The whole "feed in a separate container so they don't associate your hand with food" is bunk. It's just something people keep regurgitating because that's what they heard. Think about it, you will go into your snakes cage for many other things a lot more often(handling, cleaning, changing water, etc) - why would it only associate your hands with feeding if you only do that say 1 time a week? Some snakes won't eat out of their container - I personally don't like disturbing them with moving them back and forth while eating/digesting. However, some snakes won't eat in their tank - so you might have to move them.
Bottom line (in my opinion) is if you feed on a loose substrate you should feed out of the tank - or place a piece of carpet on top of the substrate while feeding to avoid injestion of the substrate. I also think if you can't get your snake to eat in the tank then perhaps you might try feeding outside the tank and see if that works for you.
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