Buildbot Coding Style

Symbol Names

Buildbot follows PEP8 regarding the formatting of symbol names.

The single exception in naming of functions and methods. Because Buildbot uses Twisted so heavily, and Twisted uses interCaps, Buildbot methods should do the same. That is, you should spell methods and functions with the first character in lower-case, and the first letter of subsequent words capitalized, e.g., compareToOther or getChangesGreaterThan. This point is not applied very consistently in Buildbot, but let's try to be consistent in new code.

Twisted Idioms

Programming with Twisted Python can be daunting. But sticking to a few well-defined patterns can help avoid surprises.

Prefer to Return Deferreds

If you're writing a method that doesn't currently block, but could conceivably block sometime in the future, return a Deferred and document that it does so. Just about anything might block - even getters and setters!

Helpful Twisted Classes

Twisted has some useful, but little-known classes. Brief descriptions follow, but you should consult the API documentation or source code for the full details.

twisted.internet.task.LoopingCall

Calls an asynchronous function repeatedly at set intervals. Note that this will stop looping if the function fails. In general, you will want to wrap the function to capture and log errors.

twisted.application.internet.TimerService

Similar to t.i.t.LoopingCall, but implemented as a service that will automatically start and stop the function calls when the service starts and stops. See the warning about failing functions for t.i.t.LoopingCall.

Sequences of Operations

Especially in Buildbot, we're often faced with executing a sequence of operations, many of which may block.

In all cases where this occurs, there is a danger of pre-emption, so exercise the same caution you would if writing a threaded application.

For simple cases, you can use nested callback functions. For more complex cases, deferredGenerator is appropriate.

Nested Callbacks

First, an admonition: do not create extra class methods that represent the continuations of the first:

def myMethod(self):
    d = ...
    d.addCallback(self._myMethod_2) # BAD!
def _myMethod_2(self, res):         # BAD!
    # ...

Invariably, this extra method gets separated from its parent as the code evolves, and the result is completely unreadable. Instead, include all of the code for a particular function or method within the same indented block, using nested functions:

def getRevInfo(revname):
    results = {}
    d = defer.succeed(None)
    def rev_parse(_): # note use of '_' to quietly indicate an ignored parameter
        return utils.getProcessOutput(git, [ 'rev-parse', revname ])
    d.addCallback(rev_parse)
    def parse_rev_parse(res):
        results['rev'] = res.strip()
        return utils.getProcessOutput(git, [ 'log', '-1', '--format=%s%n%b', results['rev'] ])
    d.addCallback(parse_rev_parse)
    def parse_log(res):
        results['comments'] = res.strip()
    d.addCallback(parse_log)
    def set_results(_):
        return results
    d.addCallback(set_results)
    return d

it is usually best to make the first operation occur within a callback, as the deferred machinery will then handle any exceptions as a failure in the outer Deferred. As a shortcut, d.addCallback works as a decorator:

d = defer.succeed(None)
@d.addCallback
def rev_parse(_): # note use of '_' to quietly indicate an ignored parameter
    return utils.getProcessOutput(git, [ 'rev-parse', revname ])

Be careful with local variables. For example, if parse_rev_parse, above, merely assigned rev = res.strip(), then that variable would be local to parse_rev_parse and not available in set_results. Mutable variables (dicts and lists) at the outer function level are appropriate for this purpose.

Note

do not try to build a loop in this style by chaining multiple Deferreds! Unbounded chaining can result in stack overflows, at least on older versions of Twisted. Use deferredGenerator instead.

inlineCallbacks

twisted.internet.defer.inlineCallbacks is a great help to writing code that makes a lot of asynchronous calls, particularly if those calls are made in loop or conditionals. Refer to the Twisted documentation for the details, but the style within Buildbot is as follows:

from twisted.internet import defer

@defer.inlineCallbacks
def mymethod(self, x, y):
    xval = yield getSomething(x)

    for z in (yield getZValues()):
        y += z

    if xval > 10:
        defer.returnValue(xval + y)
        return

    self.someOtherMethod()

The key points to notice here:

• Always import defer as a module, not the names within it.
• Use the decorator form of inlineCallbacks.
• In most cases, the result of a yield expression should be assigned to a variable. It can be used in a larger expression, but remember that Python requires that you enclose the expression in its own set of parentheses.
• Python does not permit returning a value from a generator, so statements like return xval + y are invalid. Instead, yield the result of defer.returnValue. Although this function does cause an immediate function exit, for clarity follow it with a bare return, as in the example, unless it is the last statement in a function.

The great advantage of inlineCallbacks is that it allows you to use all of the usual Pythonic control structures in their natural form. In particular, it is easy to represent a loop, or even nested loops, in this style without losing any readability.

Note that code using deferredGenerator is no longer acceptable in Buildbot.

Locking

Remember that asynchronous programming does not free you from the need to worry about concurrency issues. Particularly if you are executing a sequence of operations, each time you wait for a Deferred, arbitrary other actions can take place.

In general, you should try to perform actions atomically, but for the rare situations that require synchronization, the following might be useful:

• twisted.internet.defer.DeferredLock
• buildbot.util.misc.deferredLocked
• buildbot.util.misc.SerializedInvocation

Joining Sequences

It's often the case that you'll want to perform multiple operations in parallel, and re-join the results at the end. For this purpose, you'll want to use a DeferredList

def getRevInfo(revname):
    results = {}
    finished = dict(rev_parse=False, log=False)

    rev_parse_d = utils.getProcessOutput(git, [ 'rev-parse', revname ])
    def parse_rev_parse(res):
        return res.strip()
    rev_parse_d.addCallback(parse_rev_parse)

    log_d = utils.getProcessOutput(git, [ 'log', '-1', '--format=%s%n%b', results['rev'] ]))
    def parse_log(res):
        return res.strip()
    log_d.addCallback(parse_log)

    d = defer.DeferredList([rev_parse_d, log_d], consumeErrors=1, fireOnFirstErrback=1)
    def handle_results(results):
        return dict(rev=results[0][1], log=results[1][1])
    d.addCallback(handle_results)
    return d

Here the deferred list will wait for both rev_parse_d and log_d to fire, or for one of them to fail. You may attach callbacks and errbacks to a DeferredList just as for a deferred.