Python behind the scenes #6: how Python object system works

As we know from the previous parts of this series, the execution of a Python program consists of two major steps:

  1. The CPython compiler translates Python code to bytecode.
  2. The CPython VM executes the bytecode.

We've been focusing on the second step for quite a while. In part 4 we've looked at the evaluation loop, a place where Python bytecode gets executed. And in part 5 we've studied how the VM executes the instructions that are used to implement variables. What we haven't covered yet is how the VM actually computes something. We postponed this question because to answer it, we first need to understand how the most fundamental part of the language works. Today, we'll study the Python object system.

Note: In this post I'm referring to CPython 3.9. Some implementation details will certainly change as CPython evolves. I'll try to keep track of important changes and add update notes.


Consider an extremely simple piece of Python code:

def f(x):
    return x + 7

To compute the function f, CPython must evaluate the expression x + 7. The question I'd like to ask is: How does CPython do that? Special methods such as __add__() and __radd__() probably come to your mind. When we define these methods on a class, the instances of that class can be added using the + operator. So, you might think that CPython does something like this:

  1. It calls x.__add__(7) or type(x).__add__(x, 7).
  2. If x doesn't have __add__(), or if this method fails, it calls (7).__radd__(x) or int.__radd__(7, x).

The reality, though, is a bit more complicated. What really happens depends on what x is. For example, if x is an instance of a user-defined class, the algorithm described above resembles the truth. If, however, x is an instance of a built-in type, like int or float, CPython doesn't call any special methods at all.

To learn how some Python code is executed, we can do the following:

  1. Disassemble the code into bytecode.
  2. Study how the VM executes the disassembled bytecode instructions.

Let's apply this algorithm to the function f. The compiler translates the body of this function to the following bytecode:

$ python -m dis
  2           0 LOAD_FAST                0 (x)
              2 LOAD_CONST               1 (7)
              4 BINARY_ADD
              6 RETURN_VALUE

And here's what these bytecode instructions do:

  1. LOAD_FAST loads the value of the parameter x onto the stack.
  2. LOAD_CONST loads the constant 7 onto the stack.
  3. BINARY_ADD pops two values from the stack, adds them and pushes the result back onto the stack.
  4. RETURN_VALUE pops the value from the stack and returns it.

How does the VM add two values? To answer this question, we need to understand what these values are. For us, 7 is an instance of int and x is, well, anything. For the VM, though, everything is a Python object. All values the VM pushes onto the stack and pops from the stack are pointers to PyObject structs (hence the phrase "Everything in Python is an object").

The VM doesn't need to know how to add integers or strings, that is, how to do the arithmetic or concatenate sequences. All it needs to know is that every Python object has a type. A type, in turn, knows everything about its objects. For example, the int type knows how to add integers, and the float type knows how to add floats. So, the VM asks the type to perform the operation.

This simplified explanation captures the essence of the solution, but it also omits a lot of important details. To get a more realistic picture, we need to understand what Python objects and types really are and how they work.

Python objects and types

We've discussed Python objects a little in part 3. This discussion is worth repeating here.

We begin with the definition of the PyObject struct:

typedef struct _object {
    _PyObject_HEAD_EXTRA // macro, for debugging purposes only
    Py_ssize_t ob_refcnt;
    PyTypeObject *ob_type;
} PyObject;

It has two members:

  • a reference count ob_refcnt that CPython uses for garbage collection; and
  • a pointer to the object's type ob_type.

We said that the VM treats any Python object as PyObject. How is that possible? The C programming language has no notion of classes and inheritance. Nevertheless, it's possible to implement in C something that can be called a single inheritance. The C standard states that a pointer to any struct can be converted to a pointer to its first member and vice versa. So, we can "extend" PyObject by defining a new struct whose first member is PyObject.

Here's, for example, how the float object is defined:

typedef struct {
    PyObject ob_base; // expansion of PyObject_HEAD macro
    double ob_fval;
} PyFloatObject;

A float object stores everything PyObject stores plus a floating-point value ob_fval. The C standard simply states that we can convert a pointer to PyFloatObject to a pointer to PyObject and vice versa:

PyFloatObject float_object;
// ...
PyObject *obj_ptr = (PyObject *)&float_object;
PyFloatObject *float_obj_ptr = (PyFloatObject *)obj_ptr;

The reason why the VM treats every Python object as PyObject is because all it needs to access is the object's type. A type is also a Python object, an instance of the PyTypeObject struct:

// PyTypeObject is a typedef for "struct _typeobject"

struct _typeobject {
    PyVarObject ob_base; // expansion of PyObject_VAR_HEAD macro
    const char *tp_name; /* For printing, in format "<module>.<name>" */
    Py_ssize_t tp_basicsize, tp_itemsize; /* For allocation */

