Fix docs of inheritance-diagram "parts" option

The documentation now correctly describes the behavior of the ``parts``
option in an inheritance-diagram directive: it gives the number of parts
that are *kept* not dropped. The option now also accepts negative
values, which drops parts from the left (which is the what the
documentation incorrectly claimed the option would do for positive
values)

As a form of testing of the new functionality, the documentation for the
inheritance_diagram extension now includes a section "Examples" that
demonstrate the different possibilities. This would fail to build
without the patch.

Closes #4872

1 files changed, 12 insertions, 6 deletions

diff --git a/sphinx/ext/inheritance_diagram.py b/sphinx/ext/inheritance_diagram.py
index cfdac4803..df3ff01ed 100644
--- a/sphinx/ext/inheritance_diagram.py
+++ b/sphinx/ext/inheritance_diagram.py
@@ -167,11 +167,17 @@ class InheritanceGraph:
         """Return name and bases for all classes that are ancestors of
         *classes*.
 
-        *parts* gives the number of dotted name parts that is removed from the
-        displayed node names.
-
-        *top_classes* gives the name(s) of the top most ancestor class to traverse
-        to. Multiple names can be specified separated by comma.
+        *parts* gives the number of dotted name parts to include in the
+        displayed node names, from right to left. If given as a negative, the
+        number of parts to drop from the left. A value of 0 displays the full
+        dotted name. E.g. ``sphinx.ext.inheritance_diagram.InheritanceGraph``
+        with ``parts=2`` or ``parts=-2`` gets displayed as
+        ``inheritance_diagram.InheritanceGraph``, and as
+        ``ext.inheritance_diagram.InheritanceGraph`` with ``parts=3`` or
+        ``parts=-1``.
+
+        *top_classes* gives the name(s) of the top most ancestor class to
+        traverse to. Multiple names can be specified separated by comma.
         """
         all_classes = {}
         py_builtins = vars(builtins).values()
@@ -332,7 +338,7 @@ class InheritanceDiagram(SphinxDirective):
     optional_arguments = 0
     final_argument_whitespace = True
     option_spec = {
-        'parts': directives.nonnegative_int,
+        'parts': int,
         'private-bases': directives.flag,
         'caption': directives.unchanged,
         'top-classes': directives.unchanged_required,