Why do Python's empty classes and functions work as arbitrary data containers, but not other objects?

Question

I've seen two different Python objects used to group arbitrary data together: empty classes and functions.

def struct():
    pass

record = struct
record.number = 3
record.name = "Zoe"


class Struct:
    pass

record = Struct()
record.number = 3
record.name = "Zoe"

Even if the class isn't empty, it seems to work so long as it's defined at runtime.

But when I got cocky and tried to do this with built-in functions or classes, it didn't work.

record = set()
record.number = 3
AttributeError: 'set' object has no attribute 'number'

record = pow
pow.number = 3
AttributeError: 'builtin_function_or_method' object has no attribute 'number'

Is there a fundamental difference between built-in and "custom" classes and functions that accounts for this behavior?

Answer 1

The difference is that both function objects and your Struct object have a __dict__ attribute, but set instances and built-in functions do not:

>>> def struct():
...     pass
...
>>> record = struct
>>> record.number = 2
>>> struct.__dict__
{'number': 2}
>>> class Struct:
...     pass
...
>>> record = Struct()
>>> record.number = 3
>>> record.__dict__
{'number': 3}
>>> record=set()
>>> record.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'set' object has no attribute '__dict__'
>>> pow.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'builtin_function_or_method' object has no attribute '__dict__'

In classes you can emulate the behavour using slots (although only on new-style classes):

>>> class StructWithSlots(object):
...     __slots__ = []
...
>>> record = StructWithSlots()
>>> record.number = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'StructWithSlots' object has no attribute 'number'
>>> record.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'StructWithSlots' object has no attribute '__dict__'

Answer 2

Built-in types are written in C and cannot be modified like that. But after the type/class unification introduced in py2.2 you can now inherit from a built-in types and override or add your own attributes to that subclass.

You can use the forbiddenfood package to add attributes to built-in types:

This project aims to give you the way to find heaven in tests, but it might lead you to hell if you use it on production code.

>>> from forbiddenfruit import curse 
>>> def words_of_wisdom(self):
 ...     return self * "blah "
>>> curse(int, "words_of_wisdom", words_of_wisdom)
>>> assert (2).words_of_wisdom() == "blah blah "

And of course if you're cocky enough then you can create your own types in C and add such features to it.