python - What is the purpose of Py_DECREF and PY_INCREF? -

i going through tutorial defining 'new types' in python, https://docs.python.org/2/extending/newtypes.html, , did not understand purpose of using py_decref in piece of code.

static pyobject * noddy_new(pytypeobject *type, pyobject *args, pyobject *kwds) {     noddy *self;      self = (noddy *)type->tp_alloc(type, 0);     if (self != null) {         self->first = pystring_fromstring("");         if (self->first == null)           {             py_decref(self);             return null;           }          self->last = pystring_fromstring("");         if (self->last == null)           {             py_decref(self);             return null;           }          self->number = 0;     }      return (pyobject *)self; } 

my understanding of reference counting patchy , appreciated.

in case, py_decref free memory allocated tp->alloc.

tp->alloc sets ref count 1. py_decref decreases ref count 1 0; finds ref count 0, calls appropriate functions free memory (noddy_dealloc in case.)

if python c api function returns null, has gone wrong; exception set (saved in global variable).

if caller returns null again, exception chained, hence 'return null'.