Conclusion:

internal function, do not change the global variable can access the global variable internal function, modify the same global variable, python will think it is a local variable before internal function to modify a global variable with the same call variable names (such as print the sum), cause Unbound – LocalError

The sum set in the program belongs to the global variable, and there is no definition of sum in the function. According to Python’s rules for accessing local and global variables, when searching for a variable,
If the variable is not found in the local scope, then Python looks for the variable in the global variable. If it is not found, it throws an exception (NameError or Unbound-LocalError, depending on the Python version).

if you have any references to external internal function function of the same variable or a global variable, and is modified for this variable. Python assumes that it is a local variable, and because there is no definition or assignment of sum in the function, it returns an error.

from the following two procedures separate access or modify a global variable, is not an error ~

# accessing global variables #! /usr/bin/python

import
sys

sum
=
5

def
add
(
a
=
1
.
b
=
3
) :

    
print
a
.
b

    
print
sum
#
Just to visit
 

add
(
4
.
8
)

print
sum

[
root
@rac3
python
]
# python local.py

4
8

5

5

# modify a global variable of the same name
Is considered a local variable

#! /usr/bin/python

import
sys

sum
=
5

def
add
(
a
=
1
.
b
=
3
) :

    
print
a
.
b

#
An inner function has a variable of the same name or a global variable that refers to an outer function, and changes are made to that variable. Python will think of it as a local variable

    
sum
=
b
+
a
#
Modify inside the function

    
print
sum

add
(
4
.
8
)

[
root
@rac3
python
]
# python local.py

4 8

12

If the inner function has a variable of the same name or a global variable that refers to the outer function, and the variable is modified. Python assumes that it is a local variable, and because there is no definition or assignment of sum in the function, it returns an error

#! /usr/bin/python

import
sys

sum
=
5

def
add
(
a
=
1
.
b
=
3
) :

    
print
a
.
b

    
print
The sum # inner function refers to a variable of the same name and modifies that variable. Python thinks of it as a local variable. Because the sum variable was not defined before print here, an error will be reported. (It is recommended to compare with case 1, note: this is only before the above example: print sum)

    
sum
=
b
+
a

    
print
sum

add
(
4
.
8
)

print
sum

[
root
@rac3
python
]
# python local.py

4
8

Traceback
(
most
recent
call
last
) :

 
File
“local.py”
.
line
10
.
in
?

   
add
(
4
.
8
)

 
File
“local.py”
.
line
6
.
in
add

   
print
sum

UnboundLocalError
:
local
variable
‘sum’
referenced
before
assignment

You can use the: global keyword when you access a global variable in your program and you want to change the value of the global variable. Declare the variable as a global in a function

#! /usr/bin/python

import
sys

sum
=
5

print
‘Before changing: sum=’
.
sum

def
add
(
a
=
1
.
b
=
3
) :

    
global
sum

    
print
‘add ‘:sum=’
.
sum

    
sum
=
b
+
a

    
print
‘function :sum=’
.
sum

add
(
4
.
8
)

print
‘change sum=’
.
sum

[
root
@rac3
python
]
# vim local.py

Before you change:
sum
=
5

add
In the function
:
sum
=
5

When I change the function
:
sum
=
 
12

After the change
sum
=
12