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    <h1>Mercurial's Journey to and Reflections on Python 3</h1>
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    <p>Mercurial 5.2 was released on November 5, 2019. It is the first version
of Mercurial that supports Python 3. This milestone comes nearly 11 years
after Python 3.0 was first released on December 3, 2008.</p>

<p>Speaking as a maintainer of Mercurial and an avid user of Python, I
feel like the experience of making Mercurial work with Python 3 is
worth sharing because there are a number of lessons to be learned.</p>

<p>This post is logically divided into two sections: a mostly factual recount
of Mercurial's Python 3 porting effort and a more opinionated commentary
of the transition to Python 3 and the Python language ecosystem as a whole.
Those who don't care about the mechanics of porting a large Python project
to Python 3 may want to skip the next section or two.</p>

<h2>Porting Mercurial to Python 3</h2>

<p>Let's start with a brief history lesson of Mercurial's support for
Python 3 as told by its own commit history.</p>

<p>The Mercurial version control tool was first released in April 2005
(the same month that Git was initially released). Version 1.0 came out
in March 2008. The first reference to Python 3 I found in the code base
was in <a href="https://www.mercurial-scm.org/repo/hg/rev/8fee8ff13d37">September 2008</a>.
Then not much happens for a while until
<a href="https://www.mercurial-scm.org/repo/hg/rev/4494fb02d549">June 2010</a>, when
someone authors a bunch of changes to make the Python C extensions
start to recognize Python 3. Then things were again quiet for a while
until <a href="https://www.mercurial-scm.org/repo/hg/rev/56ef99fbd6f2">January 2013</a>,
when a handful of changes landed to remove 2 argument <code>raise</code>. There were
a handful of commits in 2014 but nothing worth calling out.</p>

<p>Mercurial's meaningful journey to Python 3 started in 2015. In code,
the work started in
<a href="https://www.mercurial-scm.org/repo/hg/rev/af6e6a0781d7">April 2015</a>, with
effort to make Mercurial's test harness run with Python 3. Part of
this was a <a href="https://www.mercurial-scm.org/repo/hg/rev/fefc72523491">decision</a>
that Python 3.5 (to be released several months later in September 2015)
would be the minimum Python 3 version that Mercurial would support.</p>

<p>Once the Mercurial Project decided it wanted to port to Python 3 (as opposed
to another language), one of the earliest decisions was how to perform that
port. <strong>Mercurial's code base was too large to attempt a flag day conversion</strong>
where there would be a Python 2 version and a Python 3 version and one day
everyone would switch from Python 2 to 3. <strong>Mercurial needed a way to run the
same code (or as much of the same code) on both Python 2 and 3.</strong> We would
maintain a single code base and users would gradually switch from running with
Python 2 to Python 3.</p>

<p>In <a href="https://www.mercurial-scm.org/repo/hg/rev/e1fb276d4619">May 2015</a>,
Mercurial dropped support for Python 2.4 and 2.5. Dropping support for
these older Python versions was critical, as it was effectively impossible to
write Python code that ran on this wide gamut of versions because of
incompatibilities in syntax and language features. For example, you needed
Python 2.6 to get <code>print()</code> via <code>from __future__ import print_function</code>.
The project's late start at a Python 3 port can be significantly attributed
to Python 2.4 and 2.5 compatibility holding us back.</p>

<p>The main goal with Mercurial's early porting work was just getting the code base
to a point where <code>import mercurial</code> would work. There were a myriad of places
where Mercurial used syntax that was invalid on Python 3 and Python 3
couldn't even parse the source code, let alone compile it to bytecode and
execute it.</p>

<p>This effort began in earnest in
<a href="https://www.mercurial-scm.org/repo/hg/rev/e93036747902">June 2015</a>
with global source code rewrites like using modern octal syntax,
modern exception catching syntax (<code>except Exception as e</code> instead of
<code>except Exception, e</code>), <code>print()</code> instead of <code>print</code>, and a
<a href="https://www.mercurial-scm.org/repo/hg/rev/1a6a117d0b95">modern import convention</a>
along with the use of <code>from __future__ import absolute_import</code>.</p>

