On Jan 24, 2017, at 12:48 PM, Craig Rodrigues <rodrigc@crodrigues.org> wrote:
On Mon, Jan 23, 2017 at 5:10 PM, Jean-Paul Calderone <exarkun@twistedmatrix.com <mailto:exarkun@twistedmatrix.com>> wrote: On Mon, Jan 23, 2017 at 7:08 PM, Craig Rodrigues <rodrigc@crodrigues.org <mailto:rodrigc@crodrigues.org>> wrote:
I did some more debugging. The callstack is quite deep. :/ I used this command to trigger the error:
trial buildbot.test.unit.test_process_buildrequestdistributor.TestMaybeStartBuilds.test_slow_db
I found in this function in buildbot's BuildRequestsEndpoint.get() function ( https://github.com/buildbot/buildbot/blob/master/master/buildbot/data/buildr... <https://github.com/buildbot/buildbot/blob/master/master/buildbot/data/buildr...> ) there is this line:
defer.returnValue( [(yield self.db2data(br)) for br in buildrequests])
On Python 2, this line returns: an object list each list entry is a dictionary of name/value pairs that looks like: [{'buildrequestid': 10, 'complete': False, 'waited_for': False, 'claimed_at': None, 'results': -1, 'claimed': False, 'buildsetid': 11, 'complete_at': None, 'submitted_at': datetime.datetime(1970, 1, 2, 12, 6, 40, tzinfo=tzutc()), 'builderid': 77, 'claimed_by_masterid': None, 'priority': 0}, {'buildrequestid': 11, 'complete': False, 'waited_for': False, 'claimed_at': None, 'results': -1, 'claimed': False, 'buildsetid': 11, 'complete_at': None, 'submitted_at': datetime.datetime(1970, 1, 2, 13, 30, tzinfo=tzutc()), 'builderid': 77, 'claimed_by_masterid': None, 'priority': 0}]
On Python 3, this returns: an object <generator object BuildRequestsEndpoint.get.<locals>.<listcomp> of type <class 'generator'>
Yep.
On Python 2, [(yield self.db2data(br)) for br in buildrequests] is a list comprehension. It will have len(buildrequests) elements and each will be the value sent back in to the generator via the yield expression.
On Python 3, the same expression is a list of one element which is a generator expression.
This form is much clearer, I think, and behaves as intended on both versions of Python:
results = [] for br in buildrequests: results.append((yield self.db2data(br))) defer.returnValue(results)
Wow, thanks! That fixed it. I submitted those fixes upstream to buildbot.
I'm not so familiar with generator expressions and Deferred's so have been playing around with things to try to understand.
I went back to the problem code, and changed:
defer.returnValue( [(yield self.db2data(br)) for br in buildrequests])
to:
defer.returnValue( list([(yield self.db2data(br)) for br in buildrequests])
and ran the code under Python 3 and got a return value of:
[ Deferred ]
instead of: [ {......} ]
where the Deferred.results value was the dictionary.
How does Python 2 directly go to returning the dictionary out of the Deferred, while Python 3 returns the Deferred inside the list? self.db2data() returns a Deferred.
I've encountered this before and quickly worked around it, but I think this might actually be a bug in python 3, or at least its documentation. The language docs officially say that a "list display" (which is what I believe we're looking at here) "yields a new list object". But that is not what I see here:
[(yield 1) for x in range(10)] <generator object <listcomp> at 0x10cd210f8>
That is a comprehension enclosed in square brackets, but I'm getting a generator object out rather than a list. Is there a PEP for this that just hasn't updated the language reference? -glyph