Add identity operators example

Author: phrdang

1_beginner/chapter6/examples/identity_operators.py

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+# Identity Operators
+
+message_1 = 'hello'
+message_2 = 'hello'
+
+print(message_1 is message_2)  # True
+print(message_1 is not message_2)  # False
+
+message_3 = 'world'
+message_4 = 'tahiti'
+
+print(message_3 is message_4)  # False
+print(message_3 is not message_4)  # True
+
+# Identity vs Equality
+
+# primitive values, if duplicated,
+# are stored in the same location
+x = 5
+y = 5
+if x is y:
+    print('x is y')
+else:
+    print('x is not y')
+
+# lists are objects,
+# so they are located in different
+# locations in memory
+a = [1, 2, 3]
+b = [1, 2, 3]
+
+# use the identity operator
+# to test if the 2 variables
+# reference the same location
+# in memory
+if a is b:
+    print('a is b')
+else:
+    print('a is not b')
+
+# use the equality operator
+# to test if content is equivalent
+if a == b:
+    print('a == b')
+else:
+    print('a != b')