Importing a Python module from another module at the same level

I am working on a Python package with the basic structure given below and examples of what each Python file contains in curly braces.

MAIN_PACKAGE/
    setup.py
    main_package/
        __init__.py 
        { 
        import package1
        import package2
        __all__=['main_package']
        }
        package1/
            __init__.py
            {
            import module1
            import module2
            __all__=['package1']
            }
            module1/
                __init__.py
                {
                from script1 import Class1A, Class1B
                __all__ = ['script1']
                }
                script1.py
                {contains 2 classes: Class1A and Class1B}

            module2/
                __init__.py
                {
                from script2 import Class2A, Class2B
                __all__ = ['script2']
                }
                script2.py
                {contains 2 classes: Class2A, Class2B}

            module3/
                __init__.py
                {
                from script3 import Class3A, Class3B
                __all__ = ['script3'] 
                }
                script3.py
                {contains 2 classes: Class3A, Class3B}

        package2/
                executable_script.py

      

        
        
        
      

    

This hasn't got me in trouble to date. However, I recently added a new module that does not import correctly (module3). I found several knitted strands, but none of these special tastes.

I can successfully install this using setup.py and then from the command line (any directory) import the following importers:

python
>>> from main_package.package1.module3.script3 import Class3A, Class3B

      

        
        
        
      

    

But when I copy the same line in script2 and reload the setup.py file (no errors), I get the following error on startup executable_script

:

ImportError: cannot import name Class3A

      

        
        
        
      

    

And then I also get the same error when I try to import into python via command line. BUT, I have this problem when I try to import Class3A from script3 to script2, not importing similar classes from script1 to script2. The only difference I can see is that script3 is a newer module than script1. Is there something that cannot be updated so that I can import this new module / class into an older script? Or is there something wrong with the structure that should have caught up with me?

Another detail is that this error only occurs when this line is added to another module at the same level, but the error occurs executable_script.py

in package2. For example. first line of error message

/usr/bin/executable_script, line 9, in <module>load_entry_point('MAIN_PACKAGE==0.1.0','console_scripts','executable_script')()

...

Line 9 of the executable script is part of the document line. I am using Python 2.7.