    /* Methods to implement standard operations */

    destructor tp_dealloc;
    Py_ssize_t tp_vectorcall_offset;
    getattrfunc tp_getattr;
    setattrfunc tp_setattr;
    PyAsyncMethods *tp_as_async; /* formerly known as tp_compare (Python 2)
                                    or tp_reserved (Python 3) */
    reprfunc tp_repr;

    /* Method suites for standard classes */

    PyNumberMethods *tp_as_number;
    PySequenceMethods *tp_as_sequence;
    PyMappingMethods *tp_as_mapping;

    /* More standard operations (here for binary compatibility) */

    hashfunc tp_hash;
    ternaryfunc tp_call;
    reprfunc tp_str;
    getattrofunc tp_getattro;
    setattrofunc tp_setattro;

    /* Functions to access object as input/output buffer */
    PyBufferProcs *tp_as_buffer;

    /* Flags to define presence of optional/expanded features */
    unsigned long tp_flags;

    const char *tp_doc; /* Documentation string */

    /* Assigned meaning in release 2.0 */
    /* call function for all accessible objects */
    traverseproc tp_traverse;

    /* delete references to contained objects */
    inquiry tp_clear;

    /* Assigned meaning in release 2.1 */
    /* rich comparisons */
    richcmpfunc tp_richcompare;

    /* weak reference enabler */
    Py_ssize_t tp_weaklistoffset;

    /* Iterators */
    getiterfunc tp_iter;
    iternextfunc tp_iternext;

    /* Attribute descriptor and subclassing stuff */
    struct PyMethodDef *tp_methods;
    struct PyMemberDef *tp_members;
    struct PyGetSetDef *tp_getset;
    struct _typeobject *tp_base;
    PyObject *tp_dict;
    descrgetfunc tp_descr_get;
    descrsetfunc tp_descr_set;
    Py_ssize_t tp_dictoffset;
    initproc tp_init;
    allocfunc tp_alloc;
    newfunc tp_new;
    freefunc tp_free; /* Low-level free-memory routine */
    inquiry tp_is_gc; /* For PyObject_IS_GC */
    PyObject *tp_bases;
    PyObject *tp_mro; /* method resolution order */
    PyObject *tp_cache;
    PyObject *tp_subclasses;
    PyObject *tp_weaklist;
    destructor tp_del;

    /* Type attribute cache version tag. Added in version 2.6 */
    unsigned int tp_version_tag;

    destructor tp_finalize;
    vectorcallfunc tp_vectorcall;

By the way, note that the first member of a type is not PyObject but PyVarObject, which is defined as follows:

typedef struct {
    PyObject ob_base;
    Py_ssize_t ob_size; /* Number of items in variable part */
} PyVarObject;

Nevertheless, since the first member of PyVarObject is PyObject, a pointer to a type can still be converted to a pointer to PyObject.

So, what is a type and why does it have so many members? A type determines how the objects of that type behave. Each member of a type, called slot, is responsible for a particular aspect of the object's behavior. For example:

  • tp_new is a pointer to a function that creates new objects of the type.
  • tp_str is a pointer to a function that implements str() for objects of the type.
  • tp_hash is a pointer to a function that implements hash() for objects of the type.

Some slots, called sub-slots, are grouped together in suites. A suite is just a struct that contains related slots. For example, the PySequenceMethods struct is a suite of sub-slots that implement the sequence protocol:

typedef struct {
    lenfunc sq_length;
    binaryfunc sq_concat;
    ssizeargfunc sq_repeat;
    ssizeargfunc sq_item;
    void *was_sq_slice;
    ssizeobjargproc sq_ass_item;
    void *was_sq_ass_slice;
    objobjproc sq_contains;

    binaryfunc sq_inplace_concat;
    ssizeargfunc sq_inplace_repeat;
} PySequenceMethods;

If you count all the slots and sub-slots, you'll get a scary number. Fortunately, each slot is very well documented in the Python/C API Reference Manual (I strongly recommend you to bookmark this link). Today we'll cover only a few slots. Nevertheless, it shall give us a general idea of how slots are used.

Since we're interested in how CPython adds objects, let's find the slots responsible for addition. There must be at least one such slot. After careful inspection of the PyTypeObject struct, we find that it has the "number" suite PyNumberMethods, and the first slot of this suite is a binary function called nd_add:

typedef struct {
    binaryfunc nb_add; // typedef PyObject * (*binaryfunc)(PyObject *, PyObject *)
    binaryfunc nb_subtract;
    binaryfunc nb_multiply;
    binaryfunc nb_remainder;
    binaryfunc nb_divmod;
    // ... more sub-slots
} PyNumberMethods;

It seems that the nb_add slot is what we're looking for. Two questions naturally arise regarding this slot:

  • What is it set to?

