What is tp_init_pointer in cpython?

ganeshverse2005 · April 4, 2026, 2:06pm

I have a doubt regarding the tp_init pointer in the struct PyTypeObject.
Does it point to the user defined _init_ or some other ??

JamesParrott · April 4, 2026, 2:15pm

I’m not sure: Type Object Structures — Python 3.14.3 documentation

jeff5 · April 4, 2026, 4:16pm

Yes, it points to the C implementation of __init__. Of course, it bears a slightly complex relationship to tp_new and type.__call__, but that is just the data model.

github.com/python/cpython

Objects/typeobject.c

853dafe23


      
          static int
          object_init(PyObject *self, PyObject *args, PyObject *kwds)
          {
              PyTypeObject *type = Py_TYPE(self);
              if (excess_args(args, kwds)) {
                  if (type->tp_init != object_init) {
                      PyErr_SetString(PyExc_TypeError,
                                      "object.__init__() takes exactly one argument (the instance to initialize)");
                      return -1;
                  }
                  if (type->tp_new == object_new) {
                      PyErr_Format(PyExc_TypeError,
                                   "%.200s.__init__() takes exactly one argument (the instance to initialize)",
                                   type->tp_name);
                      return -1;
                  }
              }
              return 0;
          }

Now if the type is defined in Python, with an __init__ in Python, then this slot is made to contain a pointer to a wrapper function (in C) that dispatches to that Python method.

github.com/python/cpython

Objects/typeobject.c

853dafe23


      
          static int
          slot_tp_init(PyObject *self, PyObject *args, PyObject *kwds)
          {
              PyObject *res = call_method(self, &_Py_ID(__init__), args, kwds);
              if (res == NULL)
                  return -1;
              if (res != Py_None) {
                  PyErr_Format(PyExc_TypeError,
                               "__init__() should return None, not '%.200s'",
                               Py_TYPE(res)->tp_name);
                  Py_DECREF(res);
                  return -1;
              }
              Py_DECREF(res);
              return 0;
          }

ganeshverse2005 · April 4, 2026, 5:16pm

thankyou….was looking for this…

ganeshverse2005 · April 4, 2026, 5:17pm

thankyou!! This made the picture clearer!!