = B()
and now the output is...
$ 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__()
>>> len(name)

>>> number = 10
>>> number.__add__(20)
>>> number + 20
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):

    def __len__(self):
        return len(self.people)

room = Room()
print len(room)     # 1
The documentation covers all these special methods.


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!


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