Clarify space around colon in slice (severa cases).

pep-0008.txt

@@ -408,6 +408,27 @@ Avoid extraneous whitespace in the following situations:
       Yes: if x == 4: print x, y; x, y = y, x
       No:  if x == 4 : print x , y ; x , y = y , x
 
+- However, in a slice the colon acts like a binary operator, and
+  should have equal amounts on either side (treating it as the
+  operator with the lowest priority).  In an extended slice, both
+  colons must have the same amount of spacing applied.  Exception:
+  when a slice parameter is omitted, the space is omitted.
+
+  Yes::
+
+      ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
+      ham[lower:upper], ham[lower:upper:], ham[lower::step]
+      ham[lower+offset : upper+offset]
+      ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
+      ham[lower + offset : upper + offset]
+
+  No::
+
+      ham[lower + offset:upper + offset]
+      ham[1: 9], ham[1 :9], ham[1:9 :3]
+      ham[lower : : upper]
+      ham[ : upper]
+
 - Immediately before the open parenthesis that starts the argument
   list of a function call::
 
@@ -417,8 +438,8 @@ Avoid extraneous whitespace in the following situations:
 - Immediately before the open parenthesis that starts an indexing or
   slicing::
 
-      Yes: dict['key'] = list[index]
-      No:  dict ['key'] = list [index]
+      Yes: dct['key'] = lst[index]
+      No:  dct ['key'] = lst [index]
 
 - More than one space around an assignment (or other) operator to
   align it with another.