In Python, how to write a set containing a set?

Question

We know in Python, a set can be defined by writing out all its elements like this:

a_set={1,"xyz"}

And books of Python all say elements of a set can be any datatype. So we should be able to write out a set containing a set. I tried to write it as:

a_set={1,{"xyz"}}

But IDLE reported an error:

Traceback (most recent call last):
  File "<pyshell#58>", line 1, in <module>
    a_set={1,{"xyz"}}
TypeError: unhashable type: 'set'

I think this may be because Python is trying to understand it as a dictionary. Then, how to write out a set containing a set in Python?

Answer 1

The inner most sets need to be of type frozenset which is an immutable version of a set.

>>> a_set = {1, frozenset(['xyz'])}
>>> a_set
set([1, frozenset(['xyz'])])

From the docs:

class frozenset([iterable])

Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be frozenset objects. If iterable is not specified, a new empty set is returned.