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Best Practices

Hide Twisted and Crochet

Consider some synchronous do-one-thing-after-the-other application code that wants to use event-driven Twisted-using code. We have two threads at a minimum: the application thread(s) and the reactor thread. There are also multiple layers of code involved in this interaction:

  • Twisted code: Should only be called in reactor thread. This may be code from the Twisted package itself, or more likely code you have written that is built on top of Twisted.
  • @wait_for/@run_in_reactor wrappers: The body of the functions runs in the reactor thread... but the caller should be in the application thread.
  • The application code: Runs in the application thread(s), expects synchronous/blocking calls.

Sometimes the first two layers suffice, but there are some issues with only having these. First, if you're using @run_in_reactor it requires the application code to understand Crochet's API, i.e. EventualResult objects. Second, if the wrapped function returns an object that expects to interact with Twisted, the application code will not be able to use that object since it will be called in the wrong thread.

A better solution is to have an additional layer in-between the application code and @wait_for/@run_in_reactor wrappers. This layer can hide the details of the Crochet API and wrap returned Twisted objects if necessary. As a result the application code simply seems a normal API, with no need to understand EventualResult objects or Twisted.

In the following example the different layers of the code demonstrate this separation: _ExchangeRate is just Twisted code, and ExchangeRate provides a blocking wrapper using Crochet.

.. literalinclude:: ../examples/scheduling.py

Minimize decorated code

It's best to have as little code as possible in the @wait_for/@run_in_reactor wrappers. As this code straddles two worlds (or at least, two threads) it is more difficult to unit test. Having an extra layer between this code and the application code is also useful in this regard as well: Twisted code can be pushed into the lower-level Twisted layer, and code hiding the Twisted details from the application code can be pushed into the higher-level layer.