transform if to try/except in develop_steps

pymath/calculus/expression.py

@@ -218,14 +218,14 @@ class Expression(Explicable):
 
     def develop_steps(self, tokenList):
         """ From a list of tokens, it develops steps of each tokens """
-        # TODO: Attention les étapes sont dans le mauvais sens |lun. avril 20 10:06:03 CEST 2015
         tmp_steps = []
-        with Expression.tmp_render():
-            for t in tokenList:
-                if hasattr(t, "explain"):
+        for t in tokenList:
+            try:
+                with Expression.tmp_render():
                     tmp_steps.append([i for i in t.explain()])
-                else:
-                    tmp_steps.append([t])
+            except AttributeError:
+                tmp_steps.append([t])
+
         if max([len(i) for i in tmp_steps]) == 1:
             # Cas où rien n'a dû être expliqué.
             return []