How CPython implements
__slots__ (part 1): storage
At an abstract level, each instance of a conventional class has a
__dict__ member that is a conventional Python dictionary, and
instance attributes are created and manipulated by manipulating
this dictionary; the dictionary key is the attribute name and the
value is the attribute's value.
__slots__ eliminates this
dictionary and instead has a fixed list of attributes that instances
of the class know about. All of this is in the documentation. What the
documentation won't tell you is how the machine level storage for
all of this actually works. That's what today's entry is about.
In CPython, class instances start out as a more or less opaque C
structure that is specific to the C-level type that your class inherits
from (we saw this before). However, the general
CPython type infrastructure for new-style classes reserves the right to
add some extra space on the end of your type's opaque blob for its own
If your class has a
__slots__, this code adds some extra
space after the C structure blob to store what is effectively an array
of pointers to Python objects. These entries are used to point to the
values of each
__slots__ attribute (if there is no value set, the
corresponding entry is NULL and the CPython code reacts appropriately).
While somewhat complicated, this approach minimizes the memory overhead for class instances. If you allocated the array of slot value pointers separately, you would have a second memory allocation and you'd need an extra pointer in the base object structure to point to the separate array. And because all instances of the class have exactly the same slots, you can put all information on the names of slots and how to access them on the class, instead of having to have it also attached on the instance.
If you have a class that both has a non-empty
__slots__ and tries to
inherit from certain built in types, you will get the error:
nonempty __slots__ not supported for subtype of '<type>'
The Python documentation mentions this but does not explain the details of what is going on, which have to do with this storage approach.
Most C-level types have a fixed size C structure; however, the type infrastructure has general support for types that have a fixed size header structure plus some number of (fixed size) items immediately after the header. Because the information on how to access slot values is attached to the class, not the instance, the CPython code requires that all slot value pointers have a constant offset from the start of the instance object. This requires that all instance objects for a type have the same fixed size, which is not the case for instances of 'base + items' C-level types. Hence the message you get here.
You can still have an empty
__slots__ even for 'base + items'
types, because this doesn't require allocating any slot value pointers;
it just turns off the creation of the
__dict__ itself is (usually) implemented
One might innocently think that
__dict__ would be implemented by
having something like an
ob_dict pointer in the basic Python C-level
object structure. As it happens, CPython is both more clever and more
sleazy than this. The storage for the pointer to the
dictionary is actually usually created through this same 'add things
on the end of the type's blob' code, and the C structure for the type
itself has a field that says what offset this pointer is to be found
at. This saves a pointer when
__slots__ turns off
probably has other implementation advantages that I don't know about.
You might wonder how this works for base + items types. That's where the
sleaze comes in: CPython has special magic support to make this work for
__dict__ offset. If I'm reading the code right, it switches to
indexing the offset from the end of the object instead of the start.
(If you want the gory details, see
Objects/object.c in the CPython source code.)
If you want to see some of this sausage's insides, look at the
__dictoffset__ attribute of any new-style class. For bonus points,
create a class that inherits from, say,
str and then look at its
__dictoffset__. Note that almost all built-in types will show a 0
for this value for reasons that do not fit into this sidebar.