Do not compute parenthesization twice

Previously both `PrettyPrinter._print_Mul` and `prettyForm.__mul__` tried to apply parenthesis, and the action of the former hid a typo in the latter.

sympy/printing/pretty/pretty.py

@@ -222,7 +222,7 @@ def _print_Relational(self, e):
 
         l = self._print(e.lhs)
         r = self._print(e.rhs)
-        pform = prettyForm(*stringPict.next(l, op, r))
+        pform = prettyForm(*stringPict.next(l, op, r), binding=prettyForm.OPEN)
         return pform
 
     def _print_Not(self, e):
@@ -1767,23 +1767,9 @@ def _print_Mul(self, product):
 
         from sympy import Integral, Piecewise, Product, Sum
 
-        # Convert to pretty forms. Add parens to Add instances if there
-        # is more than one term in the numer/denom
-        for i in range(0, len(a)):
-            if (a[i].is_Add and len(a) > 1) or (i != len(a) - 1 and
-                    isinstance(a[i], (Integral, Piecewise, Product, Sum))):
-                a[i] = prettyForm(*self._print(a[i]).parens())
-            elif a[i].is_Relational:
-                a[i] = prettyForm(*self._print(a[i]).parens())
-            else:
-                a[i] = self._print(a[i])
-
-        for i in range(0, len(b)):
-            if (b[i].is_Add and len(b) > 1) or (i != len(b) - 1 and
-                    isinstance(b[i], (Integral, Piecewise, Product, Sum))):
-                b[i] = prettyForm(*self._print(b[i]).parens())
-            else:
-                b[i] = self._print(b[i])
+        # Convert to pretty forms. Parentheses are added by `__mul__`.
+        a = [self._print(ai) for ai in a]
+        b = [self._print(bi) for bi in b]
 
         # Construct a pretty form
         if len(b) == 0:

sympy/printing/pretty/stringpict.py

@@ -441,18 +441,21 @@ def __mul__(self, *others):
         }
 
         if len(others) == 0:
-            return self # We aren't actually multiplying... So nothing to do here.
-        args = self
-        if args.binding > prettyForm.MUL:
-            arg = stringPict(*args.parens())
-        result = [args]
+            return self  # We aren't actually multiplying... So nothing to do here.
+
+        # add parens on args that need them
+        arg = self
+        if arg.binding > prettyForm.MUL and arg.binding != prettyForm.NEG:
+            arg = stringPict(*arg.parens())
+        result = [arg]
         for arg in others:
             if arg.picture[0] not in quantity.values():
                 result.append(xsym('*'))
             #add parentheses for weak binders
-            if arg.binding > prettyForm.MUL:
+            if arg.binding > prettyForm.MUL and arg.binding != prettyForm.NEG:
                 arg = stringPict(*arg.parens())
             result.append(arg)
+
         len_res = len(result)
         for i in range(len_res):
             if i < len_res - 1 and result[i] == '-1' and result[i + 1] == xsym('*'):