inferences raises an error on Number input

    Some atoms should raise an error (like Numbers) and others
    (like Symbols) should not. A commented-out test was uncommented.

sympy/logic/inference.py

@@ -72,34 +72,37 @@ def pl_true(expr, model={}):
         return expr
 
     expr = sympify(expr)
-    if expr.is_Atom:
+
+    if expr.is_Symbol:
         return model.get(expr)
 
     args = expr.args
-    if expr.func is Not:
+    func = expr.func
+
+    if func is Not:
         p = pl_true(args[0], model)
         if p is None: return None
         else: return not p
-    elif expr.func is Or:
+    elif func is Or:
         result = False
         for arg in args:
             p = pl_true(arg, model)
             if p == True: return True
             if p == None: result = None
         return result
-    elif expr.func is And:
+    elif func is And:
         result = True
         for arg in args:
             p = pl_true(arg, model)
             if p == False: return False
             if p == None: result = None
         return result
 
-    elif expr.func is Implies:
+    elif func is Implies:
         p, q = args
         return pl_true(Or(Not(p), q), model)
 
-    elif expr.func is Equivalent:
+    elif func is Equivalent:
         p, q = args
         pt = pl_true(p, model)
         if pt == None:

sympy/logic/tests/test_inference.py

@@ -115,7 +115,7 @@ def test_pl_true_wrong_input():
     from sympy import pi
     raises(ValueError, "pl_true('John Cleese')")
     raises(ValueError, "pl_true(42+pi+pi**2)")
-    #raises(ValueError, "pl_true(42)")  #returns None, but should it?
+    raises(ValueError, "pl_true(42)")
 
 def test_PropKB():
     A, B, C = symbols('A,B,C')