modify xthreaded and xreplace to fail gracefully

- xreplace leave args that don't implement xreplace alone
- xtrheaded allows objects that don't rebuild to return unchanged

sympy/core/basic.py

@@ -1050,7 +1050,13 @@ def xreplace(self, rule):
         if self in rule:
             return rule[self]
         elif rule:
-            args = tuple([arg.xreplace(rule) for arg in self.args])
+            args = []
+            for a in self.args:
+                try:
+                    args.append(a.xreplace(rule))
+                except AttributeError:
+                    args.append(a)
+            args = tuple(args)
             if not _aresame(args, self.args):
                 return self.func(*args)
         return self

sympy/polys/partfrac.py

@@ -65,7 +65,9 @@ def apart(f, x=None, full=False, **options):
             nc = Mul(*[apart(i, x=x, full=full, **_options) for i in nc])
             if c:
                 c = apart(Mul._from_args(c), x=x, full=full, **_options)
-            return c*nc
+                return c*nc
+            else:
+                return nc
         elif f.is_Add:
             c = []
             nc = []

sympy/utilities/decorator.py

@@ -14,7 +14,10 @@ def threaded_func(expr, *args, **kwargs):
         if isinstance(expr, Matrix):
             return expr.applyfunc(lambda f: func(f, *args, **kwargs))
         elif iterable(expr):
-            return expr.__class__([ func(f, *args, **kwargs) for f in expr ])
+            try:
+                return expr.__class__([func(f, *args, **kwargs) for f in expr])
+            except TypeError:
+                return expr
         else:
             expr = sympify(expr)