First experience implementing StreamResult

19Feb13

My last two blog posts were largely about the needs of subunit, but a key result of any protocol is how easy working with it in a high level language is.

In the weekend and evenings I’ve done an implementation of a new set of classes – StreamResult and friends – that provides:

  • Adaption to and from the existing TestResult APIs (the 2.6 and below API, 2.7 API, and the testtools extended API).
  • Multiplexing multiple streams together.
  • Adding timing data to a stream if it is absent.
  • Summarising a stream.
  • Copying a stream to multiple outputs
  • A split out API for instructing a test run to stop.
  • A simple test-at-a-time stream processor that makes it easy to just deal with tests rather than the innate complexities of an event based interface.

So far the code has been uniformly simple to write. I started with an API that included an ‘estimate’ function, which I’ve since removed – I don’t believe the complexity is justified; enumeration is not significantly more expensive than counting, and runners that want to be efficient can either not enumerate or remember the enumeration from prior runs.

The documentation in the linked pull request is a good place to start to get a handle on the API; I’d love feedback.

Next steps for me are to do a subunit protocol revision that maps to the Python API, both parser and generator and see how it feels. One wrinkle there is that the reason for doing this is to fix intrinsic limits in the existing protocol – so doing forward and backward wire protocol compatibility would defeat the point. However… we can make the output side explicitly choose a protocol version, and if we can autodetect the protocol version in the parser, even if we cannot handle mixed streams we can get the benefits of the new protocol once data has been detected. That said, I think we can start without autodetection during prototyping, and add it later. Without autodetection, programs like TestRepository will need configuration options to control what protocol variant to expect. This could be done by requiring this new protocol and providing a stream filter that can be deployed when needed.

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