First, the behavior for pwd.struct_passwd:
-----------------------------------------
--> pwd.getpwuid(1000)
pwd.struct_passwd(pw_name='ethan', pw_passwd='x', pw_uid=1000, pw_gid=1000, pw_gecos='Ethan Furman,,,', pw_dir='/home/ethan', pw_shell='/bin/bash')

--> set(pwd.getpwuid(1000))
set(['/bin/bash', 1000, 'Ethan Furman,,,', '/home/ethan', 'ethan', 'x'])

--> set([pwd.getpwuid(1000)])
set([pwd.struct_passwd(pw_name='ethan', pw_passwd='x', pw_uid=1000, pw_gid=1000, pw_gecos='Ethan Furman,,,', pw_dir='/home/ethan', pw_shell='/bin/bash')])

Now, the behavior for grp.struct_group:
--------------------------------------
--> grp.getgrgid(1000)
grp.struct_group(gr_name='ethan', gr_passwd='x', gr_gid=1000, gr_mem=[])

--> set(grp.getgrgid(1000))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

--> set([grp.getgrgid(1000)])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'


At the very least the error message is wrong (it's not a list), and at the most grp.struct_group should be hashable  -- i.e. we should be able to have a set of groups.

Test added.

This is because grp.struct_group contains unhashable component (a list).

Same behavior with tuple:

>>> tuple(grp.getgrgid(1000))
('serhiy', 'x', 1000, [])
>>> set(tuple(grp.getgrgid(1000)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> set([tuple(grp.getgrgid(1000))])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Instead a set of grp.struct_group, create a dict, which maps group name to grp.struct_group.

Thanks, Serhiy.