Here's some simple logic I've posted on here a gazillion times in answering this question:

If one feeding a snake in it's cage would expect their ball python to expect to be fed each and every time the keeper went into its cage, one could make an equally convincing arguement that removing a snake to be fed in a seperate container would condition it to expect to be fed each and every time it was removed from its enclosure (for example, for handling sessions).

What that means: If you say that feeding a snake inside its enclosure causes it to be "aggressive," you could make an equally supported claim that removing it from its enclosure to be fed would cause it to become "aggressive" as well.

In other words- it doesn't matter. I feed all of my snakes in their enclosures, and they know what is a rat (a rat) and what isn't a rat (my hand/arm). C'mon, they deserve a little more credit here....