<p>In the early days of the port, our first goal was to get all source code
parsing as valid Python 3. The next step was to get all the modules <code>import</code>ing
cleanly. This entailed fixing code that ran at <code>import</code> time to work on
Python 3. Our thinking was that we would need the code base to be <code>import</code>
clean on Python 3 before seriously thinking about run-time behavior. In reality,
we quickly ported a lot of modules to <code>import</code> cleanly and then moved on
to higher-level porting, leaving a long-tail of modules with <code>import</code> failures.</p>

<p>This initial porting effort played out over months. There weren't many
people working on it in the early days: a few people would basically hack on
Python 3 as a form of itch scratching and most of the project's energy was
focused on improving the existing Python 2 based product. You can get a rough
idea of the timeline and participation in the early porting effort through the
<a href="https://www.mercurial-scm.org/repo/hg/log/081a77df7bc6/tests/test-check-py3-compat.t?revcount=960">history of test-check-py3-compat.t</a>.
We see the test being added in <a href="https://www.mercurial-scm.org/repo/hg/rev/40eb385f798f">December 2015</a>,
By June 2016, most of the code base was ported to our modern import convention
and we were ready to move on to more meaningful porting.</p>

<p>One of the biggest early hurdles in our porting effort was how to overcome
the string literals type mismatch between Python 2 and 3. In Python 2, a
<code>''</code> string literal is a sequence of bytes. In Python 3, a <code>''</code> string literal
is a sequence of Unicode code points. These are fundamentally different types.
And in Mercurial's code base, <strong>most of our <em>string</em> types are binary by design:
use of a Unicode based <code>str</code> for representing data is flat out wrong for our use
case</strong>. We knew that Mercurial would need to eventually switch many string
literals from <code>''</code> to <code>b''</code> to preserve type compatibility. But doing so would
be problematic.</p>

<p>In the early days of Mercurial's Python 3 port in 2015, Mercurial's project
maintainer (Matt Mackall) set a ground rule that the Python 3 port shouldn't overly
disrupt others: he wanted the Python 3 port to more or less happen in the background
and not require every developer to be aware of Python 3's low-level behavior in order
to get work done on the existing Python 2 code base. This may seem like a questionable
decision (and I probably disagreed with him to some extent at the time because I was
doing Python 3 porting work and the decision constrained this work). But it was the
correct decision. Matt knew that it would be years before the Python 3 port was either
necessary or resulted in a meaningful return on investment (the value proposition of
Python 3 has always been weak to Mercurial because Python 3 doesn't demonstrate a
compelling advantage over Python 2 for our use case). What Matt was trying to do was
minimize the externalized costs that a Python 3 port would inflict on the project.
He correctly recognized that maintaining the existing product and supporting
existing users was more important than a long-term bet in its infancy.</p>

<p>This ground rule meant that a mass insertion of <code>b''</code> prefixes everywhere
was not desirable, as that would require developers to think about whether
a type was a <code>bytes</code> or <code>str</code>, a distinction they didn't have to worry about
on Python 2 because we practically never used the Unicode-based string type in
Mercurial.</p>

<p>In addition, there were some other practical issues with doing a bulk <code>b''</code>
prefix insertion. One was that the added <code>b</code> characters would cause a lot of lines
to grow beyond our length limits and we'd have to reformat code. That would
require manual intervention and would significantly slow down porting. And
a sub-issue of adding all the <code>b</code> prefixes and reformatting code is that it would
<em>break</em> annotate/blame more than was tolerable. The latter issue was addressed
by teaching Mercurial's annotate/blame feature to <em>skip</em> revisions. The project
now has a convention of annotating commit messages with <code># skip-blame &lt;reason&gt;</code>
so structural only changes can easily be ignored when performing an
annotate/blame.</p>

