main.py:

# The variable that has to be set for mymodule to work
X = 2

import mymodule

mymodule.py:

print(X)

Fel:

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    import mymodule
  File "/home/jon/tmp/mymodule.py", line 2, in <module>
    print(X)
NameError: name 'X' is not defined