3.3.2. Buildbot Coding Style

3.3.2.1. Documentation

Buildbot strongly encourages developers to document the methods, behavior, and usage of classes that users might interact with. However, this documentation should be in .rst files under master/docs/developer, rather than in docstrings within the code. For private methods or where code deserves some kind of explanatory preface, use comments instead of a docstring. While some docstrings remain within the code, these should be migrated to documentation files and removed as the code is modified.

Within the reStructuredText files, write each English sentence on its own line. While this does not affect the generated output, it makes git diffs between versions of the documentation easier to read, as they are not obscured by changes due to re-wrapping. This convention is not followed everywhere, but we are slowly migrating documentation from the old (wrapped) style as we update it.

3.3.2.2. Symbol Names

Buildbot follows PEP8 regarding the formatting of symbol names.

Due to historical reasons, most of the public API uses interCaps naming style To preserve backwards compatibility, the public API should continue using interCaps naming style. That is, you should spell public API methods and functions with the first character in lower-case, and the first letter of subsequent words capitalized, e.g., compareToOther or getChangesGreaterThan. The public API refers to the documented API that external developers can rely on. See section on the definition of the public API in Buildbot Development.

Everything else should use the style recommended by PEP8.

In summary:

Symbol Type	Format
Methods and functions	under_scores
Method and function arguments	under_scores
Public API methods and functions	interCaps
Public API method and function arguments	interCaps
Classes	InitialCaps
Variables	under_scores
Constants	ALL_CAPS

3.3.2.3. 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, inlineCallbacks is appropriate. In all cases, please prefer maintainability and readability over performance.

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):
    # for demonstration only! see below for a better implementation with inlineCallbacks
    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 can work 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 ])

Note

d.addCallback is not really a decorator as it does not return a modified function. As a result, in the previous code, rev_parse value is actually the Deferred. In general, the inlineCallbacks method is preferred inside new code as it keeps the code easier to read. As a general rule of thumb, when you need more than 2 callbacks in the same method, it’s time to switch to inlineCallbacks. This would be for example the case for the getRevInfo example. See this discussion <:pull:`2523>`_ with Twisted experts for more information.

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 inlineCallbacks instead.

In most of the cases, if you need more than two callbacks in a method, it is more readable and maintainable to use inlineCallbacks.

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:
        return xval + y

    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.

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.

The previous getRevInfo example implementation should rather be written as:

@defer.inlineCallbacks
def getRevInfo(revname):
    results = {}
    res = yield utils.getProcessOutput(git, [ 'rev-parse', revname ])
    results['rev'] = res.strip()
    res = yield utils.getProcessOutput(git, [ 'log', '-1', '--format=%s%n%b',
                                             results['rev'] ])
    results['comments'] = res.strip()
    return results

Locking

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

Joining Sequences

It’s often the case that you want to perform multiple operations in parallel and rejoin the results at the end. For this purpose, you may 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 you would with a deferred.

Functions running outside of the main thread

It is very important in Twisted to be able to distinguish functions that runs in the main thread and functions that don’t, as reactors and deferreds can only be used in the main thread. To make this distinction clearer, every function meant to be run in a secondary thread must be prefixed with thd_.