<p>A stop-gap solution to the <code>b''</code> everywhere issue came in
<a href="https://www.mercurial-scm.org/repo/hg/rev/1c22400db72d">July 2016</a>, when I
introduced a custom Python module importer that rewrote source code as part
of <code>import</code> when running on Python 3. (I have
<a href="/blog/2017/03/13/from-__past__-import-bytes_literals/">previously blogged</a>
about this hack.) What this did was transparently add <code>b''</code> prefixes to all
un-prefixed string literals as well as modify how a few common functions were
called so that we wouldn't need to modify source code so things would run natively
on Python 3. The source transformer allowed us to have the benefits of progressing
in our Python 3 port without having to rewrite tens of thousands of lines of
source code. The solution was hacky. But it enabled us to make significant
progress on the Python 3 port without externalizing a lot of cost onto others.</p>

<p>I thought the source transformer would be relatively short-lived and would be
removed shortly after the project inevitably decided to go all in on Python 3.
To my surprise, others built additional transforms over the years and the source
transformer persisted all the way until
<a href="https://www.mercurial-scm.org/repo/hg/rev/d783f945a701">October 2019</a>, when
I removed it just before the first non-alpha Python 3 compatible version
of Mercurial was released.</p>

<p>A common problem Mercurial faced with making the code base dual Python 2/3 native
was dealing with standard library differences. Most of the problems stemmed
from changes between Python 2.7 and 3.5+. But there are changes within the
versions of Python 3 that we had to wallpaper over as well. In
<a href="https://www.mercurial-scm.org/repo/hg/rev/6041fb8f2da8">April 2016</a>, the
<code>mercurial.pycompat</code> module was introduced to export aliases or wrappers around
standard library functionality to abstract the differences between Python
versions. This file <a href="https://www.mercurial-scm.org/repo/hg/log/66af68d4c751/mercurial/pycompat.py?revcount=240">grew over time</a>
and <a href="https://www.mercurial-scm.org/repo/hg/file/66af68d4c751/mercurial/pycompat.py">eventually became</a>
Mercurial's version of <a href="https://six.readthedocs.io/">six</a>. To be honest, I'm
not sure if we should have used <code>six</code> from the beginning. <code>six</code> probably would
have saved some work. But we had to eventually write a lot of shims for
converting between <code>str</code> and <code>bytes</code> and would have needed to invent a
<code>pycompat</code> layer in some form anyway. So I'm not sure <code>six</code> would have saved
enough effort to justify the baggage of integrating a 3rd party package into
Mercurial. (When Mercurial accepts a 3rd party package, downstream packagers
like Debian get all hot and bothered and end up making questionable patches
to our source code. So we prefer to minimize the surface area for
problems by minimizing dependencies on 3rd party packages.)</p>

<p>Once we had a source transforming module importer and the <code>pycompat</code>
compatibility shim, we started to focus in earnest on making core
functionality actually work on Python 3. We established a convention of
annotating changesets needed for Python 3 with <code>py3</code>, so a
<a href="https://www.mercurial-scm.org/repo/hg/log?rev=desc(py3)&amp;revcount=4000">commit message search</a>
yields a lot of the history. (But it isn't a full history since not every Python 3
oriented change used this convention). We see from that history that after
the source importer landed, a lot of porting effort was spent on things
very early in the <code>hg</code> process lifetime. This included handling environment
variables, loading config files, and argument parsing. We introduced a
<a href="https://www.mercurial-scm.org/repo/hg/log/@/tests/test-check-py3-commands.t">test-check-py3-commands.t</a>
test to track the progress of <code>hg</code> commands working in Python 3. The very early
history of that file shows the various error messages changing, as underlying
early process functionality was slowly ported to work on Python 3. By
<a href="https://www.mercurial-scm.org/repo/hg/rev/2d555d753f0e">December 2016</a>, we
had <code>hg version</code> working on Python 3!</p>

