When can you dynamically add a field to an object?

Question

I'm learning curses in python, and wanted to add an attribute to the curses window object.

my minimal program is:

import curses
try:
  stdscr = curses.initscr()
  stdscr.cur_line = 0
finally: 
  #clean-up so your terminal isn't wrecked by above error
  curses.nocbreak()
  stdscr.keypad(False)
  curses.echo()
  curses.endwin()

The error is:

$ python3 tmp
Traceback (most recent call last):
  File "tmp", line 4, in <module>
    stdscr.cur_line = 0
AttributeError: '_curses.curses window' object has no attribute 'cur_line'

However, this works:

class Temp:
    def __init__(self):
        pass
t = Temp()
t.cur_line = 0 #does not fail

My questions is: When does dynamically adding a field to an instance fail? How is python discerning between an instance of my user defined class, and an instance of the class from the curses library?

Answer 1

Most often, this happens because you're trying to add attributes to libraries written in C, and not pure python objects.

>>> import pickle, cPickle
>>> pickle.dump.a=1
>>> cPickle.dump.a=1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'builtin_function_or_method' object has no attribute 'a'