Python name mangling

Question

In other languages, a general guideline that helps produce better code is always make everything as hidden as possible. If in doubt about whether a variable should be privat

Answer 1

You shouldn't start with private data and make it public as necessary. Rather, you should start by figuring out the interface of your object. I.e. you should start by figuring out what the world sees (the public stuff) and then figure out what private stuff is necessary for that to happen.

Other language make difficult to make private that which once was public. I.e. I'll break lots of code if I make my variable private or protected. But with properties in python this isn't the case. Rather, I can maintain the same interface even with rearranging the internal data.

The difference between _ and __ is that python actually makes an attempt to enforce the latter. Of course, it doesn't try really hard but it does make it difficult. Having _ merely tells other programmers what the intention is, they are free to ignore at their peril. But ignoring that rule is sometimes helpful. Examples include debugging, temporary hacks, and working with third party code that wasn't intended to be used the way you use it.

Answer 2

The chosen answer does a good job of explaining how properties remove the need for private attributes, but I would also add that functions at the module level remove the need for private methods.

If you turn a method into a function at the module level, you remove the opportunity for subclasses to override it. Moving some functionality to the module level is more Pythonic than trying to hide methods with name mangling.

Answer 3

First: Why do you want to hide your data? Why is that so important?

Most of the time you don't really want to do it but you do because others are doing.

If you really really really don't want people using something, add one underscore in front of it. That's it... Pythonistas know that things with one underscore is not guaranteed to work every time and may change without you knowing.

That's the way we live and we're okay with that.

Using two underscores will make your class so bad to subclass that even you will not want to work that way.

Answer 4

Following code snippet will explain all different cases :

• two leading underscores (__a)
• single leading underscore (_a)

• no underscore (a)

  class Test:
  
  def __init__(self):
      self.__a = 'test1'
      self._a = 'test2'
      self.a = 'test3'
  
  def change_value(self,value):
      self.__a = value
      return self.__a
  

printing all valid attributes of Test Object

testObj1 = Test()
valid_attributes = dir(testObj1)
print valid_attributes

['_Test__a', '__doc__', '__init__', '__module__', '_a', 'a', 
'change_value']

Here, you can see that name of __a has been changed to _Test__a to prevent this variable to be overridden by any of the subclass. This concept is known as "Name Mangling" in python. You can access this like this :

testObj2 = Test()
print testObj2._Test__a

test1

Similarly, in case of _a, the variable is just to notify the developer that it should be used as internal variable of that class, the python interpreter won't do anything even if you access it, but it is not a good practise.

testObj3 = Test()
print testObj3._a

test2

a variable can be accesses from anywhere it's like a public class variable.

testObj4 = Test()
print testObj4.a

test3

Hope the answer helped you :)

Answer 5

At first glance it should be the same as for other languages (under "other" I mean Java or C++), but it isn't.

In Java you made private all variables that shouldn't be accessible outside. In the same time in Python you can't achieve this since there is no "privateness" (as one of Python principles says - "We're all adults"). So double underscore means only "Guys, do not use this field directly". The same meaning has singe underscore, which in the same time doesn't cause any headache when you have to inherit from considered class (just an example of possible problem caused by double underscore).

So, I'd recommend you to use single underscore by default for "private" members.