Variable scope

• Every Python variable has a scope, defining where it is created and who/when this variable can be accessed

• GLOBAL SCOPE: all functions have access to them.

  • These are created in the program’s main code chunk (“outside functions”)

• LOCAL SCOPE: only the function who created it has access to it

  • Parameters are also local variables
  • Local variables cease to exist when the function finishes!

a = 5 # a is a global variable
b = a + 1 # b is a global variable

def fun(d): # d, as a parameter, is a local variable
  c = 7 + a + d # c is a local variable
  print(c)


fun(1)

13

Scope rules

• Variables in different scopes can have the same names!

a = 5

def fun1():
  a = 1
  print(a)

def fun2():
  a = 3
  print(a)

print(a)

5

Scope rules

• Local variables have precedence over global variables!

a = 5

def fun1():
  a = 1
  print(a)

def fun2():
  a = 3
  print(a)

fun1()
fun2()
print(a)

1
3
5

Example

• What are the local and global variables in the following program?

x=4

def main():
  f1(3)
  f2(3)
  print(x)

def f1(a):
  x = 10
  print('f1',a+x)
  
def f2(a):
  c=10
  print('f2',a+x+c)

main()

f1 13
f2 17
4