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Porting Launchpad to Python 3: progress report

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Launchpad peaceable requires Python 2, which in 2020
is a bit of a discipline.
Unlike most of the relaxation of 2020, though, there’s upright cause to be
optimistic about growth.

I’ve been porting Python 2 code to Python 3 on and off for a really very long time, from
assist when I modified into once on the Ubuntu Foundations team and placing forward things take care of
the Ubiquity installer. When I moved to
Launchpad in 2015 it modified into once completely on my mind that this modified into once an unlimited body of
code peaceable stuck on Python 2. One option would were to aesthetic catch
that and inch away it as it’s, seemingly doing extra backporting work over time as
aid for Python 2 fades away. I’ve long been of the conception that this
would doom Launchpad to being unmaintainable within the long dash, and since I
genuinely treasure working on Launchpad – I receive it an extremely rewarding
venture – this wasn’t something I modified into once willing to honest catch. We’re already
seeing a few of our principal dependencies dropping aid for Python 2,
which is completely practical on their phrases but which is beginning to change into
a marvelous obstacle to turning in principal aspects when we want fresh
aspects from more moderen versions of those dependencies. It furthermore looks as if
it’d be hard for us to dash on Ubuntu 20.04 LTS (we’re at this time on
16.04, with an upgrade to 18.04 in growth) so long as we peaceable require
Python 2, since we’ve some procedure dependencies that 20.04 now now no longer
presents. And then there are spirited fresh aspects take care of form
hints
and
async/are looking ahead to that we’d take care of
so that you would possibly maybe maybe exhaust.

Nonetheless, till final 365 days there were so many blockers that even mad by a
port modified into once barely possible. What modified in 2019 modified into once checking out a
trifecta of core dependencies. We ported our
database layer, Storm. We
upgraded
to in style versions of our Zope Toolkit dependencies
(after contributing varied fixes upstream, including some huge
adjustments to Zope’s take a look at runner
that we’d carried as local patches for some years). And we
ported
our Bazaar code web hosting infrastructure to
Breezy. With all that in effect, a port
regarded extra of a pragmatic possibility.

Restful, even with this, it modified into once never going to be a subject of aesthetic following
some customary porting recommendation and calling it
upright. Launchpad has practically 1,000,000 strains of Python code in its principal git
tree
, and spherical 250 dependencies of
which a amount are reasonably Launchpad-specific. In a venture that dimension, now no longer
only is following customary porting recommendation an extraordinarily time-ingesting project
in its delight in factual, but aesthetic about every odd nook case goes to level to
up somewhere. (Create you know that StringIO.StringIO(None) and
io.StringIO(None) enact diversified things even after you yarn for the
native string vs. Unicode textual thunder material distinction? How about the behaviour of
.union() on a subclass of
frozenset
?)
Launchpad’s take a look at suite is fortunately extraordinarily thorough, but even aesthetic
environment out the take a look at suite entails importing most of the records mannequin code,
so earlier than you are going to be ready to begin up taking real thing about it it’s a ways fundamental to compose an unlimited
half of the codebase be at the least syntactically-aesthetic Python 3 code and
exhaust only modules that exist in Python 3 while peaceable working in Python 2; in
a venture this dimension that turns out to be an unlimited effort on its delight in, and would possibly maybe maybe maybe
be reasonably
unstable
in areas.

Canonical’s product engineering groups work on a six-month cycle, but it surely aesthetic
isn’t that you’re going to be ready to assume of to cram this make of thing into six months except you enact
actually nothing else, and “please enact we effect all characteristic style on
aid while we dash to face peaceable” is a reasonably well-known promote to even basically the most
conception management. Thankfully, we’ve been ready to develop the
Launchpad team within the final 365 days or so,
and so it’s been that you’re going to be ready to assume of to effect “Python 3” on our roadmap on the
conception that we aren’t going to receive all of the very best design there in one cycle,
while peaceable being ready to enact diversified huge characteristic style work as neatly.

So, with all that preamble, what have we carried out this cycle? We’ve taken a
two-pronged design. From one discontinue, we identified 147 classes that wished
to be ported away from some compatibility code in our database layer that
modified into once substantially much less superior to Python 3: we’ve ported 38 of those, so
there’s clearly a aesthetic bit extra to enact, but we were ready to distribute this
determine among the team reasonably successfully. From the diversified discontinue, it modified into once certain
that it’d be very inefficient to enact customary porting work when any
strive and even dash the take a look at suite would dash straight into the same crashes
within the same command, so I space myself a intention of getting the take a look at suite to
beginning up up, and started hacking on an substantial git
department
that I never
anticipated to strive and land without delay: as a replacement, I felt free to commit aesthetic about
anything that regarded practical and moved things forward even though it modified into once very
tough, and every so most ceaselessly went assist to trim things up and cherry-eradicate
particular particular person commits into a make that incorporated some form of explanation and
handed present exams so that I would possibly maybe maybe maybe imply them for evaluation.

This diagram has been dramatically extra successful than anything I’ve tried
earlier than at this scale. To this point this cycle, mad by only Launchpad’s principal
git tree, we’ve landed 137 Python-3-relevant merge proposals for a complete of
39552 strains of git diff output, conserving our present exams passing alongside
the very best design and deploying incrementally to manufacturing. Now we have gotten about 27000 extra
strains of patch at varying degrees of quality to trim up and merge. Our principal
style department is simply seemingly 10 or 20 extra patches away from the take a look at
suite being ready to begin up up, at which level we’ll be ready to receive a buildbot
running so that lots of builders can work on this noteworthy extra without explain and
look the construct of their work. With the beefy unlanded patch stack, about 75%
of the take a look at suite passes on Python 3! This peaceable leaves a long tail of
lots of thousand exams to determine and fix, but it surely’s a noteworthy extra
incrementally-tractable form of discipline than where we began.

In a roundabout design: the funniest (to me) bug I’ve encountered in this effort modified into once the
one I encountered within the take a look at runner and fastened in
zopefoundation/zope.testrunner#106:
IDs of failing exams were written to a pipe, so within the event you have a take a look at suite
that’s huge ample and damaged ample then in the end that pipe would reach
its ability and your take a look at runner would aesthetic quit and dangle. Slightly
tense when it intended an overnight take a look at dash didn’t give principal outcomes, but
furthermore eloquent commentary of kinds.

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