Fibonacci sequence

[Darbandiman123]

Hey i have this code which should print out fibonaccis sequence yet encounter the following error.

Error:
Traceback (most recent call last):
  File "C:\Users\Louis\csyr9\fibonacci.py", line 12, in <module>
    fibonacci()
  File "C:\Users\Louis\csyr9\fibonacci.py", line 5, in fibonacci
    while a <= 20:
UnboundLocalError: local variable 'a' referenced before assignment

here is the code:

a = 1
b = 2

def fibonacci():
    while a <= 20:
        c = a+b
        d = b+c
        print (a,b,c,d)
        a = c
        b = d
fibonacci()

how do i fix this? thanks