call a private method in an imported class [duplicate]

Question

I am having an unexpected issue in some code, and could reproduce it in a more simple example:

file1.py

class FirstClass: def function1(self): print("hello from function1") def __function2(self): print("hello from function2")

file2.py

from file1 import FirstClass fc = FirstClass() fc.function1() fc.__function2()

..and here's what happens:

$ python file2.py hello from function1 Traceback (most recent call last): File "file2.py", line 7, in <module> fc.__function2() AttributeError: FirstClass instance has no attribute '__function2'

What can you do so that the call to __function2 works? I am not really supposed to go into that imported class and make that private method public.

Well, it is the first time it has happened to me, that's why I wasn't expecting it. — cardamom
– cardamom, Commented Jan 11, 2019 at 15:54
You can do it with _FirstClass__function2. If you must. I won't bother with the health warnings. See the section Private Variables here: docs.python.org/3.7/tutorial/classes.html — BoarGules
– BoarGules, Commented Jan 11, 2019 at 15:56
Note that def __somefunction is not a private method, it's used for avoiding namespace collisions, as python has no privacy model for access — C.Nivs
– C.Nivs, Commented Jan 11, 2019 at 16:02
You can do it like was said, but should you? Someone has modelled the class thinking that client code should not access that "private" attribute. — progmatico
– progmatico, Commented Jan 11, 2019 at 16:06

Serge Ballesta · Accepted Answer · 2019-01-11 15:59:17Z

A function with a name starting with 2 underscore characters is not intented to be called from outside its class. And in order to allow users to redefine it in a subclass with each class calling its one (not the normal method override), its name is mangled to _className__methodName.

So here, you really should not use it directly, but if you really need to, you should be able to do:

fc._FirstClass__function2()

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