What is so special about += operator? [duplicate]

I wrote a code similar to the following and it gives me the local variable 'a' referenced before assignment error. When I changed that a += [2] into a.append(2), it worked.

def f():
   a = [1]
   def f1():
      a += [2] # => no error with a.append(2)
   f1()
   print(a)

Why? Why the parser can't recognize the outside a with +=?

Solution 1:

It's an assignment to a. It's basically syntactic sugar for

a = a.__iadd__([2])

The assignment makes a a local variable when the code is generated, but then the RHS of the assignment tries to access that variable before at runtime it is defined.

a.append(2), on the other hand, is not an assignment. a is a free variable whose value is taken from the closest enclosing scope.

If you want to assign to a non-local variable, you need to declare the name as non-local first.

def f():
   a = [1]
   def f1():
      nonlocal a
      a += [2]
   f1()
   print(a)