How to check if a function has not returned a certain variable in the past second

def collideY(player, object):
if man.x <= object.x + object.width and man.x + man.width >= object.x: # Checks if  the player is inside the bounderies of a list of objects
    return "F"

cY = collideY(Player, object)
if not cY == "F":
    print("Not F")

This prints "not F" all the time and i want it to print only when F is not returned