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There may be confusion between class privates and module privates.

A module private starts with one underscore
Such a element is not copied along when using the from <module_name> import * form of the import command; it is however imported if using the import <moudule_name> syntax (see Ben Wilhelm's answer)
Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py using the from a import * syntax.

A class private starts with two underscores (aka dunder i.e. d-ouble under-score)
Such a variable has its name "mangled" to include the classname etc.
It can still be accessed outside of the class logic, through the mangled name.
Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>> 

There may be confusion between class privates and module privates.

A module private starts with one underscore
Such a element is not copied along when using the from <module_name> import * form of the import command; it is however imported if using the import <module_name> syntax (see Ben Wilhelm's answer)

Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py using the from a import * syntax.

A class private starts with two underscores (aka dunder i.e. d-ouble under-score)

Such a variable has its name "mangled" to include the classname etc.

It can still be accessed outside of the class logic, through the mangled name.

Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>> 

There may be confusion between class privates and module privates.

A module private starts with one underscore
Such a element is not copied along when using the from <module_name> import * form of the import command; it is however imported if using the import <moudule_name> syntax (see Ben Wilhelm's answer)
Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py.

A class private starts with two underscores (aka dunder i.e. d-ouble under-score)
Such a variable has its name "mangled" to include the classname etc.
It can still be accessed outside of the class logic, through the mangled name.
Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>> 

There may be confusion between class privates and module privates.

A module private starts with one underscore
Such a element is not copied along when using the from <module_name> import * form of the import command; it is however imported if using the import <moudule_name> syntax (see Ben Wilhelm's answer)
Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py using the from a import * syntax.

A class private starts with two underscores (aka dunder i.e. d-ouble under-score)
Such a variable has its name "mangled" to include the classname etc.
It can still be accessed outside of the class logic, through the mangled name.
Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>> 

There may be confusion between class privates and module privates.

A module private begins with a single underscore
Such a element is not copied along when using import.
Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py.

A class private begins with two underscores (aka dunder i.e. d-ouble under-score)
Such a variable has its name "mangled" to include the classname etc.
It can still be accessed outside of the class logic, through the mangled name.
Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>> 

There may be confusion between class privates and module privates.

A module private starts with one underscore
Such a element is not copied along when using the from <module_name> import * form of the import command; it is however imported if using the import <moudule_name> syntax (see Ben Wilhelm's answer)
Simply remove one underscore from the a.__num of the question's example and it won't show in modules that import a.py.

A class private starts with two underscores (aka dunder i.e. d-ouble under-score)
Such a variable has its name "mangled" to include the classname etc.
It can still be accessed outside of the class logic, through the mangled name.
Although the name mangling can serve as a mild prevention device against unauthorized access, its main purpose is to prevent possible name collisions with class members of the ancestor classes. See Alex Martelli's funny but accurate reference to consenting adults as he describes the convention used in regards to these variables.

>>> class Foo(object): ... __bar = 99 ... def PrintBar(self): ... print(self.__bar) ... >>> myFoo = Foo() >>> myFoo.__bar #direct attempt no go Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'Foo' object has no attribute '__bar' >>> myFoo.PrintBar() # the class itself of course can access it 99 >>> dir(Foo) # yet can see it ['PrintBar', '_Foo__bar', '__class__', '__delattr__', '__dict__', '__doc__', '__ format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__ ', '__subclasshook__', '__weakref__'] >>> myFoo._Foo__bar #and get to it by its mangled name ! (but I shouldn't!!!) 99 >>>