  • How is it used?

I think it's better to start with the second. We should expect that the VM calls nb_add to execute the BINARY_ADD opcode. So, let's, for a moment, suspend our discussion about types and take a look at how the BINARY_ADD opcode is implemented.


Like any other opcode, BINARY_ADD is implemented in the evaluation loop in Python/ceval.c:

    PyObject *right = POP();
    PyObject *left = TOP();
    PyObject *sum;
    /* NOTE(haypo): Please don't try to micro-optimize int+int on
        CPython using bytecode, it is simply worthless.
        See and for the discussion. In short,
        no patch shown any impact on a realistic benchmark, only a minor
        speedup on microbenchmarks. */
    if (PyUnicode_CheckExact(left) &&
                PyUnicode_CheckExact(right)) {
        sum = unicode_concatenate(tstate, left, right, f, next_instr);
        /* unicode_concatenate consumed the ref to left */
    else {
        sum = PyNumber_Add(left, right);
    if (sum == NULL)
        goto error;

This code requires some comments. We can see that it calls PyNumber_Add() to add two objects, but if the objects are strings, it calls unicode_concatenate() instead. Why so? This is an optimization. Python strings seem immutable, but sometimes CPython mutates a string and thus avoids creating a new string. Consider appending one string to another:

output += some_string

If the output variable points to a string that has no other references, it's safe to mutate that string. This is exactly the logic that unicode_concatenate() implements.

It might be tempting to handle other special cases in the evaluation loop as well and optimize, for example, integers and floats. The comment explicitly warns against it. The problem is that a new special case comes with an additional check, and this check is only useful when it succeeds. Otherwise, it may have a negative effect on performance.

After this little digression, let's look at PyNumber_Add():

PyObject *
PyNumber_Add(PyObject *v, PyObject *w)
    // NB_SLOT(nb_add) expands to "offsetof(PyNumberMethods, nb_add)"
    PyObject *result = binary_op1(v, w, NB_SLOT(nb_add));
    if (result == Py_NotImplemented) {
        PySequenceMethods *m = Py_TYPE(v)->tp_as_sequence;
        if (m && m->sq_concat) {
            return (*m->sq_concat)(v, w);
        result = binop_type_error(v, w, "+");
    return result;

I suggest to step into binary_op1() straight away and figure out what the rest of PyNumber_Add() does later:

static PyObject *
binary_op1(PyObject *v, PyObject *w, const int op_slot)
    PyObject *x;
    binaryfunc slotv = NULL;
    binaryfunc slotw = NULL;

    if (Py_TYPE(v)->tp_as_number != NULL)
        slotv = NB_BINOP(Py_TYPE(v)->tp_as_number, op_slot);
    if (!Py_IS_TYPE(w, Py_TYPE(v)) &&
        Py_TYPE(w)->tp_as_number != NULL) {
        slotw = NB_BINOP(Py_TYPE(w)->tp_as_number, op_slot);
        if (slotw == slotv)
            slotw = NULL;
    if (slotv) {
        if (slotw && PyType_IsSubtype(Py_TYPE(w), Py_TYPE(v))) {
            x = slotw(v, w);
            if (x != Py_NotImplemented)
                return x;
            Py_DECREF(x); /* can't do it */
            slotw = NULL;
        x = slotv(v, w);
        if (x != Py_NotImplemented)
            return x;
        Py_DECREF(x); /* can't do it */
    if (slotw) {
        x = slotw(v, w);
        if (x != Py_NotImplemented)
            return x;
        Py_DECREF(x); /* can't do it */

The binary_op1() function takes three parameters: the left operand, the right operand and an offset that identifies the slot. Types of both operands can implement the slot. Therefore, binary_op1() looks up both implementations. To calculate the result, it calls one implementation or another relying on the following logic:

  1. If the type of one operand is a subtype of another, call the slot of the subtype.

  2. If the left operand doesn't have the slot, call the slot of the right operand.

  3. Otherwise, call the slot of the left operand.

The reason to prioritize the slot of a subtype is to allow the subtypes to override the behavior of their ancestors:

$ python -q
>>> class HungryInt(int):
...     def __add__(self, o):
...             return self
>>> x = HungryInt(5)
>>> x + 2
>>> 2 + x
>>> HungryInt.__radd__ = lambda self, o: self
>>> 2 + x

Let's turn back to PyNumber_Add(). If binary_op1() succeeds, PyNumber_Add() simply returns the result of binary_op1(). If, however, binary_op1() returns the NotImplemented constant, which means that the operation cannot be performed for a given combination of types, PyNumber_Add() calls the sq_concat "sequence" slot of the first operand and returns the result of this call:

PySequenceMethods *m = Py_TYPE(v)->tp_as_sequence;
if (m && m->sq_concat) {
    return (*m->sq_concat)(v, w);

A type can support the + operator either by implementing nb_add or sq_concat. These slots have different meanings:

  • nb_add means algebraic addition with properties like a + b = b + a.
  • sq_concat means the concatenation of sequences.

