Yu1709$!%yuyu!$%wutu!5=126+

                
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      \_/     \/    @ligmakokz

Editing: object_layout.md

# Object layout

## Common header

Each Python object starts with two fields:

* ob_refcnt
* ob_type

which form the header common to all Python objects, for all versions,
and hold the reference count and class of the object, respectively.

## Pre-header

Since the introduction of the cycle GC, there has also been a pre-header.
Before 3.11, this pre-header was two words in size.
It should be considered opaque to all code except the cycle GC.

## 3.11 pre-header

In 3.11 the pre-header was extended to include pointers to the VM managed ``__dict__``.
The reason for moving the ``__dict__`` to the pre-header is that it allows
faster access, as it is at a fixed offset, and it also allows object's
dictionaries to be lazily created when the ``__dict__`` attribute is
specifically asked for.

In the 3.11 the non-GC part of the pre-header consists of two pointers:

* dict
* values

The values pointer refers to the ``PyDictValues`` array which holds the
values of the objects's attributes.
Should the dictionary be needed, then ``values`` is set to ``NULL``
and the ``dict`` field points to the dictionary.

## 3.12 pre-header

In 3.12, the pointer to the list of weak references is added to the
pre-header. In order to make space for it, the ``dict`` and ``values``
pointers are combined into a single tagged pointer:

* weakreflist
* dict_or_values

If the object has no physical dictionary, then the ``dict_or_values``
has its low bit set to one, and points to the values array.
If the object has a physical dictioanry, then the ``dict_or_values``
has its low bit set to zero, and points to the dictionary.

The untagged form is chosen for the dictionary pointer, rather than
the values pointer, to enable the (legacy) C-API function
`_PyObject_GetDictPtr(PyObject *obj)` to work.

## Layout of a "normal" Python object in 3.12:

* weakreflist
* dict_or_values
* GC 1
* GC 2
* ob_refcnt
* ob_type

For a "normal" Python object, one that doesn't inherit from a builtin
class or have slots, the header and pre-header form the entire object.

![Layout of "normal" object in 3.12](./object_layout_312.png)

There are several advantages to this layout:

* It allows lazy `__dict__`s, as described above.
* The regular layout allows us to create tailored traversal and deallocation
  functions based on layout, rather than inheritance.
* Multiple inheritance works properly,
  as the weakrefs and dict are always at the same offset.

The full layout object, with an opaque part defined by a C extension,
and `__slots__` looks like this:

![Layout of "full" object in 3.12](./object_layout_full_312.png)

Name	Type	Actions
clinic	Directory
stringlib	Directory
README	File
abstract.c	File
abstract.gcda	File
abstract.o	File
boolobject.c	File
boolobject.gcda	File
boolobject.o	File
bytearrayobject.c	File
bytearrayobject.gcda	File
bytearrayobject.o	File
bytes_methods.c	File
bytes_methods.gcda	File
bytes_methods.o	File
bytesobject.c	File
bytesobject.gcda	File
bytesobject.o	File
call.c	File
call.gcda	File
call.o	File
capsule.c	File
capsule.gcda	File
capsule.o	File
cellobject.c	File
cellobject.gcda	File
cellobject.o	File
classobject.c	File
classobject.gcda	File
classobject.o	File
codeobject.c	File
codeobject.gcda	File
codeobject.o	File
complexobject.c	File
complexobject.gcda	File
complexobject.o	File
descrobject.c	File
descrobject.gcda	File
descrobject.o	File
dictnotes.txt	File
dictobject.c	File
dictobject.gcda	File
dictobject.o	File
enumobject.c	File
enumobject.gcda	File
enumobject.o	File
exception_handling_notes.txt	File
exceptions.c	File
exceptions.gcda	File
exceptions.o	File
fileobject.c	File
fileobject.gcda	File
fileobject.o	File
floatobject.c	File
floatobject.gcda	File
floatobject.o	File
frame_layout.md	File
frameobject.c	File
frameobject.gcda	File
frameobject.o	File
funcobject.c	File
funcobject.gcda	File
funcobject.o	File
genericaliasobject.c	File
genericaliasobject.gcda	File
genericaliasobject.o	File
genobject.c	File
genobject.gcda	File
genobject.o	File
interpreteridobject.c	File
interpreteridobject.gcda	File
interpreteridobject.o	File
iterobject.c	File
iterobject.gcda	File
iterobject.o	File
listobject.c	File
listobject.gcda	File
listobject.o	File
listsort.txt	File
lnotab_notes.txt	File
locations.md	File
longobject.c	File
longobject.gcda	File
longobject.o	File
memoryobject.c	File
memoryobject.gcda	File
memoryobject.o	File
methodobject.c	File
methodobject.gcda	File
methodobject.o	File
moduleobject.c	File
moduleobject.gcda	File
moduleobject.o	File
namespaceobject.c	File
namespaceobject.gcda	File
namespaceobject.o	File
object.c	File
object.gcda	File
object.o	File
object_layout.md	File
object_layout_312.gv	File
object_layout_312.png	File
object_layout_full_312.gv	File
object_layout_full_312.png	File
obmalloc.c	File
obmalloc.gcda	File
obmalloc.o	File
odictobject.c	File
odictobject.gcda	File
odictobject.o	File
picklebufobject.c	File
picklebufobject.gcda	File
picklebufobject.o	File
rangeobject.c	File
rangeobject.gcda	File
rangeobject.o	File
setobject.c	File
setobject.gcda	File
setobject.o	File
sliceobject.c	File
sliceobject.gcda	File
sliceobject.o	File
structseq.c	File
structseq.gcda	File
structseq.o	File
tupleobject.c	File
tupleobject.gcda	File
tupleobject.o	File
typeobject.c	File
typeobject.gcda	File
typeobject.o	File
typeslots.inc	File
typeslots.py	File
typevarobject.c	File
typevarobject.gcda	File
typevarobject.o	File
unicodectype.c	File
unicodectype.gcda	File
unicodectype.o	File
unicodeobject.c	File
unicodeobject.gcda	File
unicodeobject.o	File
unicodetype_db.h	File
unionobject.c	File
unionobject.gcda	File
unionobject.o	File
weakrefobject.c	File
weakrefobject.gcda	File
weakrefobject.o	File