Private Variables and Methods in Python
PythonPrivatePrivate MembersPrivate MethodsDouble UnderscorePython Problem Overview
> Possible Duplicate:
> The meaning of a single- and a double-underscore before an object name in Python
Which should I use _foo
(an underscore) or __bar
(double underscore) for private members and methods in Python?
Python Solutions
Solution 1 - Python
Please note that there is no such thing as "private method" in Python. Double underscore is just name mangling:
>>> class A(object):
... def __foo(self):
... pass
...
>>> a = A()
>>> A.__dict__.keys()
['__dict__', '_A__foo', '__module__', '__weakref__', '__doc__']
>>> a._A__foo()
So therefore __
prefix is useful when you need the mangling to occur, for example to not clash with names up or below inheritance chain. For other uses, single underscore would be better, IMHO.
EDIT, regarding confusion on __
, PEP-8 is quite clear on that:
> 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. This invokes > Python's name mangling algorithm, where the name of the class is > mangled into the attribute name. This helps avoid attribute name > collisions should subclasses inadvertently contain attributes with the > same name. > > Note 3: Not everyone likes name mangling. Try to balance the > need to avoid accidental name clashes with potential use by > advanced callers.
So if you don't expect subclass to accidentally re-define own method with same name, don't use it.
Solution 2 - Python
The double underscore. It mangles the name in such a way that it can't be accessed simply through __fieldName
from outside the class, which is what you want to begin with if they're to be private. (Though it's still not very hard to access the field.)
class Foo:
def __init__(self):
self.__privateField = 4;
print self.__privateField # yields 4 no problem
foo = Foo()
foo.__privateField
# AttributeError: Foo instance has no attribute '__privateField'
It will be accessible through _Foo__privateField
instead. But it screams "I'M PRIVATE DON'T TOUCH ME", which is better than nothing.
Solution 3 - Python
Double underscore. That mangles the name. The variable can still be accessed, but it's generally a bad idea to do so.
Use single underscores for semi-private (tells python developers "only change this if you absolutely must") and doubles for fully private.
Solution 4 - Python
Because thats coding convention. See here for more.