Sort the args of Xor.

Complement to issue 4081.

sympy/logic/boolalg.py

@@ -386,6 +386,31 @@ class Xor(BooleanFunction):
     x
 
     """
+    def __new__(cls, *args, **options):
+        args = [_sympify(arg) for arg in args]
+
+        argset = set(args)
+        truecount = 0
+        for x in args:
+            if isinstance(x, Number) or x in [True, False]: # Includes 0, 1
+                argset.discard(x)
+                if x:
+                    truecount += 1
+        if len(argset) < 1:
+            return true if truecount % 2 != 0 else false
+        if truecount % 2 != 0:
+            return Not(Xor(*argset))
+        _args = frozenset(argset)
+        obj = super(Xor, cls).__new__(cls, *_args, **options)
+        if isinstance(obj, Xor):
+            obj._argset = _args
+        return obj
+
+    @property
+    @cacheit
+    def args(self):
+        return tuple(ordered(self._argset))
+
     @classmethod
     def eval(cls, *args):
         if not args:

sympy/printing/tests/test_str.py

@@ -5,7 +5,7 @@
     Interval, Lambda, Limit, Matrix, nan, O, oo, pi, Rational, Float, Rel,
     S, sin, SparseMatrix, sqrt, summation, Sum, Symbol, symbols, Wild,
     WildFunction, zeta, zoo, Dummy, Dict, Tuple, FiniteSet, factor,
-    MatrixSymbol, subfactorial, true, false, Equivalent)
+    MatrixSymbol, subfactorial, true, false, Equivalent, Xor)
 from sympy.core import Expr
 from sympy.physics.units import second, joule
 from sympy.polys import Poly, RootOf, RootSum, groebner, ring, field, ZZ, QQ, lex, grlex
@@ -698,4 +698,7 @@ def test_true_false():
     assert str(false) == repr(false) == sstr(false) == "False"
 
 def test_Equivalent():
-    assert str(Equivalent(x, y)) == "Equivalent(x, y)"
+    assert str(Equivalent(y, x)) == "Equivalent(x, y)"
+
+def test_Xor():
+    assert str(Xor(y, x, evaluate=False)) == "Xor(x, y)"