replace TRUE, FALSE with True, False

norvig · norvig · commit 75617fa18b29 · 2016-04-11T15:57:27.000-07:00
diff --git a/logic.py b/logic.py
@@ -144,9 +144,8 @@ def is_var_symbol(s):
 
 
 def is_prop_symbol(s):
-    """A proposition logic symbol is an initial-uppercase string other than
-    TRUE or FALSE."""
-    return is_symbol(s) and s[0].isupper() and s != 'TRUE' and s != 'FALSE'
+    """A proposition logic symbol is an initial-uppercase string."""
+    return is_symbol(s) and s[0].isupper()
 
 
 def variables(s):
@@ -184,7 +183,6 @@ def parse_definite_clause(s):
         return conjuncts(antecedent), consequent
 
 # Useful constant Exprs used in examples and code:
-TRUE, FALSE = Symbol('TRUE'), Symbol('FALSE')
 A, B, C, D, E, F, G, P, Q, x, y, z = map(Expr, 'ABCDEFGPQxyz')
 
 
@@ -233,20 +231,18 @@ def tt_true(s):
     True
     """
     s = expr(s)
-    return tt_entails(TRUE, s)
+    return tt_entails(True, s)
 
 
 def pl_true(exp, model={}):
     """Return True if the propositional logic expression is true in the model,
     and False if it is false. If the model does not specify the value for
     every proposition, this may return None to indicate 'not obvious';
     this may happen even when the expression is tautological."""
+    if exp == True or exp == False:
+        return exp
     op, args = exp.op, exp.args
-    if exp == TRUE:
-        return True
-    elif exp == FALSE:
-        return False
-    elif is_prop_symbol(op):
+    if is_prop_symbol(op):
         return model.get(exp)
     elif op == '~':
         p = pl_true(args[0], model)
@@ -364,7 +360,7 @@ def distribute_and_over_or(s):
         if s.op != '|':
             return distribute_and_over_or(s)
         if len(s.args) == 0:
-            return FALSE
+            return False
         if len(s.args) == 1:
             return distribute_and_over_or(s.args[0])
         conj = first(arg for arg in s.args if arg.op == '&')
@@ -397,7 +393,7 @@ def associate(op, args):
     else:
         return Expr(op, *args)
 
-_op_identity = {'&': TRUE, '|': FALSE, '+': 0, '*': 1}
+_op_identity = {'&': True, '|': False, '+': 0, '*': 1}
 
 
 def dissociate(op, args):
@@ -447,7 +443,7 @@ def pl_resolution(KB, alpha):
                  for i in range(n) for j in range(i+1, n)]
         for (ci, cj) in pairs:
             resolvents = pl_resolve(ci, cj)
-            if FALSE in resolvents:
+            if False in resolvents:
                 return True
             new = new.union(set(resolvents))
         if new.issubset(set(clauses)):
