Local scope gone wrong

a = 42
def f():
    print(a)
    a = 23

print('ok')
print(a)
f()
print(a)

ok
42
Traceback (most recent call last):
  File "scoping_external_variable.py", line 8, in <module>
    f()
  File "scoping_external_variable.py", line 3, in f
    print(a)
UnboundLocalError: local variable 'a' referenced before assignment

Accessing a global variable inside a function works, but if I change it (make it refer to another piece of data), then it is disallowed. If I only change the data inside (for mutable variables), that works, but is a bad practice.