Built-in types such as int and float implement nb_add, and built-in types such as str and list implement sq_concat. Technically, there's no much difference. The main reason to choose one slot over another is to indicate the appropriate meaning. In fact, the sq_concat slot is so unnecessary that it's set to NULL for all user-defined types (i.e. classes).

We saw how the nb_add slot is used: it's called by the binary_op1() function. The next step is to see what it's set to.

What nb_add can be

Since addition is a different operation for different types, the nb_add slot of a type must be one of two things:

  • it's either a type-specific function that adds object of that type; or
  • it's a type-agnostic function that calls some type-specific functions, such as type's __add__() special method.

It's indeed one of these two, and which one depends on the type. For example, built-in types such as int and float have their own implementations of nb_add. In contrast, all classes share the same implementation. Fundamentally, built-in types and classes are the same thing – instances of PyTypeObject. The important difference between them is how they are created. This difference effects the way the slots are set, so we should discuss it.

Ways to create a type

There are two ways to create a type object:

  • by statically defining it; or
  • by dynamically allocating it.

Statically defined types

An example of a statically defined type is any built-in type. Here's, for instance, how CPython defines the float type:

PyTypeObject PyFloat_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    (destructor)float_dealloc,                  /* tp_dealloc */
    0,                                          /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    (reprfunc)float_repr,                       /* tp_repr */
    &float_as_number,                           /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    (hashfunc)float_hash,                       /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,   /* tp_flags */
    float_new__doc__,                           /* tp_doc */
    0,                                          /* tp_traverse */
    0,                                          /* tp_clear */
    float_richcompare,                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    float_methods,                              /* tp_methods */
    0,                                          /* tp_members */
    float_getset,                               /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    0,                                          /* tp_dictoffset */
    0,                                          /* tp_init */
    0,                                          /* tp_alloc */
    float_new,                                  /* tp_new */

The slots of a statically defined type are specified explicitly. We can easily see how the float type implements nb_add by looking at the "number" suite:

static PyNumberMethods float_as_number = {
    float_add,          /* nb_add */
    float_sub,          /* nb_subtract */
    float_mul,          /* nb_multiply */
    // ... more number slots

where we find the float_add() function, a straightforward implementation of nb_add:

static PyObject *
float_add(PyObject *v, PyObject *w)
    double a,b;
    a = a + b;
    return PyFloat_FromDouble(a);

The floating-point arithmetic is not that important for our discussion. This example demonstrates how to specify the behavior of a statically defined type. It turned out to be quite easy: just write the implementation of slots and point each slot to the corresponding implementation.

If you want to learn how to statically define your own types, check out Python's tutorial for C/C++ programmers.

Dynamically allocated types

Dynamically allocated types are the types we define using the class statement. As we've already said, they are instances of PyTypeObject, just like statically defined types. Traditionally, we call them classes but we might call them user-defined types as well.

From the programmer's perspective, it's easier to define a class in Python than a type in C. This is because CPython does a lot of things behind the scenes when it creates a class. Let's see what's involved in this process.

If we wouldn't know where to start, we could apply the familiar method:

 1. Define a simple class

class A:

 2. Run the disassembler:

$ python -m dis

 3. Study how the VM executes the produced bytecode instructions.

Feel free to do that if you find the time, or read the article on classes by Eli Bendersky. We'll take a shortcut.

