mirror of https://github.com/python/cpython
83 lines
2.6 KiB
Markdown
83 lines
2.6 KiB
Markdown
# Object layout
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## Common header
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Each Python object starts with two fields:
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* ob_refcnt
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* ob_type
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which the form the header common to all Python objects, for all versions,
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and hold the reference count and class of the object, respectively.
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## Pre-header
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Since the introduction of the cycle GC, there has also been a pre-header.
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Before 3.11, this pre-header was two words in size.
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It should be considered opaque to all code except the cycle GC.
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## 3.11 pre-header
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In 3.11 the pre-header was extended to include pointers to the VM managed ``__dict__``.
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The reason for moving the ``__dict__`` to the pre-header is that it allows
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faster access, as it is at a fixed offset, and it also allows object's
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dictionaries to be lazily created when the ``__dict__`` attribute is
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specifically asked for.
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In the 3.11 the non-GC part of the pre-header consists of two pointers:
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* dict
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* values
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The values pointer refers to the ``PyDictValues`` array which holds the
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values of the objects's attributes.
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Should the dictionary be needed, then ``values`` is set to ``NULL``
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and the ``dict`` field points to the dictionary.
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## 3.12 pre-header
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In 3.12 the the pointer to the list of weak references is added to the
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pre-header. In order to make space for it, the ``dict`` and ``values``
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pointers are combined into a single tagged pointer:
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* weakreflist
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* dict_or_values
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If the object has no physical dictionary, then the ``dict_or_values``
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has its low bit set to one, and points to the values array.
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If the object has a physical dictionary, then the ``dict_or_values``
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has its low bit set to zero, and points to the dictionary.
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The untagged form is chosen for the dictionary pointer, rather than
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the values pointer, to enable the (legacy) C-API function
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`_PyObject_GetDictPtr(PyObject *obj)` to work.
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## Layout of a "normal" Python object in 3.12:
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* weakreflist
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* dict_or_values
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* GC 1
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* GC 2
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* ob_refcnt
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* ob_type
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For a "normal" Python object, that is one that doesn't inherit from a builtin
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class or have slots, the header and pre-header form the entire object.
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There are several advantages to this layout:
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* It allows lazy `__dict__`s, as described above.
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* The regular layout allows us to create tailored traversal and deallocation
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functions based on layout, rather than inheritance.
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* Multiple inheritance works properly,
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as the weakrefs and dict are always at the same offset.
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The full layout object, with an opaque part defined by a C extension,
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and `__slots__` looks like this:
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