BrandonLMorris · February 12, 2016 14:54 · Feb 12, 2016
diff --git a/mutable_probs.py b/mutable_probs.py
@@ -0,0 +1,69 @@
+"""
+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')
+
+