replace letter in abstact_poly and minor change in polynom

pymath/calculus/abstract_polynom.py

@@ -426,6 +426,31 @@ class AbstractPolynom(Explicable):
         ans.this_append_before(steps)
         return ans
 
+    def replace_letter(self, letter):
+        """ Replace the letter in the expression
+
+        :param letter: the new letter.
+        :returns: The expression with the new letter.
+
+        >>> A = AbstractPolynom([1, 2])
+        >>> A
+        < AbstractPolynom x [1, 2]>
+        >>> B = A.replace_letter("y")
+        >>> B
+        < Expression [2, < Polynom y [0, 1]>, *, 1, +]>
+        >>> C = A.replace_letter(2)
+        >>> C
+        < Expression [2, 2, *, 1, +]>
+
+        """
+        postfix_exp = [
+                Expression(letter) if i == self._letter
+                else i
+                for i in self.postfix_tokens
+                ]
+
+        return Expression(postfix_exp)
+
     def __eq__(self, other):
         try:
             o_poly = self.conv2poly(other)

pymath/calculus/polynom.py

@@ -147,13 +147,9 @@ class Polynom(AbstractPolynom):
         3 h^{  2 } + 8 h + 6
         >>> R = P(Q)
         """
-        postfix_exp = [
-                Expression(value) if i == self._letter
-                else i
-                for i in self.postfix_tokens
-                ]
+        eval_exp = self.replace_letter(value)
 
-        return Expression(postfix_exp).simplify()
+        return eval_exp.simplify()
 
     def derivate(self):
         """ Return the derivated polynom