Global and local variables

Question

Can anyone please explain why below gives the error “ local variable ‘a’ referenced before assignment” ?

a=[1,2,3]
def add(n):
a+=[n]
add(4)

Answer 1

a=[1,2,3]
def add(n):
global a
a+=[n]
print(a)
add(4)

you have to use global keyword if you have to code like you asked in question or you have to code like choe said

Answer 2

better to do this

a = [1,2,3]
def add(n):
a.append(n)
print(a)
add(4)