solve bug in polynom with Expression.set_render

pymath/expression.py

@@ -20,6 +20,10 @@ class Expression(object):
     def set_render(cls, render):
         cls.STR_RENDER = render
 
+    @classmethod
+    def get_render(cls ):
+        return cls.STR_RENDER
+
     @classmethod
     def set_default_render(cls):
         cls.set_render(cls.DEFAULT_RENDER)

pymath/polynom.py

@@ -254,9 +254,10 @@ class Polynom(object):
                 # On converti en Expression
                 coef_exp = Expression(postfix_add)
 
+                old_render = Expression.get_render()
                 Expression.set_render(lambda _,x:Expression(x))
                 coef_steps = list(coef_exp.simplify())
-                Expression.set_default_render()
+                Expression.set_render(old_render)
 
             elif type(coef) == Expression: