Change global variable in function python

Global Variables

Variables that are created outside of a function (as in all of the examples above) are known as global variables.

Global variables can be used by everyone, both inside of functions and outside.

Example

Create a variable outside of a function, and use it inside the function

x = "awesome"

def myfunc():
  print("Python is " + x)

myfunc()

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If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the function. The global variable with the same name will remain as it was, global and with the original value.

Example

Create a variable inside a function, with the same name as the global variable

x = "awesome"

def myfunc():
  x = "fantastic"
  print("Python is " + x)

myfunc()

print("Python is " + x)

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The global Keyword

Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function.

To create a global variable inside a function, you can use the global keyword.

Example

If you use the global keyword, the variable belongs to the global scope:

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

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Also, use the global keyword if you want to change a global variable inside a function.

Example

To change the value of a global variable inside a function, refer to the variable by using the global keyword:

x = "awesome"

def myfunc():
  global x
  x = "fantastic"

myfunc()

print("Python is " + x)

Try it Yourself »

Within a Python scope, any assignment to a variable not already declared within that scope creates a new local variable unless that variable is declared earlier in the function as referring to a globally scoped variable with the keyword global.

Let's look at a modified version of your pseudocode to see what happens:

# Here, we're creating a variable 'x', in the __main__ scope.
x = 'None!'

def func_A():
  # The below declaration lets the function know that we
  #  mean the global 'x' when we refer to that variable, not
  #  any local one

  global x
  x = 'A'
  return x

def func_B():
  # Here, we are somewhat mislead.  We're actually involving two different
  #  variables named 'x'.  One is local to func_B, the other is global.

  # By calling func_A(), we do two things: we're reassigning the value
  #  of the GLOBAL x as part of func_A, and then taking that same value
  #  since it's returned by func_A, and assigning it to a LOCAL variable
  #  named 'x'.     
  x = func_A() # look at this as: x_local = func_A()

  # Here, we're assigning the value of 'B' to the LOCAL x.
  x = 'B' # look at this as: x_local = 'B'

  return x # look at this as: return x_local

In fact, you could rewrite all of func_B with the variable named x_local and it would work identically.

The order matters only as far as the order in which your functions do operations that change the value of the global x. Thus in our example, order doesn't matter, since func_B calls func_A. In this example, order does matter:

def a():
  global foo
  foo = 'A'

def b():
  global foo
  foo = 'B'

b()
a()
print foo
# prints 'A' because a() was the last function to modify 'foo'.

Note that global is only required to modify global objects. You can still access them from within a function without declaring global. Thus, we have:

x = 5

def access_only():
  return x
  # This returns whatever the global value of 'x' is

def modify():
  global x
  x = 'modified'
  return x
  # This function makes the global 'x' equal to 'modified', and then returns that value

def create_locally():
  x = 'local!'
  return x
  # This function creates a new local variable named 'x', and sets it as 'local',
  #  and returns that.  The global 'x' is untouched.

Note the difference between create_locally and access_only -- access_only is accessing the global x despite not calling global, and even though create_locally doesn't use global either, it creates a local copy since it's assigning a value.

The confusion here is why you shouldn't use global variables.

Global & local variables with same name

Checkout this example,

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total = 100 

def func1(): 
   total = 15 

print('Total = ', total) 

func1() 

print('Total = ', total)

Output:

Total = 100 
Total = 100

Here 'total' is a global variable and func() function has a local variable with same name. By default a function gives preference to
local variable over global variable if both are of same name. Therefore in above code when we modified 'total' variable inside the function then it was not reflected outside the function. Because inside function func() total variable is treated as local variable.

But what if want to access global variable inside a function that has local variable with same name ?

Use of “global†keyword to modify global variable inside a function

If your function has a local variable with same name as global variable and you want to modify the global variable inside function then use 'global' keyword before the variable name at start of function i.e.

global total

It will make the function to refer global variable total whenever accessed. Checkout this example,

total = 100
def func():
    # refer to global variable 'total' inside function
    global total
    if total > 10:
        total = 15

print('Total = ', total)
func()
print('Total = ', total)

Output:

Total =  100
Total =  15

As you can see modification done to global variable total is now visible outside the function too.

When we use global keyword with a variable inside the function then the local variable will be hidden. But what if we want to keep bot the local & global variable with same and modify both in the function ? Let's see how to do that,

Using globals() to access global variables inside the function

As 'global' keywords hide the local variable with same name, so to access both the local & global variable inside a function there is an another way i.e. global() function.
globals() returns a dictionary of elements in current module and we can use it to access / modify the global variable without using 'global' keyword i,e.

total = 100

def func3():
    listOfGlobals = globals()
    listOfGlobals['total'] = 15
    total = 22
    print('Local Total = ', total)

print('Total = ', total)
func3()
print('Total = ', total)

Output:

Total =  15
Local Total =  22
Total =  11

As you can see that we have local variable and global variable with same name i.e. total and we modified both inside the function. By using dictionary returned by globals() to refer global variable instead of keyword 'global'. It will not hide local variable inside the function.

Handling UnboundLocalError Error

If we try to access a global variable with 'global' keyword or globals() inside a function i.e.

total = 22
def func2():
    if total > 10:
        total = 15

It will throw an error like this,

UnboundLocalError: local variable 'total' referenced before assignment

To prevent this error we either need to use 'global' keyword or global() function i.e.

total = 22 

def func2():
    global total
    if total > 10:
        total = 15

The Complete example of global variable and globals() in Python

# Global variable
total = 100

def test():
    # Local variable
    marks = 19
    print('Marks = ', marks)
    print('Total = ', total)

def func1():
    total = 15

def func():
    # refer to global variable 'total' inside function
    global total
    if total > 10:
        total = 15

def func2():
    global total
    if total > 10:
        total = 15

def func3():
    listOfGlobals = globals()
    listOfGlobals['total'] = 11
    total = 22
    print('Local Total = ', total)


def main():
    print('Total = ', total)
    func1()
    print('Total = ', total)
    func()
    print('Total = ', total)
    func2()
    print('Total = ', total)
    func3()
    print('Total = ', total)

if __name__ == '__main__':
    main()

Output:

Total =  100
Total =  100
Total =  15
Total =  15
Local Total =  22
Total =  11

Can you change a global variable in a function Python?

Can you change global variables in a function?

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