Updating a variable problem.

[dn]

On 04/08/2020 20:38, Steve wrote:
> Why should line 6 fail until line 7 is commented out?
> Python complains that MSN is "referenced before assignment".
> 
> def ReadTheEQfile():
>    global MSN
>    MSN = ("1 Monitor") #This line works every time.
>      
> def EditTheEQlist():
>    print("MSN2 = " + MSN) # Works if the next line is commented out.
>    MSN = ("3 Monitor")
>          
> # Main()
> ReadTheEQfile()
> print("MSN1 = " + MSN) # This line works every time
> EditTheEQlist()

NB there are no lineNRs above!
(added comment/guide == good job!)


Others have answered the question. Here is some reading to consolidate 
your understanding:-

What are the rules for local and global variables in Python?
https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python

Assignment is defined recursively depending on the form of the target 
(list). When a target is part of a mutable object (an attribute 
reference, subscription or slicing), the mutable object must ultimately 
perform the assignment and decide about its validity, and may raise an 
exception if the assignment is unacceptable. The rules observed by 
various types and the exceptions raised are given with the definition of 
the object types (see section The standard type hierarchy).
...
If the target is an identifier (name):

     If the name does not occur in a global or nonlocal statement in the 
current code block: the name is bound to the object in the current local 
namespace.

     Otherwise: the name is bound to the object in the global namespace 
or the outer namespace determined by nonlocal, respectively.
https://docs.python.org/3/reference/simple_stmts.html#assignment-statements

...we have the function modifying x. It may appear somewhat confusing 
since x is being used in multiple locations...
https://pythonprogramming.net/global-local-variables/

"Scope", particularly in its applications to Classes and Namespaces:
https://docs.python.org/3/tutorial/classes.html

The "LEGB" 'rule' (NB not a term you'll find in the Python docs) ie 
Local, Enclosing, Global, and Built-in scopes:
https://realpython.com/python-scope-legb-rule/
and more in:
https://realpython.com/python-namespaces-scope/
-- 
Regards =dn