An object is created by a call to a type, e.g. list() or MyClass(). A class is created by a call to a metatype. A metatype is just a type whose instances are types. Python has one built-in metatype called PyType_Type, which is known to us simply as type. Here's how it's defined:

PyTypeObject PyType_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "type",                                     /* tp_name */
    sizeof(PyHeapTypeObject),                   /* tp_basicsize */
    sizeof(PyMemberDef),                        /* tp_itemsize */
    (destructor)type_dealloc,                   /* tp_dealloc */
    offsetof(PyTypeObject, tp_vectorcall),      /* tp_vectorcall_offset */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_as_async */
    (reprfunc)type_repr,                        /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    (ternaryfunc)type_call,                     /* tp_call */
    0,                                          /* tp_str */
    (getattrofunc)type_getattro,                /* tp_getattro */
    (setattrofunc)type_setattro,                /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_HAVE_VECTORCALL,                 /* tp_flags */
    type_doc,                                   /* tp_doc */
    (traverseproc)type_traverse,                /* tp_traverse */
    (inquiry)type_clear,                        /* tp_clear */
    0,                                          /* tp_richcompare */
    offsetof(PyTypeObject, tp_weaklist),        /* tp_weaklistoffset */
    0,                                          /* tp_iter */
    0,                                          /* tp_iternext */
    type_methods,                               /* tp_methods */
    type_members,                               /* tp_members */
    type_getsets,                               /* tp_getset */
    0,                                          /* tp_base */
    0,                                          /* tp_dict */
    0,                                          /* tp_descr_get */
    0,                                          /* tp_descr_set */
    offsetof(PyTypeObject, tp_dict),            /* tp_dictoffset */
    type_init,                                  /* tp_init */
    0,                                          /* tp_alloc */
    type_new,                                   /* tp_new */
    PyObject_GC_Del,                            /* tp_free */
    (inquiry)type_is_gc,                        /* tp_is_gc */

The type of all built-in types is type, and the type of all classes defaults to type. So, type determines how types behave. For example, what happens when we call a type, like list() or MyClass(), is specified by the tp_call slot of type. The implementation of the tp_call slot of type is the type_call() function. Its job is to create new objects. It calls two other slots to do that:

  1. It calls tp_new of a type to create an object.
  2. It calls tp_init of a type to initialize the created object.

The type of type is type itself. So, when we call type(), the type_call() function is invoked. It checks for the special case when we pass a single argument to type(). In this case, type_call() simply returns the type of the passed object:

$ python -q
>>> type(3)
<class 'int'>
>>> type(int)
<class 'type'>
>>> type(type)
<class 'type'>

But when we pass three arguments to type(), type_call() creates a new type by calling tp_new and tp_init of type as described above. The following example demonstrates how to use type() to create a class:

$ python -q
>>> MyClass = type('MyClass', (), {'__str__': lambda self: 'Hey!'})
>>> instance_of_my_class = MyClass()
>>> str(instance_of_my_class)

The arguments we pass to type() are:

  1. the name of a class
  2. a tuple of its bases; and
  3. a namespace.

Other metatypes take arguments in this form as well.

We saw that we can create a class by calling type(), but that's not what we typically do. Typically, we use the class statement to define a class. It turns out that in this case too the VM eventually calls some metatype, and most often it calls type().

To execute the class statement, the VM calls the __build_class__() function from the builtins module. What this function does can be summarized as follows:

  1. Decide which metatype to call to create the class.
  2. Prepare the namespace. The namespace will be used as a class's dictionary.
  3. Execute the body of the class in the namespace, thus filling the namespace.
  4. Call the metatype.

We can instruct __build_class__() which metatype it should call using the metaclass keyword. If no metaclass is specified, __build_class__() calls type() by default. It also takes metatypes of bases into account. The exact logic of choosing the metatype is nicely described in the docs.

Suppose we define a new class and do not specify metaclass. Where does the class actually get created? In this case, __build_class__() calls type(). This invokes the type_call() function that, in turn, calls the tp_new and tp_init slots of type. The tp_new slot of type points to the type_new() function. This is the function that creates classes. The tp_init slot of type points to the function that does nothing, so all the work is done by type_new().

The type_new() function is nearly 500 lines long and probably deserves a separate post. Its essence, though, can be briefly summarized as follows:

  1. Allocate new type object.
  2. Set up the allocated type object.

To accomplish the first step, type_new() must allocate an instance of PyTypeObject as well as suites. Suites must be allocated separately from PyTypeObject because PyTypeObject contains only pointers to suites, not suites themselves. To handle this inconvenience, type_new() allocates an instance of the PyHeapTypeObject struct that extends PyTypeObject and contains the suites:

/* The *real* layout of a type object when allocated on the heap */
typedef struct _heaptypeobject {
    PyTypeObject ht_type;
    PyAsyncMethods as_async;
    PyNumberMethods as_number;
    PyMappingMethods as_mapping;
    PySequenceMethods as_sequence;
    PyBufferProcs as_buffer;
    PyObject *ht_name, *ht_slots, *ht_qualname;
    struct _dictkeysobject *ht_cached_keys;
    PyObject *ht_module;
    /* here are optional user slots, followed by the members. */
} PyHeapTypeObject;

To set up a type object means to set up its slots. This is what type_new() does for the most part.

