contents

• related file
• memory layout
• how element stored inside
• free list

related file

• cpython/Include/cpython/tupleobject.c
• cpython/Include/tupleobject.h
• cpython/Objects/tupleobject.c

memory layout

The structure of the tuple object is simpler than other Python objects. Obviously, ob_item is an array of PyObject* pointers, and all elements will be stored inside the ob_item array. But how exactly is each element stored in PyTupleObject? Does the first element begin at index 0? What is the resize strategy?

Let's see

how element stored inside

t = ("aa", "bb", "cc", "dd")

ob_size represents the size of the PyTupleObject object. Because a tuple object is immutable, ob_item is the start address of an array of pointers to PyObject, and the size of this array is ob_size. There's no need for a resize operation.

free list

The free_list mechanism used here is more interesting than free_list in list

#define PyTuple_MAXSAVESIZE     20
#define PyTuple_MAXFREELIST  2000
static PyTupleObject *free_list[PyTuple_MAXSAVESIZE];
static int numfree[PyTuple_MAXSAVESIZE];

Let's examine what free_list and numfree are. We assume that the Python interpreter doesn't create/deallocate any tuple objects internally during the following code

>>> t = tuple()
>>> id(t)
4458758208
>>> del t
>>> t = tuple()
>>> id(t) # same as previous deleted one
4458758208
>>> t2 = tuple()
>>> id(t2) # same as t
4458758208

>>> t = ("aa", )
>>> id(t)
4459413360
>>> del t
>>> t = ("bb", )
>>> id(t) # it's not repeatable, because I assume that the Python interpreter doesn't create/deallocate any tuple objects during execution
4459413360
>>> t2 = ("cc", )
>>> del t
>>> del t2

num_free[i] indicates how many objects are left in free_list[i]. When you create a new tuple with size i, CPython will use the top object at free_list[i]

>>> t2 = ("aa", "bb")
>>> t3 = ("cc", "dd")
>>> t4 = ("ee", "ff")
>>> t5 = ("gg", "hh")
>>> del t2
>>> del t3
>>> del t4
>>> del t5