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you can break one of these in about 5 seconds but it will prevent the most basic crime of opportunity like your situation, and it's compact enough to fit in a jersey or saddle bag. If someone has cable cutters though its game over.
>take 0 locks and stay within sight of the bike at all times
I would be even more extreme and say the bike should be locked if its not inbetween your legs or inside your house (assuming its a nice bike).
If I plan on leaving my bike out of my sight I use two ulocks, and a mini cable lock. If your bike isn't expensive pro bike thieves won't want it, and if its locked up heavily crackheads / drug addicts won't bother