Adding in the evaluate postfix notation using Stack

data_structures/stacks/evaluate_postfix_notations.py

@@ -1,12 +1,24 @@
-'''
-The Reverse Polish Nation also known as Polish postfix notation or simply postfix notation
-https://en.wikipedia.org/wiki/Reverse_Polish_notation
-Classic examples of simple stack implementations
-Valid operators are +, -, *, /. Each operand may be an integer or another expression.
-'''
+# The Reverse Polish Nation also known as Polish postfix notation or simply postfix notation
+# https://en.wikipedia.org/wiki/Reverse_Polish_notation
+# Classic examples of simple stack implementations
+# Valid operators are +, -, *, /. Each operand may be an integer or another expression.
 
 
 def evaluate_postfix(postfix_notation):
+    """
+    >>> array = ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
+    >>> evaluate_postfix(array)
+    22
+    >>> array = ["2", "1", "+", "3", "*"]
+    >>> evaluate_postfix(array)
+    9
+    >>> array = ["4", "13", "5", "/", "+"]
+    >>> evaluate_postfix(array)
+    6
+    >>> array = []
+    >>> evaluate_postfix(array)
+    0
+    """
     operations = {'+', '-', '*', '/'}
     stack = []
 
@@ -26,10 +38,11 @@ def evaluate_postfix(postfix_notation):
                     stack.append(a // b)
         else:
             stack.append(int(token))
-    return stack.pop()
 
+    return stack.pop() if len(stack) != 0 else 0
 
-assert evaluate_postfix(
-    ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]) == 22
-assert evaluate_postfix(["2", "1", "+", "3", "*"]) == 9
-assert evaluate_postfix(["4", "13", "5", "/", "+"]) == 6
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()