Assertions should be used for expressing invariants, or preconditions.
In your example, you are using them for checking unexpected input – and that’s a completely different class of exceptions.
Depending on the requirements, it may be perfectly OK to raise an exception on wrong input, and stop the application; however the code should always be tailored for expressiveness, and raising an AssertionError is not that explicit.
Much better would be to raise your own exception, or a ValueError.