Python 3 - Second Edition

44.6    Copying Instances: copy

As you already know, in Python an assignment only creates a new reference to the same instance instead of creating a copy of the instance.

In the following example, s and t reference the same list, as the comparison with is reveals:

>>> s = [1, 2, 3]>>> t = s>>> t is sTrue  

This approach doesn't always behave as intended because changes to the list referenced by s also affect t due to side effects and vice versa.

For example, if a method of a class returns a list that’s also used within the class, the list can also be modified via the returned reference, which is usually not what we want:

class MyClass:    def __init__(self):        self.list = [1, 2, 3]    def get_list(self):        return self.list    def show_list(self):        print(