== Some notes on what ((__dictoffset__)) on types means in CPython

I mentioned ((__dictoffset__)) in passing in HowSlotsWorkI. Today I feel
like expanding on that passing mention with some notes. All of this is
specific to CPython.

As mentioned in passing in HowSlotsWorkI, the ((__dictoffset__))
attribute tells you the offset to where you find the pointer to the
((__dict__)) object in any instance object that has one. It is in
bytes. A positive value is an offset from the start of the object; a
negative value is an offset from the end of the object, and is used only
for classes derived from types (such as _str_ and _long_) that have a
variable-sized component. A ((__dictoffset__)) value of zero means that
the type (or class) doesn't have a ((__dict__)) attribute.

(You can tell which types have a variable-sized component by looking at
their ((__itemsize__)) attribute; zero means that they don't have such a
component.)

As sort of discussed in the sidebar in HowSlotsWorkI, if you inherit
from something with a zero ((__dictoffset__)) your subclass will
normally have a non-zero ((__dictoffset__)) and the pointer to the
((__dict__)) object will be glued on the end of the C-level blob of your
basic type.

Most built-in types have a ((__dictoffset__)) of zero, as you'd
expect. However, a few types have a non-zero ((__dictoffset__));
the ones I know of are exceptions, functions, modules, and _type_
itself. What is going on is that all of these types already have to
have some sort of dictionary for their contents, along with a
pointer to this dictionary in their basic C-level blob. So they
reuse this pointer (and associated dictionary) as their ((__dict__)),
by pointing ((__dictoffset__)) directly to this internal field.  One
consequence of this is that subclasses of these classes always have a
((__dict__)), even if your subclass uses ((__slots__)).

(In general, once a class has a non-zero ((__dictoffset__)) all of
its subclasses will always have a ((__dict__)). I think that you can
sometimes still save space and allocations by using ((__slots__)), but
you don't get any of the other features of ((__slots__)) that people
are sometimes unwisely attracted to.)