I'm trying to understand about private variable and about name mangling in python.
When I defined class like this, it throws AttributeError
>>> class E():
def __test():
print("__test()")
>>> c = E()
>>> c.__test()
AttributeError: 'E' object has no attribute '__test'
I want to understand the exact reasons, for this error. Can any one explain them clearly?
I'll refer you to Python's documentation on name mangling.
Any identifier of the form
__spam(at least two leading underscores, at most one trailing underscore) is textually replaced with_classname__spam, where classname is the current class name with leading underscore(s) stripped.
So this raises an AttributeError because the __test methods has been implicitly renamed:
class E:
@staticmethod
def __test():
print("foo")
E.__test() # AttributeError
But can be made to work by using the mangled name _E__test:
class E:
@staticmethod
def __test():
print("foo")
E._E__test() # prints 'foo'
Notice that I had to add a staticmethod decorator due to your method not taking a self argument. This is out of scope, but you can read about it here, here and here.
E() and E? Please explain this.
selfvs.something: the name doesn't matter, as long as you understand that the first argument will be the object that invoked the method.