def test_pop_back():
l = SingleLinkedList()
l.push_back(1)
assert l.head.data == 1
assert l.tail.data == 1
l.push_back(2)
assert l.head.data == 1
assert l.tail.data == 2
l.push_back(3)
assert l.head.data == 1
assert l.tail.data == 3
assert l.pop_back(), 3
assert l.pop_back(), 2
assert l.pop_back(), 1
