the following code :

def test():
    code=''
    def sub(n):
        for i in range(n):
            code+=str(i)
    sub(5)
    sub(10)
    return code

>>> test()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "<console>", line 6, in test
  File "<console>", line 5, in sub
UnboundLocalError: local variable 'code' referenced before assignment

error came from the += operator.
Tested for code initialized to '', to 0
I guess it's the same for all inline operators.

I agree that global variables CANNOT be assigned, it's ok.

But += (and I guess *= etc) operators are not assignements, and are not 
different from .append(), or .extend() methods.

I was expecting += to work the same as append() method

Is code a global variable?

Inplace operators ("+=", "-=" etc) are assignments since in the case of
immutable types like str

code += "some string"

is equivalent to

code = code + "some string"