My bike chain lock is like a necklace or a bracelet. I bought the most common one for sale at the store. It's light, comes in a variety of colors and has an attached fastener.
Here it is as an anklet:
I don't know why I even bother... oh, yes I do. I'm paranoid.
Most people only use the "o-lock." It's attached to the back wheel and pushes a little bar through the spokes to make a complete O upon itself. It is effective at preventing the bike from moving, unless someone picks up the bike and carries it to a nearby van. But that never seems to happen... despite my being told that bike theft is soooo common here. Ha. Look at that dinky little cable lock. Ha ha ha.
This bike, in spite of it's paint job, would be gone within minutes in New York. Just for practice.
Check out this silly little lock. (and the "o-lock" under it.)
It must be worse in Amsterdam. There's a market for
"High-Tech Antitheft Bicycle Spray"
Be sure to look at the other pretty bikes on that site.