Type initialization

Before any type can be used, it should be initialized with the PyType_Ready() function. For a class, PyType_Ready() is called by type_new(). For a statically defined type, PyType_Ready() must be called explicitly. When CPython starts, it calls PyType_Ready() for each built-in type.

The PyType_Ready() function does a number of things. For example, it does slot inheritance.

Slot inheritance

When we define a class that inherits from other type, we expect the class to inherit some behavior of that type. For example, when we define a class that inherits from int, we expect it to support the addition:

$ python -q
>>> class MyInt(int):
...     pass
>>> x = MyInt(2)
>>> y = MyInt(4)
>>> x + y

Does MyInt inherit the nb_add slot of int? Yes, it does. It's pretty straightforward to inherit the slots from a single ancestor: just copy those slots that the class doesn't have. It's a little bit more complicated when a class has multiple bases. Since bases, in turn, may inherit from other types, all these ancestor types combined form an hierarchy. The problem with the hierarchy is that it doesn't specify the order of inheritance. To solve this problem, PyType_Ready() converts this hierarchy into a list. The Method Resolution Order (MRO) determines how to perform this conversion. Once the MRO is calculated, it becomes easy to implement the inheritance in the general case. The PyType_Ready() function iterates over ancestors according to the MRO. From each ancestor, it copies those slots that haven't been set on the type before. Some slots support the inheritance and some don't. You can check in the docs whether a particular slot is inherited.

In contrast to a class, a statically defined type can specify at most one base. This is done by implementing the tp_base slot.

If no bases are specified, PyType_Ready() assumes that the object type is the only base. Every type directly or indirectly inherits from object. Why? Because it implements the slots that every type is expected to have. For example, it implements tp_alloc, tp_init and tp_repr slots.

The ultimate question

So far we've seen two ways in which a slot can be set:

  • It can be specified explicitly (if a type is a statically defined type).
  • It can be inherited from an ancestor.

It's still unclear how slots of a class are connected to its special methods. Moreover, we have a reverse problem for built-in types. How do they implement special methods? They certainly do:

$ python -q
>>> (3).__add__(4)

We come to the ultimate question of this post: What's the connection between special methods and slots?

Special methods and slots

The answer lies in the fact that CPython keeps a mapping between special methods and slots. This mapping is represented by the slotdefs array. It looks like this:

    {NAME, offsetof(PyTypeObject, SLOT), (void *)(FUNCTION), WRAPPER, \

static slotdef slotdefs[] = {
    TPSLOT("__getattribute__", tp_getattr, NULL, NULL, ""),
    TPSLOT("__getattr__", tp_getattr, NULL, NULL, ""),
    TPSLOT("__setattr__", tp_setattr, NULL, NULL, ""),
    TPSLOT("__delattr__", tp_setattr, NULL, NULL, ""),
    TPSLOT("__repr__", tp_repr, slot_tp_repr, wrap_unaryfunc,
           "__repr__($self, /)\n--\n\nReturn repr(self)."),
    TPSLOT("__hash__", tp_hash, slot_tp_hash, wrap_hashfunc,
           "__hash__($self, /)\n--\n\nReturn hash(self)."),
    // ... more slotdefs

Each entry of this array is a slotdef struct:

// typedef struct wrapperbase slotdef;

struct wrapperbase {
    const char *name;
    int offset;
    void *function;
    wrapperfunc wrapper;
    const char *doc;
    int flags;
    PyObject *name_strobj;

Four members of this struct are important for our discussion:

  • name is a name of a special method.
  • offset is an offset of a slot in the PyHeapTypeObject struct. It specifies the slot corresponding to the special method.
  • function is an implementation of a slot. When a special method is defined, the corresponding slot is set to function. Typically, function calls special methods to do the work.
  • wrapper is a wrapper function around a slot. When a slot is defined, wrapper provides an implementation for the corresponding special method. It calls the slot to do the work.

Here's, for example, an entry that maps __add__() special method to the nb_add slot:

  • name is "__add__".
  • offset is offsetof(PyHeapTypeObject, as_number.nb_add).
  • function is slot_nb_add().
  • wrapper is wrap_binaryfunc_l().

The slotdefs array is a many-to-many mapping. For example, as we'll see, both the __add__() and __radd__() special methods map to the same nb_add slot. Conversely, both the mp_subscript "mapping" slot and the sq_item "sequence" slot map to the same __getitem__() special method.

CPython uses the slotdefs array in two ways:

  • to set slots based on special methods; and
  • to set special methods based on slots.