<p>With basic <code>hg</code> command dispatch ported to Python 3 at the end of 2016,
2017 represented an inflection point in the Python 3 porting effort. With the
early process functionality working, different people could pick up different
commands and code paths and start making code work with Python 3. By
<a href="https://www.mercurial-scm.org/repo/hg/rev/52ee1b5ac277">March 2017</a>, basic
repository opening and <code>hg files</code> worked. Shortly thereafter,
<a href="https://www.mercurial-scm.org/repo/hg/rev/ed23f929af38">hg init started working as well</a>.
And <a href="https://www.mercurial-scm.org/repo/hg/rev/935a1b1117c7">hg status</a> and
<a href="https://www.mercurial-scm.org/repo/hg/rev/aea8ec3f7dd1">hg commit</a> did as well.</p>

<p>Within a few months, enough of Mercurial's functionality was working with Python
3 that we started to <a href="https://www.mercurial-scm.org/repo/hg/rev/7a877e569ed6">track which tests passed on Python 3</a>.
The <a href="https://www.mercurial-scm.org/repo/hg/log/@/contrib/python3-whitelist?revcount=480">evolution of this file</a>
shows a reasonable history of the porting velocity.</p>

<p>In <a href="https://www.mercurial-scm.org/repo/hg/rev/feb910d2f59b">May 2017</a>, we dropped
support for Python 2.6. This significantly reduced the complexity of supporting
Python 3, as there was tons of functionality in Python 2.7 that made it easier
to target both Python 2 and 3 and now our hands were untied to utilize it.</p>

<p>In <a href="https://www.mercurial-scm.org/repo/hg/rev/bd8875b6473c">November 2017</a>, I
landed a test harness feature to report exceptions seen during test runs. I
later <a href="https://www.mercurial-scm.org/repo/hg/rev/8de90e006c78">refined the output</a>
so the most frequent failures were reported more prominently. This feature
greatly enabled our ability to target the most common exceptions, allowing
us to write patches to fix the most prevalent issues on Python 3 and uncover
previously unknown failures.</p>

<p>By the end of 2017, we had most of the structural pieces in place to complete
the port. Essentially all that was required at that point was time and labor.
We didn't have a formal mechanism in place to target porting efforts. Instead,
people would pick up a component or test that they wanted to hack on and then
make incremental changes towards making that work. All the while, we didn't
have a strict policy on not regressing Python 3 and regressions in Python 3
porting progress were semi-frequent. Although we did tend to correct
regressions quickly. And over time, developers saw a flurry of Python 3
patches and slowly grew awareness of how to accommodate Python 3, and the
number of Python 3 regressions became less frequent.</p>

<p>As useful as the source-transforming module importer was, it incurred some
additional burden for the porting effort. The source transformer effectively
converted all un-prefixed string literals (<code>''</code>) to bytes literals (<code>b''</code>)
to preserve string type behavior with Python 2. But various aspects of Python
3 didn't like the existence of <code>bytes</code>. Various standard library functionality
now wanted unicode <code>str</code> and didn't accept <code>bytes</code>, even though the Python
2 implementation used the equivalent of <code>bytes</code>. So our <code>pycompat</code> layer
grew pretty large to accommodate calling into various standard library
functionality. Another side-effect which we didn't initially anticipate
was the <code>**kwargs</code> calling convention. Python allows you to use <code>**</code>
with a dict with string keys to turn those keys into named arguments
in a function call. But Python 3 requires these <code>dict</code> keys to be
<code>str</code> and outright rejects <code>bytes</code> keys, even if the <code>bytes</code> instance
is ASCII safe and has the same underlying byte representation of the
string data as the <code>str</code> instance would. So we had to invent support
functions that would convert <code>dict</code> keys from <code>bytes</code> to <code>str</code> for
use with <code>**kwargs</code> and another to convert a <code>**kwargs</code> dict from
<code>str</code> keys to <code>bytes</code> keys so we could use <code>''</code> syntax to access keys
in our source code! Also on the string type front, we had to sprinkle
the codebase with raw string literals (<code>r''</code>) to force the use of
<code>str</code> irregardless of which Python version you were running on (our
source transformer only changed unprefixed string literals, so existing
<code>r''</code> strings would be preserved as <code>str</code>).</p>

