Changing values inside Python Modules

Question

I have a simple Python module file First.py :

a = 50
b = [100,200,300]

I try to import this module into another file Test.py :

import First
First.a = 420
First.b[0] = 420
print (First.a)

My purpose is to change the list values inside the First module.

Once the script Test.py completes when I print the values inside the module , I find that the values have not changed.

Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 25 2016, 22:01:18) [MSC v.1900 32 bit (I
tel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import First
>>> dir(First)
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__',
'__package__', '__spec__', 'a', 'b']
>>> First.a
50
>>> First.b
[100, 200, 300]

What am I missing here? Can someone kindly guide me?

Answer 1

Once the script Test.py completes when I print the values inside the module , i find that the values have not changed.

If you executed python Test.py , then fired up your interpreter interactively and checked the values, of course, modifications won't be visible. Python just loads First.py when the import is found, executing it and initializing a and b with the values in First.py ; previous executions won't affect this.

If you import Test in your interactive interpreter and then import First changes will be reflected:

Python 3.5.2 |Anaconda 4.2.0 (64-bit)| (default, Jul  2 2016, 17:53:06) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import Test
420
>>> import First
>>> First.a
420
>>> First.b
[420, 200, 300]

During the import of Test , First was loaded and executed, then its values modified. When you re-import python will just look in a table of imported modules ( sys.modules ) and return it without executing its content (and re-initializing a and b )