Error when refering to class defined in 'main' in an imported module

[HeRo]

Hi!

In my 'main.py' I have this:

class ScalableLabel(Label):
    pass

from sharebox_ios_etc import ShareBox

And in 'sharebox_ios_etc.py' I have this:

class ShareBox(ScalableLabel):

But that line produces this error:

Error:
 Traceback (most recent call last):
   File "/mnt/4AF15A0435E762B4/mypython/GeoESP/main.py", line 70, in <module>
     from sharebox_ios_etc import ShareBox
   File "/mnt/4AF15A0435E762B4/mypython/GeoESP/sharebox_ios_etc.py", line 15, in <module>
     class ShareBox(ScalableLabel):
 NameError: name 'ScalableLabel' is not defined

So how can I refer to a class defined in the main module in an imported module?
Do I have to but "class ScalableLabel(Label):" in a third 'common.py' module and import that in both of the others?
In that case I guess I would have to import Label in 'common.py'.