从不同文件中的同一 class 访问 function

Question

I am trying to access a function from within a class that is within a different file.

My layout is:

Directory
   -> ClassFile.py
   -> _AFile.py

in ClassFile:

class TestClass:

    from ._AFile import test_1, test_2, test_3

    def __init__(self):


    def RunAllTests(self):
        self.test_1()
        self.test_2()
        self.test_3()


    @staticmethod
    def __DoSomething(a, b):
        return a + b

in _AFile:

def test_1(self):
    self.__DoSomething

def test_2(self):
    self.__DoSomething

def test_3(self):
    self.__DoSomething

This is what I want to do but cannot as I get: AttributeError: 'TestClass' object has no attribute '__DoSomething'

How can I access the static method from TestClass within AFile?

Answer 1

You are calling a private method from outside its class declaration, and it happens that python is doing some transformations under the hood .

As explained in the link above, the private attributes and methods of a python class are modified under the form _ClassName__attributename . This is a protection to avoid collision with subclasses defining potentially the same attributes or methods.

As a result, to make your import work as you want, you have two possibilities:

• keep the method private, but use the generated method name from python
• Use protected methods to use in your secondary file.

The following files will show you both possibilities:

_AFile.py

def test_1(self):
    # Express which private method you want to use
    self._TestClass__DoSomething()

def test_2(self):
    # Use a protected attribute only
    self._do_another_thing()

ClassFile.py

class TestClass:

    from _AFile import test_1, test_2

    def __init__(self):
        pass

    def RunAllTests(self):
        self.test_1()
        self.test_2()

    @staticmethod
    def __DoSomething():
        print("done something")

    @staticmethod
    def _do_another_thing():
        print("done another thing")

Execution:

>>> from ClassFile import TestClass
>>> TestClass().RunAllTests()
done something
done another thing

Answer 2

Methods which start with __ are class-private and they are mangled and so you cant access them when importing form a different file.

Official Docs:

__ *

Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between “private” attributes of base and derived classes. See section Identifiers (Names).

Fix

Change __DoSomething to _DoSomething

You are spreading the implementation of class TestClass into two files ClassFile.py and _AFile.py . In such a case you need to put __init__.py (empty file) inside the module.

Fix

Your directory structure should look like below

mymodule
   --> __init__.py
   --> ClassFile.py
   --> _AFile.py

ClassFile.py

class TestClass:

    from ._AFile import test_1, test_2, test_3

    def __init__(self):
        pass


    def RunAllTests(self):
        self.test_1()
        self.test_2()
        self.test_3()


    @staticmethod
    def _DoSomething(a, b):
        result = a+b
        print (result)
        return result

_AFile.py

def test_1(self):
    self._DoSomething(1,2)

def test_2(self):
    self._DoSomething(2,3)

def test_3(self):
    self._DoSomething(4,5)

Testing:

from mymodule.ClassFile import TestClass
test = TestClass()
test.RunAllTests()

Output:

3
5
9