What exactly are "containers" in python? (And what are all the python container types?)

The python documentation frequently speaks of "containers". E.g. :

If check_circular is False (default: True), then the circular reference check for container types will be skipped and a circular reference will result in an OverflowError (or worse).

But I can't find any official definition of containers, neither a list of them.

Edit

For Python 2.7.3:

Checked builtin types which are containers:

(isinstance(object, collections.Container) returns True)

1. Containers which have a __contains__ method defined:

   • All builtin sequence types: Lists, bytearrays, strings, unicode strings and tuples.
   • Dictionaries
   • All builtin set types: sets and frozensets

2. Containers which do not have a __contains__ method defined:

   • xrange objects

Checked builtin types which are not containers:

(isinstance(object, collections.Container) returns False):

• Int objects
• Float objects
• Long objects
• Boolean objects
• Module objects
• File objects
• Buffer objects
• The None object

Tell me which other builtin types you have checked for isinstance(object, collections.Container) and I'll add them to the list.