need a simple way to add meta information/class to a python list/tuple variable?

Question

All,

I want simple meta information to be enclosed on an list object, see below code.

>>> a = []
>>> a.foo = 100
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
AttributeError: 'list' object has no attribute 'foo'
>>> setattr(a,"foo",100)
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
AttributeError: 'list' object has no attribute 'foo'
>>> dir(a)
...
 '__setattr__',
 '__setitem__',
...

my quesions are

1. why I can not use setattr() for variable "a" as the list should have 'setattr' function already?

2. is there a simple way to attach meta info to list/tuple variable instead of overload 'setattr' or embedded that list into a dict?

thanks!

Answer 1

The base types defined in C cannot have arbitrary attributes added; you must derive a Python type from them in order to change this.

>>> class MyList(list):
...   pass
... 
>>> m = MyList((1, 2, 3))
>>> len(m)
3
>>> m.foo = 42
>>> m.foo
42

Answer 2

1. it does have setattr. it does not have attribute 'foo', as the message says.
2. yes, subclass it, e.g.

class mylist(list):
 pass

l = mylist()
l.foo = 'bar'