Re: Updating a variable problem.
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
--
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Re: Updating a variable problem.
On 2020-08-04 18:25, Steve wrote:
How is MSN a new variable? It is not intended to be.
If line 8 is commented out, it is printable in all three locations.
If line 8 is not commented out, then MSN in the previous line is determined to
be undeclared.
It looks as if I am not allowed to change the contents of the variable MSN.
If there's an assignment to a variable anywhere in a function, then that
variable is assumed to be local to that function unless it's explicitly
declared global.
In 'EditTheEQlist', you're assigning to 'MSN' on the third line, and
you're not declaring that it's global in that function, so it's assumed
to be local. However, on the second line you're trying to use its value,
but you haven't assigned to it yet.
You declared that 'MSN' was global in 'ReadTheEQfile', but that's a
different function. 'global' applies only to the function it's used in.
From: Souvik Dutta
Sent: Tuesday, August 4, 2020 5:12 AM
To: Steve
Subject: Re: Updating a variable problem.
I don't know your exact meaning of fail. But as much as I can say there is a
new variable MSN being declared in the function that is only seen locally that
is inside the function. Now python sees this and probably says variable used
before assigning. You might try declaring a global msn in the function again.
And then changing msn after the print statement. Also this error occurred
because python first searches the variable in the local scope which is absent
earlier and so it searches for the variable in the global scope where it is
present and so no errors are raised.
Souvik flutter dev
On Tue, Aug 4, 2020, 2:30 PM Steve mailto:Gronicus@sga.ninja>
> wrote:
The print statement works in the EditTheEQlist() therefore the variable is seen
as having been declared. If I replace the value in the variable in the next
line then the print statement fails. Is this still a matter of globality?
If it is, how do I fix it?
FootNote:
If money does not grow on trees, then why do banks have branches?
From: Souvik Dutta mailto:souvik.vik...@gmail.com> >
Sent: Tuesday, August 4, 2020 4:50 AM
To: Steve mailto:Gronicus@sga.ninja> >
Cc: Python List mailto:python-list@python.org> >
Subject: Re: Updating a variable problem.
Probably because the MSN variable in the second function is not global.
On Tue, Aug 4, 2020, 2:08 PM Steve mailto:Gronicus@sga.ninja> > 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()
--
https://mail.python.org/mailman/listinfo/python-list
RE: Updating a variable problem.
How is MSN a new variable? It is not intended to be.
If line 8 is commented out, it is printable in all three locations.
If line 8 is not commented out, then MSN in the previous line is determined to
be undeclared.
It looks as if I am not allowed to change the contents of the variable MSN.
FootNote:
If money does not grow on trees, then why do banks have branches?
From: Souvik Dutta
Sent: Tuesday, August 4, 2020 5:12 AM
To: Steve
Subject: Re: Updating a variable problem.
I don't know your exact meaning of fail. But as much as I can say there is a
new variable MSN being declared in the function that is only seen locally that
is inside the function. Now python sees this and probably says variable used
before assigning. You might try declaring a global msn in the function again.
And then changing msn after the print statement. Also this error occurred
because python first searches the variable in the local scope which is absent
earlier and so it searches for the variable in the global scope where it is
present and so no errors are raised.
Souvik flutter dev
On Tue, Aug 4, 2020, 2:30 PM Steve mailto:Gronicus@sga.ninja> > wrote:
The print statement works in the EditTheEQlist() therefore the variable is seen
as having been declared. If I replace the value in the variable in the next
line then the print statement fails. Is this still a matter of globality?
If it is, how do I fix it?
FootNote:
If money does not grow on trees, then why do banks have branches?
From: Souvik Dutta mailto:souvik.vik...@gmail.com> >
Sent: Tuesday, August 4, 2020 4:50 AM
To: Steve mailto:Gronicus@sga.ninja> >
Cc: Python List mailto:python-list@python.org> >
Subject: Re: Updating a variable problem.
Probably because the MSN variable in the second function is not global.
On Tue, Aug 4, 2020, 2:08 PM Steve mailto:Gronicus@sga.ninja> > 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()
=
Footnote:
Genie: You have three wishes.
Me: I wish I had more wishes.
Genie: You cannot wish for more wishes.
Me: I wish I could.
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Re: Updating a variable problem.
Probably because the MSN variable in the second function is not global.
On Tue, Aug 4, 2020, 2:08 PM 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()
>
>
>
> =
> Footnote:
> Genie: You have three wishes.
> Me: I wish I had more wishes.
> Genie: You cannot wish for more wishes.
> Me: I wish I could.
>
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
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Updating a variable problem.
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()
=
Footnote:
Genie: You have three wishes.
Me: I wish I had more wishes.
Genie: You cannot wish for more wishes.
Me: I wish I could.
--
https://mail.python.org/mailman/listinfo/python-list
