Python object in python source code analysis notes

 

 

1. in python, the object is a piece of memory applied for on the heap for the struct in C. Objects cannot be statically initialized, nor can they survive in the stack space. However, all built-in type objects are statically initialized.

2. The object size remains unchanged after being created. A variable object has a pointer pointing to a variable-size memory area.

3. The cornerstone of the object mechanism: PyObject

Fixed Length object:

typedef struct _object{           PyObject_HEAD}PyObject;

Release mode Compilation -->

 

Typedef struct _ object {intob_refcnt; // reference count, garbage collection mechanism struct_typeobject * ob_type; // type information, specified object type} PyObject;

PyObject is a total of python objects, and the most initial bytes of the object in memory.

Variable-length object:

 

# Define PyObject_VAR_HEAD PyObject_HEAD contains ob_size; // The number of elements that can be accommodated in the Variable Length section. typedef struct {PyObject_VAR_HEAD} PyVarObject;

In Python, each object has the same object header. We only need to use a PyObject * pointer to reference any object.

Eg:

 

// Pyobjecbject object typedef struct {PyObject_HEAD longob_ival; // save value} // pythonStringObject typedef struct {symbol longob_shash; into B _sstate; charob_sval [1];} PyStringObject; // PyListObject object typeDef struct {struct PyObjct ** ob_item; intallocated;} PyListObject; // PyDictObject object typedef struct {struct; PyObject * me_key; PyObject * me_value;} PyDictEntry;

4. Type object (_ typeobject type --> Implementation of the "class" concept in object-oriented theory)

 

Typedef struct _ typeobject {PyObject_VAR_HEAD // The type object is also an object, and it is also a variable-length object char * tp_name; // type name, inttp_basicsize, tp_itemsize will be used for printing; // The size of memory occupied by this type. The/* operation information associated with this type of object */destructor tp_dealloc; printfunc tp_print; //…} will be used for memory allocation ;//...} PyTypeObject;

PyType_Type (metaclass), PyInt_Type, PyString_Type, PyDict_Type, and PyList_Type are the instantiation objects of _ typeobject.

5. Reference count

 

Py_INCREF (op) // Add an object reference count Py_DECREF (op) // reduce the reference count of an object

When the reference count is reduced to 0, Py_DECREF calls the destructor of the object to release the memory and system resources occupied by the object. This destructor is specified by a function pointer defined in the corresponding type object of the object, that is, tp_dealloc

Calling the Destructor does not mean that the free space will be called in the end. In Python, a large number of Memory Object pool technologies are used to avoid frequent application and release of memory space.

Type objects will never be destructed.

6. Create an object

6.1 Python c api (this method is generally used for built-in types)

 

// APIPyObject * parameter bj = PyObject_New (PyObject, & PyInt_Type); // APIPyObject * parameter bj = PyInt_FromLong (10)

 

6.2 type object PyInt_Type (the custom type can only be in this way)

Int (10) is an integer object created through PyInt_Type.

All classes are object-based

Tp_new --> apply for memory corresponding to the new operator in C ++

Tp_init --> corresponding to the constructor in C ++, complete the "initialization" Object operation

7. Object Behavior

PyTypeObject defines a large number of function pointers, such as tp_new, tp_init, tp_dealloc, and tp_hash. Three of them are very important operation families.

Tp_as_number --> PyNumberMethods

Tp_as_sequence --> PySequenceMethods

Tp_as_mapping --> PyMappingMethods

 

typedef PyObject *(*binaryfunc) (PyObject *, PyObject *);typedef struct{binaryfunc nb_add;binaryfunc nb_subtract;//…} PyNumberMethods;typedef struct {lenfunc sq_length;binaryfunc sq_concat;ssizeargfunc sq_repeat;ssizeargfunc sq_item;ssizessizeargfunc sq_slice;ssizeobjargproc sq_ass_item;ssizessizeobjargproc sq_ass_slice;objobjproc sq_contains;/* Added in release 2.0 */binaryfunc sq_inplace_concat;ssizeargfunc sq_inplace_repeat;} PySequenceMethods;typedef struct {lenfunc mp_length;binaryfunc mp_subscript;objobjargproc mp_ass_subscript;} PyMappingMethods;

 

 

8. Type (PyType_Type)

The PyTypeObject corresponding to the custom class is created through PyType_Type. All classes are PyType_Type.

 

PyTypeObject PyType_Type = {           PyObject_HEAD_INIT(&PyType_Type)           0,           “type”,           //…};

Relationship between a common type object and a PyType_Type object

 

9. Python object Polymorphism

Through PyObject and PyTypect, Python uses the C language to complete the polymorphism of the objects provided by C ++. Python uses a PyObject * variable instead of a py1_bject * variable (or other specific types) to maintain this object. We don't know what type the pointer refers, however, it can be dynamically determined by the ob_type field of the object.

Eg.

 

Void Print (PyObject * object) {object-> ob_type-> tp_print (object); // call the corresponding tp_print Based on the object type. A function shows different behaviors under different circumstances, that is, polymorphism}