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Yeah take it off while its still bolted to the car/engine,and re press it on to the new pump bolted onto the car/engine...if you cant do it that way,you need a bench vise to hold the pump and with the proper tool,take pulley off/repress it on to the new pump in bench vise.
, But - the answer to your Question is: No, the pump can be removed from the engine with the pulley on it. BTW - I find it easier to remove the pulley from the pump (with a puller and a bit of heat from a torch) After it's out of the car and in a bench vise.