UnboundLocalError while using += but not append list [duplicate]

Question

I do not quite understand the difference between the following two similar codes:

def y(x):
    temp=[]
    def z(j):
        temp.append(j)
    z(1)
    return temp

calling y(2) returns [1]

def y(x):
    temp=[]
    def z(j):
        temp+=[j]
    z(1)
    return temp

calling y(2) returns UnboundLocalError: local variable 'temp' referenced before assignment. Why + operator generates the error? Thanks

Answer 1

Answer to the heading, the difference between + and "append" is:

[11, 22] + [33, 44,] 

will give you:

[11, 22, 33, 44]

and.

b = [11, 22, 33]
b.append([44, 55, 66]) 

will give you

[11, 22, 33 [44, 55, 66]] 

Answer to the error

This is because when you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope

The problem here is temp+=[j] is equal to temp = temp +[j]. The temp variable is read here before its assigned. This is why it's giving this problem. This is actually covered in python FAQ's.

For further readings, click here. :)