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

First - What is name mangling?

Name mangling is invoked when you are in a class definition and use __any_name or __any_name_, that is, two (or more) leading underscores and at most one trailing underscore.

class Demo:
    __any_name = "__any_name"
    __any_other_name_ = "__any_other_name_"

And now:

>>> [n for n in dir(Demo) if 'any' in n]
['_Demo__any_name', '_Demo__any_other_name_']
>>> Demo._Demo__any_name
'__any_name'
>>> Demo._Demo__any_other_name_
'__any_other_name_'

When in doubt, do what?

The ostensible use is to prevent subclassers from using an attribute that the class uses.

A potential value is in avoiding name collisions with subclassers who want to override behavior, so that the parent class functionality keeps working as expected. However, the example in the Python documentation is not Liskov substitutable, and no examples come to mind where I have found this useful.

The downsides are that it increases cognitive load for reading and understanding a code base, and especially so when debugging where you see the double underscore name in the source and a mangled name in the debugger.

My personal approach is to intentionally avoid it. I work on a very large code base. The rare uses of it stick out like a sore thumb and do not seem justified.

You do need to be aware of it so you know it when you see it.

PEP 8

PEP 8, the Python standard library style guide, currently says (abridged):

There is some controversy about the use of __names.

If your class is intended to be subclassed, and you have attributes that you do not want subclasses to use, consider naming them with double leading underscores and no trailing underscores.

1. Note that only the simple class name is used in the mangled name, so if a subclass chooses both the same class name and attribute name, you can still get name collisions.

2. Name mangling can make certain uses, such as debugging and __getattr__() , less convenient. However the name mangling algorithm is well documented and easy to perform manually.

3. Not everyone likes name mangling. Try to balance the need to avoid accidental name clashes with potential use by advanced callers.

How does it work?

If you prepend two underscores (without ending double-underscores) in a class definition, the name will be mangled, and an underscore followed by the class name will be prepended on the object:

>>> class Foo(object):
...     __foobar = None
...     _foobaz = None
...     __fooquux__ = None
... 
>>> [name for name in dir(Foo) if 'foo' in name]
['_Foo__foobar', '__fooquux__', '_foobaz']

Note that names will only get mangled when the class definition is parsed:

>>> Foo.__test = None
>>> Foo.__test
>>> Foo._Foo__test
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: type object 'Foo' has no attribute '_Foo__test'

Also, those new to Python sometimes have trouble understanding what's going on when they can't manually access a name they see defined in a class definition. This is not a strong reason against it, but it's something to consider if you have a learning audience.

One Underscore?

If the convention is to use only one underscore, I'd also like to know the rationale.

When my intention is for users to keep their hands off an attribute, I tend to only use the one underscore, but that's because in my mental model, subclassers would have access to the name (which they always have, as they can easily spot the mangled name anyways).

If I were reviewing code that uses the __ prefix, I would ask why they're invoking name mangling, and if they couldn't do just as well with a single underscore, keeping in mind that if subclassers choose the same names for the class and class attribute there will be a name collision in spite of this.