Use of Globals

[CWatters]

Looking for some clarification on the use of global variables.

Suppose I need a single variable called "Logged_In" that's available everywhere in my program (eg in multiple functions).

Can I simply declare it as global in the main code or must I also declare it as global in all the functions that need to use it?

Thanks

[wavic]

It is not recommended to use globals if you can avoid this. If you want a function to use a variable, pass it as a parameter. There is another way - make a class

class MyClass:
    num = 10

Then:

def func():
    print(MyClass.num)

[CWatters]

Thanks. Understood it's not good practice to use global variables but just trying to get my head around how python expect them to work.

[wavic]

Well, they work like this:

va1 = 1
var2 = 2

def func():
    var1 = 3
    var2 = 4
    print(var1, var2)

func () # output: 3 4

In order to print 1 2 you have to declare that you want to use the global variables.

def func_global():
    global var1
    global var2
    print(var1, var2)

func_global() # output: 1 2

[buran]

Understood it's not good practice to use global variables but just trying to get my head around how python expect them to work.

Hope this example helps

var1 = 1
var2 = 2
var3 = 3
var4 = 4

def foo():
    global var1
    print('var1 value within foo, before addition is {}'.format(var1))
    var1 += 10
    print('var1 value within foo, after addition is {}'.format(var1))
    
def bar():
    # variable with same name, but local namespace
    var3 = 0 # this is actually local var, there is also 'global' one with different value
    print('var3 value within bar, before addition is {}'.format(var3))
    var3 += 1
    print('var3 value within bar, after addition is {}'.format(var3))
    
def foo2():
    print('var4 value from within foo2 is {}'.format(var4))

def bar2():
    var2 += 10 # try to change value of var2 this will rise error
    print('var2 value within bar, before addition is {}'.format(var2))
       

    
print('var1 value before call of the foo/bar is {}'.format(var1))
print('var2 value before call of the foo/bar is {}'.format(var2))
print('var3 value before call of the foo/bar is {}\n\n'.format(var3))
foo()
print('var1 value after call of the foo is {}\n\n'.format(var1))
bar()
print('var3 value after call of the foo is {}\n\n'.format(var3)) # this refer to the 'global' one, value is still 3.
print('var4 value before call of the foo2 is {}'.format(var4))
foo2()
print('var4 value after call of the foo2 is {}\n\n'.format(var4))
bar2()

Output:
var1 value before call of the foo/bar is 1
var2 value before call of the foo/bar is 2
var3 value before call of the foo/bar is 3


var1 value within foo, before addition is 1
var1 value within foo, after addition is 11
var1 value after call of the foo is 11


var3 value within bar, before addition is 0
var3 value within bar, after addition is 1
var3 value after call of the foo is 3


var4 value before call of the foo2 is 4
var4 value from within foo2 is 4
var4 value after call of the foo2 is 4


Traceback (most recent call last):
File "C:\myfile.py", line 38, in <module>
bar2()
File "C:\myfile.py", line 23, in bar2
var2 += 10 # try to change value of var2 this will rise error
UnboundLocalError: local variable 'var2' referenced before assignment
>>>

[CWatters]

Sorry I forgot to say thank you for the replies. They make thinks a lot clearer.