add Interpreter pattern

Author: JakubVojvoda

interpreter/Interpreter.py

@@ -0,0 +1,81 @@
+#
+# Python Design Patterns: Interpreter
+# Author: Jakub Vojvoda [github.com/JakubVojvoda]
+# 2016
+#
+# Source code is licensed under MIT License
+# (for more details see LICENSE)
+# 
+
+import sys
+
+#
+# Context
+# contains information that's global to the interpreter
+#
+class Context:
+  def __init__(self):
+    self._vars = {}
+  
+  def set(self, var, value):
+    self._vars[var] = value
+  
+  def get(self, exp):
+    return self._vars[exp]
+
+#
+# Abstract Expression
+# declares an abstract Interpret operation that is common to all nodes
+# in the abstract syntax tree
+#
+class AbstractExpression:
+  def interpret(self, context):
+    return False
+
+#
+# Terminal Expression
+# implements an Interpret operation associated with terminal symbols
+# in the grammar (an instance is required for every terminal symbol
+# in a sentence)
+#
+class TerminalExpression(AbstractExpression):
+  def __init__(self, value):
+    AbstractExpression.__init__(self)
+    self._value = value
+  
+  def interpret(self, context):
+    return context.get(self._value)
+
+#
+# Nonterminal Expression
+# implements an Interpret operation for nonterminal symbols
+# in the grammar (one such class is required for every rule in the grammar)
+#
+class NonterminalExpression(AbstractExpression):
+  def __init__(self, expr_left, expr_right):
+    self._lop = expr_left
+    self._rop = expr_right
+
+  def interpret(self, context):
+    return self._lop.interpret(context) and self._rop.interpret(context)
+
+
+# An example of very simple expression tree
+# that corresponds to expression (A AND B)
+if __name__ == "__main__":
+  A = TerminalExpression("A")
+  B = TerminalExpression("B")
+  exp = NonterminalExpression(A, B)
+
+  context = Context()
+  context.set("A", True)
+  context.set("B", True)
+
+  print(str(context.get("A")) + " AND " + str(context.get("B")), end = "" )
+  print(" = " + str(exp.interpret(context)))
+
+  context.set("A", True)
+  context.set("B", False)
+
+  print(str(context.get("A")) + " AND " + str(context.get("B")), end = "" )
+  print(" = " + str(exp.interpret(context)))