c - Diffrence between instance variables and attributes in python? -

so, python docs writing extension says this:

"we want expose our instance variables attributes. there number of ways that. simplest way define member definitions:

static pymemberdef noddy_members[] = {     {"first", t_object_ex, offsetof(noddy, first), 0,      "first name"},     {"last", t_object_ex, offsetof(noddy, last), 0,      "last name"},     {"number", t_int, offsetof(noddy, number), 0,      "noddy number"},     {null}  /* sentinel */ }; 

and put definitions in tp_members slot:

noddy_members,             /* tp_members */" 

however, have put instance variables in noddy struct:

 typedef struct {     pyobject_head     pyobject *first;     pyobject *last;     int number; } noddy; 

so question why put them in both places. impression that, because want both type , instance have them preserve types values once instance updated. if thats case, how instance value updated if change class attribute? this:

>>> class foo(object): x = 4 ... >>> f = foo() >>> f.x 4 >>> foo.x = 5 >>> f.x 5 

writing c extension complex process, because have write c code, , have provide enough information python can manipulate c data if python data. have mention members of struct twice: once c compiler know how make struct, , again python knows data is, , called.

c has no introspection ability, can't (for example), list of members of struct @ runtime. array of pymemberdef provides information.