Just for future readers it is not impossible to pull them out backwards - it is merely a slow, careful process.
You should not try this at all if there are rough, sharp edges on the inside of the hole they are going through - in that case just hold until you can find an alternative way in or give up and retrieve them later.
But if the only thing holding them in is their own muscle strength wrapped around something then here is how I have done it in the past;
You cannot force them out when they are actively fighting but if you keep hold to prevent them going further in they will eventually relax a little - at that point you can usually retrieve a couple fo centimeters before they tense up again. When they tense up stop pulling immediately and just wait until the next relaxation.
It will take a very, very long time for a decent sized snake (I was doing this with a six foot king snake going into an ornamental fireplace with holes directly into other flats through the floor and no way to dismantle it short of a sledgehammer. Not a good idea in a rented flat ) it took me just over four hours to finally get him loose - at which point he bit the bejessus out of me and pooped everywhere.
But remember - if there are sharp edges it is definately worth trying to find plan B.
dr del
LOL oh man. That'd drive me insane. Sorry sorry I'm not laughing at you! But I lawl'd so hard I almost fell out of my chair