Slots based on special methods

The type_new() function calls fixup_slot_dispatchers() to set slots based on special methods. The fixup_slot_dispatchers() function calls update_one_slot() for each slot in the slotdefs array, and update_one_slot() sets the slot to function if a class has the corresponding special method.

Let's take the nb_add slot as an example. The slotdefs array has two entries corresponding to that slot:

static slotdef slotdefs[] = {
    // ...
    BINSLOT("__add__", nb_add, slot_nb_add, "+"),
    RBINSLOT("__radd__", nb_add, slot_nb_add,"+"),
    // ...

BINSLOT() and RBINSLOT() are macros. Let's expand them:

static slotdef slotdefs[] = {
    // ...
    // {name, offset, function,
    //     wrapper, doc}
    {"__add__", offsetof(PyHeapTypeObject, as_number.nb_add), (void *)(slot_nb_add),
        wrap_binaryfunc_l, PyDoc_STR("__add__" "($self, value, /)\n--\n\nReturn self" "+" "value.")},

    {"__radd__", offsetof(PyHeapTypeObject, as_number.nb_add), (void *)(slot_nb_add),
        wrap_binaryfunc_r, PyDoc_STR("__radd__" "($self, value, /)\n--\n\nReturn value" "+" "self.")},
    // ...

What update_one_slot() does is look up class.__add__() and class.__radd__(). If either is defined, it sets nb_add of the class to slot_nb_add(). Note that both entries agree on slot_nb_add() as function. Otherwise, we would have a conflict when both are defined.

Now, what is slot_nb_add(), you ask? This function is defined with a macro that expands as follows:

static PyObject *
slot_nb_add(PyObject *self, PyObject *other) {
    PyObject* stack[2];
    PyThreadState *tstate = _PyThreadState_GET();
    _Py_static_string(op_id, "__add__");
    _Py_static_string(rop_id, "__radd__");
    int do_other = !Py_IS_TYPE(self, Py_TYPE(other)) && \
        Py_TYPE(other)->tp_as_number != NULL && \
        Py_TYPE(other)->tp_as_number->nb_add == slot_nb_add;
    if (Py_TYPE(self)->tp_as_number != NULL && \
        Py_TYPE(self)->tp_as_number->nb_add == slot_nb_add) {
        PyObject *r;
        if (do_other && PyType_IsSubtype(Py_TYPE(other), Py_TYPE(self))) {
            int ok = method_is_overloaded(self, other, &rop_id);
            if (ok < 0) {
                return NULL;
            if (ok) {
                stack[0] = other;
                stack[1] = self;
                r = vectorcall_maybe(tstate, &rop_id, stack, 2);
                if (r != Py_NotImplemented)
                    return r;
                Py_DECREF(r); do_other = 0;
        stack[0] = self;
        stack[1] = other;
        r = vectorcall_maybe(tstate, &op_id, stack, 2);
        if (r != Py_NotImplemented || Py_IS_TYPE(other, Py_TYPE(self)))
            return r;
    if (do_other) {
        stack[0] = other;
        stack[1] = self;
        return vectorcall_maybe(tstate, &rop_id, stack, 2);

You don't need to study this code carefully. Recall the binary_op1() function that calls the nb_add slot. The slot_nb_add() function basically repeats the logic of binary_op1(). The main difference is that slot_nb_add() eventually calls __add__() or __radd__().

Setting special method on existing class

Suppose that we create a class without the __add__() and __radd__() special methods. In this case, the nb_add slot of the class is set to NULL. As expected, we cannot add instances of that class. If we, however, set __add__() or __radd__() after the class has been created, the addition works as if the method was a part of the class definition. Here's what I mean:

$ python -q
>>> class A:
...     pass
>>> x = A()
>>> x + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'A' and 'int'
>>> A.__add__ = lambda self, o: 5
>>> x + 2

How does that work? To set an attribute on an object, the VM calls the tp_setattro slot of the object's type. The tp_setattro slot of type points to the type_setattro() function, so when we set an attribute on a class, this function gets called. It stores the value of the attribute in the class's dictionary. Then it checks if the attribute is a special method and, if so, sets the corresponding slots by calling the update_one_slot() function.

Before we can learn how CPython does the reverse, that is, how it adds special methods to built-in types, we need to understand what a method is.


A method is an attribute, but a peculiar one. When we call a method from an instance, the method implicitly receives the instance as its first parameter, which we usually denote self:

$ python -q
>>> class A:
...     def method(self, x):
...             return self, x
>>> a = A()
>>> a.method(1)
(<__main__.A object at 0x10d10bfd0>, 1)

But when we call the same method from a class, we have to pass all arguments explicitly:

>>> A.method(a, 1)
(<__main__.A object at 0x10d10bfd0>, 1)

In our example, the method takes one argument in one case and two arguments in another. How is that possible that the same attribute is a different thing depending on how we access it?

