The odds are slim that it will happen but occasionally it does...however assignment does not change your risk....it only changes your buying power.

Example....if you are short a 10$ call and long the 11$ call (vertical credit spread) and you get assigned on the 10$ you are then short that stock at 10$ but are long with the 11$ calls....your directional risk has not changed. If it doesn't throw your buying power out of whack you can chose to keep it that way or simply put an exercise order on your long 11$ strike and that will put it all flat....and wherever you were with profit and loss will not have changed.

If it does throw your buying power below your available funds then you don't have a choice...just exercise the 11$ call and all will snap back into place. You will be flat on the position and your P/L will not have changed.

Strangles are only different in that to flatten the position you buy back or sell the shares you are assigned....doing so leaves you short on the other side so you would have to buy those back also to be completely flat.
But I would learn verticals and their variations (iron condors, butterflys etc.) before I jumped into strangles....as they have undefined risk which you will want to know how to manage when something moves against you.

DO NOT sell calls (in spreads or otherwise) on anything with earnings that occur before expiration of those calls....at least until you understand why and how to work it out. That is the one that could nail you if you aren't paying attention.

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