Scope


Scope
Variables can only reach the area in which they are defined, which is called scope. Think of it as the area of code where variables can be used. Python supports global variables (usable in the entire program) and local variables.

By default, all variables declared in a function are local variables. To access a global variable inside a function, it’s required to explicitly define ‘global variable’.

Related course
Complete Python Bootcamp: Go from zero to hero in Python

Example
Below we’ll examine the use of local variables and scope. This will not work:

#!/usr/bin/python
 
def f(x,y):
    print('You called f(x,y) with the value x = ' + str(x) + ' and y = ' + str(y))
    print('x * y = ' + str(x*y))
    z = 4 # cannot reach z, so THIS WON'T WORK
 
z = 3
f(3,2)

but this will:

#!/usr/bin/python
 
def f(x,y):
    z = 3
    print('You called f(x,y) with the value x = ' + str(x) + ' and y = ' + str(y))
    print('x * y = ' + str(x*y))
    print(z) # can reach because variable z is defined in the function
 
f(3,2)

Let’s examine this further:

#!/usr/bin/python
 
def f(x,y,z):
    return x+y+z # this will return the sum because all variables are passed as parameters
 
sum = f(3,2,1)
print(sum)

Calling functions in functions
We can also get the contents of a variable from another function:

#!/usr/bin/python
 
def highFive():
    return 5
 
def f(x,y):
    z = highFive() # we get the variable contents from highFive()
    return x+y+z # returns x+y+z. z is reachable becaue it is defined above
 
result = f(3,2)
print(result)

If a variable can be reached anywhere in the code is called a global variable. If a variable is known only inside the scope, we call it a local variable.


Global and Local variables
Loops: For loop, while loop

4 thoughts on “Scope

  1. Carl Wainwright - August 28, 2016

    I ran this code in v2.7.10 and it executed without any errors

    def f(x,y):
        print('You called f(x,y) with the value x = ' + str(x) + ' and y = ' + str(y))
        print('x * y = ' + str(x*y))
        z = 4 # cannot reach z, so THIS WON'T WORK
        print(z)
     
    z = 3
    f(3,2)
    MacBookAir:.ssh carlwainwright$ python ~/Projects/Python/helloworld.py 
    You called f(x,y) with the value x = 3 and y = 2
    x * y = 6
    4
    1. Frank - August 28, 2016

      Try this program:

      def showz():
          print(z)
       
      def f(x,y):
          print('You called f(x,y) with the value x = ' + str(x) + ' and y = ' + str($
          print('x * y = ' + str(x*y))
          z = 4 # cannot reach z, so THIS WON'T WORK
          print(z)
       
      z = 3
      f(3,2)
      showz()

      In this example z will still be 3 after calling f(3,2). Sorry for the confusion, ill update

      1. Carl Wainwright - August 29, 2016

        So without “global z” in function ‘f’ z is local to the function and when you return back the global z is scoped.

        1. Frank - August 31, 2016

          Correct, otherwise the changed occur only locally