Hide Python 2 comment.

source/Collection_Data_Types.rst

@@ -100,25 +100,35 @@ If we modify the state of the list itself, but not the references to this object
 Exercise
 --------
 
-wihout using python shell, what is the results of the following statements:
-
 .. note::
-   sum is a function which return the sum of each elements of a list.
+   ``sum`` is a function which return the sum of all the elements of a list.
+
+Wihout using the Python shell, tell what are the effects of the following statements::
 
-::
 
    x = [1, 2, 3, 4]
-   x[3] = -4 # what is the value of x now ?
-   y = sum(x)/len(x) #what is the value of y ? why ?
+   x[3] = -4  # What is the value of x now?
+   y = sum(x) / len(x)  # What is the value of y? Why?
+
+Solution (using the Python shell ;) )::
+
+    >>> x = [1, 2, 3, 4]
+    >>> x[3] = -4
+    >>> x
+    [1, 2, 3, -4]
+    >>> y = sum(x) / len(x)
+    >>> y
+    0.5
+
+Here, we compute the mean of the values contained in the list ``x``, after having changed its last element to -4.
 
-   y = 0.5
-.. warning::
+.. .. warning::
 
-    In python2 the result is ::
+..    In python2 the result is ::
 
-        y = 0
+..        y = 0
 
-    because sum(x) is an integer, len(x) is also an integer so in python2.x the result is an integer,
+..    because sum(x) is an integer, len(x) is also an integer so in python2.x the result is an integer,
     all the digits after the periods are discarded.