Py4J 0.6 is almost ready to be released, thanks to Jakub L. Gustak who submitted important bug reports, feature requests, and patches. I have been trying to polish Py4J in the latest releases to make the API more consistent and predictable and the biggest “feature” of 0.6 will no doubt be how Py4J treats Exceptions.
Currently, exceptions can be raised in four places: (1) in the Py4J Python code, (2) in the Py4J Java code, (3) in the Java client code, and (4) in the network stack. An exception might be raised in the Py4J code if the client code is not correct, for example, if the client tries to call from Python a Java method that does not exist. Before 0.6, Py4J raised a Py4JError in cases 1,2,3 and a Py4JNetworkError (a subtype of Py4JError) in case 4. Moreover, if the Java exception was raised on the Java side, the Java stack trace was copied, as a string, in the Py4JError.
There are two issues with this approach. First, the client does not have access to the exception instance on the Java side, and this exception may have some important fields and methods that can help the error recovery. Second, it is very difficult for the client to determine at runtime the source of the error.
Starting from 0.6, Py4J will raise three types of exceptions: Py4JNetworkError in case #4, Py4JJavaError in case #3, Py4JError in cases #1 and #2. Py4JNetworkError and Py4JJavaError will be a subtype of Py4JError (so a client can implement a catch all). Py4JJavaError will also have a method that will return the instance of the Java exception and Py4JError will still display the Java stack trace for case #2.
Stay tuned for 0.6!