solve unittest bugs and undetected develop_steps bug

pymath/calculus/expression.py

@@ -256,13 +256,19 @@ class Expression(Explicable):
         for t in tokenList:
             try:
                 with Expression.tmp_render():
-                    tmp_steps.append([i for i in t.explain()])
+                    steps = [i for i in t.explain()]
+                    if steps:
+                        tmp_steps.append(steps)
+                    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 []
+        elif min([len(i) for i in tmp_steps]) == 0:
+            raise ValueError('tmp_steps: {} \ntokenList: {}'.format(tmp_steps, tokenList))
         else:
             tmp_steps = expand_list(tmp_steps)[:-1]
             steps = [Expression(s) for s in tmp_steps]