Step init from a Renderable. Step in Expression

pymath/calculus/expression.py

@@ -98,7 +98,7 @@ class Expression(Explicable):
 
         else:
             self.simplified = self.child.simplify()
-            self.simplified.steps = self.child.steps + self.simplified.steps
+            self.simplified.this_append_before(self.child.steps)
         return self.simplified
 
     def compute_exp(self):
@@ -106,7 +106,7 @@ class Expression(Explicable):
         if len(self.postfix_tokens) == 1:
             raise ComputeError("Nothing to compute in {}".format(self.postfix_tokens))
         else:
-            ini_step = Expression(self.postfix_tokens)
+            ini_step = Step(self.postfix_tokens)
 
             tokenList = self.postfix_tokens.copy()
             tmpTokenList = []

pymath/calculus/step.py

@@ -41,7 +41,7 @@ class Step(Renderable):
         """
         return super(Step, cls).tmp_render(Step)
 
-    def __init__(self, pstf_tokens):
+    def __init__(self, exp):
         """Initiate the renderable objet
 
         :param pstf_tokens: the postfix list of tokens
@@ -54,12 +54,20 @@ class Step(Renderable):
         < Step [2, 3, '+', 5, '*']>
 
         """
+        if isinstance(exp, Renderable):
+            self.postfix_tokens = exp.postfix_tokens
+        elif isinstance(exp, list):
             self.postfix_tokens = []
-        for t in pstf_tokens:
+            for t in exp:
                 try:
                     self.postfix_tokens += t.postfix_tokens
                 except AttributeError:
                     self.postfix_tokens.append(t)
+        else:
+            raise ValueError(
+                    "Can't initiate Step with {}".format(
+                        exp
+                        ))
 
 
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