<p>Blind transformation of all string literals to <code>bytes</code> was less than ideal
and it did impose some unwanted side-effects. But, again, most <em>strings</em>
in Mercurial are bytes by design, so we thought it would be easier to
<em>byteify</em> all strings then selectively undo that where native strings
were actually warranted (like keys in most <code>dict</code>s) than to take the
up-front cost to examine every string and make an intelligent determination
as to what type it should be. I go back and forth as to whether this was the
correct call. But when you factor in that the source transforming
module importer unblocked Python 3 porting at a time in the project's
history when there was so much focus on improving the core product and it
did so without externalizing many costs onto the people doing the critical
core product work, I think it was the right call.</p>

<p>By mid 2019, the number of test failures in Python 3 had been whittled
down to a reasonable, less daunting number. It felt like victory was
in grasp and inevitable. But a few significant issues lingered.</p>

<p>One remaining question was around addressing differences between Python
3 versions. At the time, Python 3.5, 3.6, and 3.7 were released and 3.8
was scheduled for release by the end of the year. We had a surprising
number of issues with differences in Python 3 versions. Many of us
were running Python 3.7, so it had the fewest failures. We had to spend
extra effort to get Python 3.5 and 3.6 working as well as 3.7. Same for
3.8.</p>

<p>Another task we deferred until the second half of 2019 was standing up
robust CI for Python 3. We had some coverage, but it was minimal. Wanting
a distraction from PyOxidizer for a bit and wanting to overhaul Mercurial's
CI system (which is officially built on Buildbot), I cobbled together a
<em>serverless</em> CI system built on top of AWS DynamoDB and S3 for storage,
Lambda functions and CloudWatch events for all business logic, and EC2 spot
instances for job execution. This CI system executed Python 3.5, 3.6, 3.7,
and 3.8 variants of our test harness on Linux and Python 3.7 on Windows.
This gave developers insight into version-specific failures. More
importantly, it also gave insight into Windows failures, which was
previously not well tested. It was discovered that Python 3 on Windows was
lagging significantly behind POSIX.</p>

<p>By the time of the Mercurial developer meetup in October 2019, nearly
all tests were passing on POSIX platforms and we were confident that
we could declare Python 3 support as at least beta quality for the
Mercurial 5.2 release, planned for early November.</p>

<p>One of our blockers for ripping off the alpha label on Python 3 support
was removing our source-transforming module importer. It had performance
implications and it wasn't something we wanted to ship because it felt
too hacky. A blocker for this was we wanted to automatically format
our source tree with <a href="https://black.readthedocs.io/en/stable/">black</a>
because if we removed the source transformer, we'd have to rewrite
a lot of source code to apply changes the transformer was performing,
which would necessitate wrapping a lot of lines, which would involve a lot
of manual effort. We wanted to <em>blacken</em> our code base first so that
mass rewriting source code wouldn't involve a lot of tedious reformatting
since <code>black</code> would handle that for us automatically. And rewriting the
source tree with <code>black</code> was blocked on a specific feature landing in
<code>black</code>! (We did not agree with <code>black</code>'s behavior of
unwrapping comma-delimited lists of items if they could fit on a single
line. So one of our core contributors wrote a patch to <code>black</code> that
changed its behavior so a trailing <code>,</code> in a list of items will force
items to be formatted on multiple lines. I personally find the multiple line
formatting much easier to read. And the behavior is arguably better for
code review and <em>annotation</em>, which is line based.) Once this feature
landed in <code>black</code>, we reformatted our source tree and started ripping
out the source transformations, starting by inserting <code>b''</code> literals
everywhere. By late October, the source transformer was no more and
we were ready to release beta quality support for Python 3 (at least
on UNIX-like platforms).</p>

