Demonstrating a gotcha of Python mutable types

mutable_probs.py

"""
Simple demonstration of the gotcha that can occur in Python with mutable
types being shared across instances of a class

Taken with slight modification from the Python Guide
http://docs.python-guide.org/en/latest/writing/gotchas/

(The same thing applies to default arguments)
"""

class Test(object):
    lst = []

    def print_lst(self):
        print(self.lst)

    def add_to_lst(self, x):
        self.lst.append(x)


class OtherTest(object):
    
    def __init__(self, *args, **kwargs):
        self.lst = []

    def print_lst(self):
        print(self.lst)

    def add_to_lst(self, x):
        self.lst.append(x)


""" Program driver """
if __name__ == '__main__':
    test1, test2 = Test(), Test()
    print('Appending 1, 2, and 3 to test1')
    test1.add_to_lst(1); test1.add_to_lst(2); test1.add_to_lst(3)

    print('test1.print_lst()')
    test1.print_lst()
    print()

    print('test2.print_lst()')
    test2.print_lst()
    print()

    print('As you can see, the list in test2 contained the same elements\n'
          'as the list in test1 because they point to the same object\n' 
          'since they were defined at the class level\n')

    print('Now we\'ll do the same thing with the OtherTest class, where\n'
          'the list was defined in the __init__ method\n')

    test1, test2 = OtherTest(), OtherTest()
    print('Appending 1, 2, and 3 to test1')
    test1.add_to_lst(1); test1.add_to_lst(2); test1.add_to_lst(3)

    print('test1.print_lst()')
    test1.print_lst()
    print()

    print('test2.print_lst()')
    test2.print_lst()
    print()

    print('Now the two objects have two separate lists, independent of\n'
          'each other')