From c314b55376332b3666a4c9cd5d13aeb3a91f38eb Mon Sep 17 00:00:00 2001
From: Benjamin Bertrand <lafrite@poneyworld.net>
Date: Thu, 10 Mar 2016 16:40:52 +0300
Subject: [PATCH] Step init from a Renderable. Step in Expression

---
 pymath/calculus/expression.py |  4 ++--
 pymath/calculus/step.py       | 22 +++++++++++++++-------
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/pymath/calculus/expression.py b/pymath/calculus/expression.py
index 324bb5d..a4d12c9 100644
--- a/pymath/calculus/expression.py
+++ b/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 = []
diff --git a/pymath/calculus/step.py b/pymath/calculus/step.py
index 5b48418..b66172a 100644
--- a/pymath/calculus/step.py
+++ b/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, '*']>
 
         """
-        self.postfix_tokens = []
-        for t in pstf_tokens:
-            try:
-                self.postfix_tokens += t.postfix_tokens
-            except AttributeError:
-                self.postfix_tokens.append(t)
+        if isinstance(exp, Renderable):
+            self.postfix_tokens = exp.postfix_tokens
+        elif isinstance(exp, list):
+            self.postfix_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
+                        ))
 
 
 # -----------------------------