/usr/lib/python2.7/dist-packages/sage/sets/ is in sagemath-common 8.1-7ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/python2.7/dist-packages/sage/sets/__init__.py
/usr/lib/python2.7/dist-packages/sage/sets/all.py
/usr/lib/python2.7/dist-packages/sage/sets/cartesian_product.py
/usr/lib/python2.7/dist-packages/sage/sets/disjoint_set.pxd
/usr/lib/python2.7/dist-packages/sage/sets/disjoint_union_enumerated_sets.py
/usr/lib/python2.7/dist-packages/sage/sets/family.py
/usr/lib/python2.7/dist-packages/sage/sets/finite_enumerated_set.py
/usr/lib/python2.7/dist-packages/sage/sets/finite_set_map_cy.pxd
/usr/lib/python2.7/dist-packages/sage/sets/finite_set_maps.py
/usr/lib/python2.7/dist-packages/sage/sets/integer_range.py
/usr/lib/python2.7/dist-packages/sage/sets/non_negative_integers.py
/usr/lib/python2.7/dist-packages/sage/sets/positive_integers.py
/usr/lib/python2.7/dist-packages/sage/sets/primes.py
/usr/lib/python2.7/dist-packages/sage/sets/real_set.py
/usr/lib/python2.7/dist-packages/sage/sets/recursively_enumerated_set.pxd
/usr/lib/python2.7/dist-packages/sage/sets/set.py
/usr/lib/python2.7/dist-packages/sage/sets/set_from_iterator.py
/usr/lib/python2.7/dist-packages/sage/sets/totally_ordered_finite_set.py