"extending and embedding" tutorial.

sq_ass_slice instead of the sq_slice slot.

it.

We add some new rules that are required for preserving internal
invariants of types.

1.  If type (or a subclass of type) appears in bases, it must appear
    before any non-type bases.  If a non-type base (like a regular
    new-style class) occurred first, it could trick type into
    allocating the new class an __dict__ which must be impossible.

2. There are several checks that are made of bases when creating a
   type.  Those checks are now repeated when assigning to __bases__.
   We also add the restriction that assignment to __bases__ may not
   change the metaclass of the type.

Add new tests for these cases and for a few other oddball errors that
were no previously tested.  Remove a crasher test that was fixed.

Also some internal refactoring:  Extract the code to find the most
derived metaclass of a type and its bases.  It is now needed in two
places.  Rewrite the TypeError checks in test_descr to use doctest.
The tests now clearly show what exception they expect to see.

Note that ast.c still has a mix of tabs and spaces, because it
attempts to use four-space indents for more of the new code.

Reported by Mike Verdone.

are gone.

This duplicates the behavior whening normally printing exceptions.

Fixes bug 1569356, but at the cost of a minor incompatibility in
locals().  Add test that verifies that the class namespace is not
polluted.  Also clarify the behavior in the library docs.

Along the way, cleaned up the dict_to_map and map_to_dict
implementations and added some comments that explain what they do.

 * use %r instead of backticks since backticks are going away in Py3k
 * PyArena_Malloc() already sets PyErr_NoMemory so we don't need to do it again
 * the signature for ast2obj_int incorrectly used a bool, rather than a long

Don't use a fixed temporary name (gdbm).
Don't use our own temp name creation (dbm).
Should be backported to 2.5.

of some of the common builtin types.

Use a bit in tp_flags for each common builtin type.  Check the bit
to determine if any instance is a subclass of these common types.
The check avoids a function call and O(n) search of the base classes.
The check is done in the various Py*_Check macros rather than calling
PyType_IsSubtype().

All the bits are set in tp_flags when the type is declared
in the Objects/*object.c files because PyType_Ready() is not called
for all the types.  Should PyType_Ready() be called for all types?
If so and the change is made, the changes to the Objects/*object.c files
can be reverted (remove setting the tp_flags).  Objects/typeobject.c
would also have to be modified to add conditions
for Py*_CheckExact() in addition to each the PyType_IsSubtype check.