There's much back and forth in the cycling world over whether D locks or folding locks are better. While it's undeniable that some D locks are harder to break, there are a lot of places where you can't use those locks to secure your bike in the first place. Folding locks are great because you can manipulate how well they can work around different kinds of obstacles to lock your bike in place. These locks aren't often any more or less expensive than their competing D lock counterparts, but today you can save $15 on one of the better folding locks you can buy today.
Kryptonite, a company known for its quality bike locks, has a great folding lock with six 3mm steel bars you can fold however you need. When not in use, this lock tucks away into an included transport bracket you can either clip to your belt or keep in your jersey pocket if you'd rather not mount it directly to your bike. It's a simple, reliable lock for a lot less than you would normally pay, and will keep your bike secure in a bunch of different places.