Python: derived classes access dictionary of base class in the same memory location

Question

I\'m wondering why a dictionary, that is defined in a base class and is accessed from derived classes, is obviously present only in one memory location. A short example:

Answer 1

In python, int is immutable, therefore the += operation will rebound the class variable into an instance variables. On the other hand, a dictionary indexing mutates the dictionary in place. A more comparable example would be

def add_dict_entry(self):
    # create a new dict
    tmp = dict(self._testdict)
    tmp["first"] = 1

    # shadow the class variable with an instance variables
    self._testdict = tmp