basic_types.rst: Clearer list comprehensions

This changes the list comprehension example to use color names
instead of plain integers to remove ambiguity for beginners.
The current example is confusing because the list members
are actually shifted list indices.

This is also an attempt to fix scipy-lectures#32

intro/language/basic_types.rst

@@ -98,21 +98,21 @@ Lists
 A list is an ordered collection of objects, that may have different
 types. For example ::
 
-    >>> l = [1, 2, 3, 4, 5]
+    >>> l = ['red', 'blue', 'green', 'black', 'white']
     >>> type(l)
     <type 'list'>
 
 * Indexing: accessing individual objects contained in the list::
 
     >>> l[2]
-    3
+    'green'
 
   Counting from the end with negative indices::
 
     >>> l[-1]
-    5
+    'white'
     >>> l[-2]
-    4
+    'black'
 
 .. warning::
 
@@ -123,9 +123,9 @@ types. For example ::
 ::
 
     >>> l
-    [1, 2, 3, 4, 5]
+    ['red', 'blue', 'green', 'black', 'white']
     >>> l[2:4]
-    [3, 4]
+    ['green', 'black']
 
 .. Warning::
 
@@ -137,31 +137,33 @@ types. For example ::
 
 All slicing parameters are optional::
 
+    >>> l
+    ['red', 'blue', 'green', 'black', 'white']
     >>> l[3:]
-    [4, 5]
+    ['black', 'white']
     >>> l[:3]
-    [1, 2, 3]
+    ['red', 'blue', 'green']
     >>> l[::2]
-    [1, 3, 5]
+    ['red', 'green', 'white']
 
 Lists are *mutable* objects and can be modified::
 
-    >>> l[0] = 28
+    >>> l[0] = 'yellow'
     >>> l
-    [28, 2, 3, 4, 5]
-    >>> l[2:4] = [3, 8] 
+    ['yellow', 'blue', 'green', 'black', 'white']
+    >>> l[2:4] = ['gray', 'purple']
     >>> l
-    [28, 2, 3, 8, 5]
+    ['yellow', 'blue', 'gray', 'purple', 'white']
 
 .. Note::
 
     The elements of a list may have different types::
 
-	>>> l = [3, 2, 'hello']
+	>>> l = [3, -200, 'hello']
 	>>> l
-	[3, 2, 'hello']
+	[3, -200, 'hello']
 	>>> l[1], l[2]
-	(2, 'hello')
+	(-200, 'hello')
 
     For collections of numerical data that all have the same type, it
     is often **more efficient** to use the ``array`` type provided by
@@ -182,7 +184,7 @@ Add and remove elements::
     >>> l.append(6)
     >>> l
     [1, 2, 3, 4, 5, 6]
-    >>> l.pop()
+    >>> l.pop() # removes and returns the last item
     6
     >>> l
     [1, 2, 3, 4, 5]
@@ -196,7 +198,7 @@ Add and remove elements::
 
 Reverse `l`::
 
-    >>> r = l[::-1]
+    >>> r = l[::-1] # or use l.reverse()
     >>> r
     [5, 4, 3, 2, 1]