As with any function call, your call to an Algorithm may cause an error. Usually this happens because you've sent it unexpected input, or a resource the Algorithm depends on (such as a public data feed) is unavailable, or occasionally because of a coding error in the Algorithm itself.
When this happens, the call will raise (throw) an Exception, usually of the type AlgorithmiaException:
Here, the error is caused because demo/Hello expects a String as input, but was passed an empty List . The exception raised in the local Python block is an Algorithmia.errors.AlgorithmException, but its message (generated from inside the Algorithm we called) hints at the problem: "Expected STRING but was BEGIN_ARRAY", indicating that the (Java) code of the actual Algorithm we called received an Array (cast from the List we sent) instead of a String.
To learn more, see the API Docs.
Now it's time to learn how to set Timeouts and other Query Parameters.