Allow _ for generating expressions

pymath/calculus/random_expression.py

@@ -27,7 +27,7 @@ class RdExpression(object):
         :param conditions: condition on variables (/!\ variables need to be in brackets {})
 
         """
-        self._form = form
+        self._form = self.mod_underscores(form)
         self._conditions = conditions
 
         self._letters = self.get_letters()
@@ -70,6 +70,20 @@ class RdExpression(object):
 
         return varia
 
+    def mod_underscores(self, form):
+        """ Transform underscores of string to {...} forme with capital letters
+
+        :param form: the form string with _ to replace
+        :returns: the string with _ replaced
+
+        """
+        i = 64
+        new_form = form
+        while "_" in new_form:
+            i += 1
+            new_form = new_form.replace("_", "{"+chr(i)+"}",1)
+
+        return new_form
 
     def __call__(self, val_min = -10, val_max = 10):
         """RdExpression once it is initiate act like a function which create random expressions.