bpo-8525: help() now shows exceptions' builtin subclasses

Lib/pydoc.py

@@ -1254,6 +1254,17 @@ def makename(c, m=object.__module__):
                 push('    ' + makename(base))
             push('')
 
+        # List the built-in subclasses, if any:
+        subclasses = [
+            cls.__name__ for cls in object.__subclasses__()
+            if cls.__module__ == 'builtins'
+        ]
+        if subclasses:
+            push("Built-in subclasses:")
+            for subclassname in sorted(subclasses):
+                push('    ' + subclassname)
+            push('')
+
         # Cute little class to pump out a horizontal rule between sections.
         class HorizontalRule:
             def __init__(self):

Lib/test/test_pydoc.py

@@ -515,6 +515,93 @@ def test_stripid(self):
         self.assertEqual(stripid("<type 'exceptions.Exception'>"),
                          "<type 'exceptions.Exception'>")
 
+    def test_builtin_with_child(self):
+        """Tests help on builtin object which have only child classes.
+
+        When running help() on a builtin class which has child classes, it
+        should contain a "Built-in subclasses" section. For example:
+
+        >>> help(ArithmeticError)
+        Help on class ArithmeticError in module builtins:
+
+        class ArithmeticError(Exception)
+         |  Base class for arithmetic errors.
+         |
+         ...
+         |
+         |  Built-in subclasses:
+         |      FloatingPointError
+         |      OverflowError
+         |      ZeroDivisionError
+        """
+        doc = pydoc.TextDoc()
+        text = doc.docclass(ArithmeticError)
+        snip = (" |  Built-in subclasses:\n"
+                " |      FloatingPointError\n"
+                " |      OverflowError\n"
+                " |      ZeroDivisionError")
+        self.assertIn(snip, text)
+
+    def test_builtin_with_grandchild(self):
+        """Tests help on builtin classes which have grandchild classes.
+
+        When running help() on a builtin class which has child classes, it
+        should contain a "Built-in subclasses" section. However, if it also has
+        grandchildren, these should not show up on the subclasses section.
+        For example:
+
+        >>> help(Exception)
+        Help on class Exception in module builtins:
+
+        class Exception(BaseException)
+         |  Common base class for all non-exit exceptions.
+         |
+         ...
+         |
+         |  Built-in subclasses:
+         |      ArithmeticError
+         |      AssertionError
+         |      AttributeError
+         ...
+        """
+        doc = pydoc.TextDoc()
+        text = doc.docclass(Exception)
+        snip = (" |  Built-in subclasses:\n"
+                " |      ArithmeticError\n"
+                " |      AssertionError\n"
+                " |      AttributeError")
+        self.assertIn(snip, text)
+        # Testing that the grandchild ZeroDivisionError does not show up
+        self.assertNotIn('ZeroDivisionError', text)
+
+    def test_builtin_no_child(self):
+        """Tests help on builtin object which have no child classes.
+
+        When running help() on a builtin class which has no child classes, it
+        should not contain any "Built-in subclasses" section. For example:
+
+        >>> help(ZeroDivisionError)
+
+        Help on class ZeroDivisionError in module builtins:
+
+        class ZeroDivisionError(ArithmeticError)
+         |  Second argument to a division or modulo operation was zero.
+         |
+         |  Method resolution order:
+         |      ZeroDivisionError
+         |      ArithmeticError
+         |      Exception
+         |      BaseException
+         |      object
+         |
+         |  Methods defined here:
+         ...
+        """
+        doc = pydoc.TextDoc()
+        text = doc.docclass(ZeroDivisionError)
+        # Testing that the subclasses section does not appear
+        self.assertNotIn('Built-in subclasses', text)
+
     @unittest.skipIf(sys.flags.optimize >= 2,
                      'Docstrings are omitted with -O2 and above')
     @unittest.skipIf(hasattr(sys, 'gettrace') and sys.gettrace(),

Misc/NEWS.d/next/Library/2018-01-01-00-16-59.bpo-8525.Dq8s63.rst

@@ -0,0 +1 @@
+help() now displays exception's builtin subclasses.