First of all, realize that a method we define on a class is just a function. A function accessed through an instance differs from the same function accessed through the instance's type because the function type implements the descriptor protocol. If you're unfamiliar with descriptors, I highly recommend you to read Descriptor HowTo Guide by Raymond Hettinger. In a nutshell, a descriptor is an object that, when used as an attribute, determines by itself how you get, set and delete it. Technically, a descriptor is an object that implements __get__(), __set__(), or __delete__() special methods.

The function type implements __get__(). When we look up some method, what we get is the result of a call to __get__(). Three arguments are passed to it:

  • an attribute, i.e. a function
  • an instance
  • the instance's type.

If we look up a method on a type, the instance is NULL, and __get__() simply returns the function. If we look up a method on an instance, __get__() returns a method object:

>>> type(A.method)
<class 'function'>
>>> type(a.method)
<class 'method'>

A method object stores a function and an instance. When called, it prepends the instance to the list of arguments and calls the function.

Now we're ready to tackle the last question.

Special methods based on slots

Recall the PyType_Ready() function that initializes types and does slot inheritance. It also adds special methods to a type based on the implemented slots. PyType_Ready() calls add_operators() to do that. The add_operators() function iterates over the entries in the slotdefs array. For each entry, it checks whether the special method specified by the entry should be added to the type's dictionary. A special method is added if it's not already defined and if the type implements the slot specified by the entry. For example, if the __add__() special method is not defined on a type, but the type implements the nb_add slot, add_operators() puts __add__() in the type's dictionary.

What is __add__() set to? Like any other method, it must be set to some descriptor to behave like a method. While methods defined by a programmer are functions, methods set by add_operators() are wrapper descriptors. A wrapper descriptor is a descriptor that stores two things:

  • It stores a wrapped slot. A wrapped slot "does the work" for a special method. For example, the wrapper descriptor of the __add__() special method of the float type stores float_add() as a wrapped slot.
  • It stores a wrapper function. A wrapper function "knows" how to call the wrapped slot. It is wrapper of a slotdef entry.

When we call a special method that was added by add_operators(), we call a wrapper descriptor. When we call a wrapper descriptor, it calls a wrapper function. A wrapper descriptor passes to a wrapper function the same arguments that we pass to a special methods plus the wrapped slot. Finally, the wrapper function calls the wrapped slot.

Let's see how a built-in type that implements the nb_add slot gets its __add__() and __radd__() special methods. Recall the slotdef entries corresponding to nb_add:

static slotdef slotdefs[] = {
    // ...
    // {name, offset, function,
    //     wrapper, doc}
    {"__add__", offsetof(PyHeapTypeObject, as_number.nb_add), (void *)(slot_nb_add),
        wrap_binaryfunc_l, PyDoc_STR("__add__" "($self, value, /)\n--\n\nReturn self" "+" "value.")},

    {"__radd__", offsetof(PyHeapTypeObject, as_number.nb_add), (void *)(slot_nb_add),
        wrap_binaryfunc_r, PyDoc_STR("__radd__" "($self, value, /)\n--\n\nReturn value" "+" "self.")},
    // ...

If a type implements the nb_add slot, add_operators() sets __add__() of the type to a wrapper descriptor with wrap_binaryfunc_l() as a wrapper function and nb_add as a wrapped slot. It similarly sets __radd__() of the type with one exception: a wrapper function is wrap_binaryfunc_r().

Both wrap_binaryfunc_l() and wrap_binaryfunc_r() take two operands plus a wrapped slot as their parameters. The only difference is how they call the slot:

  • wrap_binaryfunc_l(x, y, slot_func) calls slot_func(x, y)
  • wrap_binaryfunc_r(x, y, slot_func) calls slot_func(y, x).

The result of this call is what we get when we call the special method.


Today we've demystified perhaps the most magical aspect of Python. We've learned that the behavior of a Python object is determined by the slots of the object's type. The slots of a statically defined type can be specified explicitly, and any type can inherit some slots from its ancestors. The real insight was that the slots of a class are set up automatically by CPython based on the defined special methods. CPython does the reverse too. It adds special methods to the type's dictionary if the type implements the corresponding slots.

We've learned a lot. Nevertheless, the Python object system is such a vast subject that at least as much remains to be covered. For example, we haven't really discussed how attributes work. This is what we're going to do next time.

If you have any questions, comments or suggestions, feel free to contact me at