You cannot apply an expression to an attribute already animated with any of the following techniques:

• keys
• set driven key
• constraint
• motion path
• another expression
• any other direct connection

If you do so, you’ll see an error message in the Script Editor and the Command Line’s response area.

Though you can’t control a single attribute with two of the preceding techniques, you can control one attribute with keyframes, another with an expression, another with a constraint, and so on.

Also, you can use a single expression to assign values to several attributes of one or more objects.