<p>Having described a mostly factual overview of Mercurial's port to Python
3, it is now time to shift gears to the speculative and opinionated
parts of this post. <strong>I want to underscore that the opinions reflected
here are my own and do not reflect the overall Mercurial Project or even
a consensus within it.</strong></p>

<h2>The Future of Python 3 and Mercurial</h2>

<p>Mercurial's port to Python 3 is still ongoing. While we've shipped
Python 3 support and the test harness is clean on Python 3, I view shipping
as only a milestone - arguably <em>the</em> most important one - in a longer
journey. There's still a lot of work to do.</p>

<p>It is now 2020 and Python 2 support is now officially dead from the
perspective of the Python language maintainers. Linux distributions are
starting to rip out Python 2. Packages are dropping Python 2 support in
new versions. The world is moving to Python 3 only. But <strong>Mercurial still
officially supports Python 2</strong>. And it is still yet to be determined how
long we will retain support for Python 2 in the code base. We've only had
one release supporting Python 3. Our users still need to port their
extensions (implemented in Python). Our users still need to start widely
using Mercurial with Python 3. Even our own developers need to switch to
Python 3 (old habits are hard to break).</p>

<p>I anticipate a long tail of random bugs in Mercurial on Python 3. While
the tests may pass, our code coverage is not 100%. And even if it were,
Python is a dynamic language and there are tons of invariants that aren't
caught at compile time and can only be discovered at run time. <strong>These
invariants cannot all be detected by tests, no matter how good your test
coverage is.</strong> This is a <em>feature</em>/<em>limitation</em> of dynamic languages. Our
users will likely be finding a long tail of miscellaneous bugs on Python
3 for <em>years</em>.</p>

<p>At present, our code base is littered with tons of random hacks to bridge
the gap between Python 2 and 3. Once Python 2 support is dropped, we'll
need to remove these hacks and make the source tree Python 3 native, with
minimal shims to wallpaper over differences in Python 3 versions. <strong>Removing
this Python version bridge code will likely require hundreds of commits and
will be a non-trivial effort.</strong> It's likely to be deemed a low priority (it
is glorified busy work after all), and code for the express purpose of
supporting Python 2 will likely linger for years.</p>

<p>We are also still shoring up our packaging and distribution story on
Python 3. This is easier on some platforms than others. I created
<a href="https://github.com/indygreg/PyOxidizer">PyOxidizer</a> partially because
of the poor experience I had with Python application packaging and
distribution through the Mercurial Project. The Mercurial Project has
already signed off on using PyOxidizer for distributing Mercurial in
the future. So look for an <em>oxidized</em> Mercurial distribution in the
near future! (You could argue PyOxidizer is an epic yak shave to better
support Mercurial. But that's for another post.)</p>

<p>Then there's Windows support. A Python 3 powered Mercurial on Windows
still has a handful of known issues. It may require a few more releases
before we consider Python 3 on Windows to be stable.</p>

<p>Because we're still on a code base that must support Python 2, our
adoption of Python 3 features is very limited. The only Python 3
feature that Mercurial developers seem to almost universally get excited
about is type annotations. We already have some people playing around
with <code>pytype</code> using comment-based annotations and <code>pytype</code> has already
caught a few bugs. We're eager to go all in on type annotations and
uncover lots of dynamic typing bugs and poorly implemented APIs.
Beyond type annotations, I can't name any feature that people are screaming
to adopt and which makes a lot of sense for Mercurial. There's a long
tail of minor features I'm sure will get utilized. But none of the
marquee features that define major language releases seem that interesting
to us. Time will tell.</p>

<h2>Commentary on Python 3</h2>

<p>Having described Mercurial's ongoing journey to Python 3, I now want to
focus more on Python itself. Again, the opinions here are my own and
don't reflect those of the Mercurial Project.</p>

<p><strong>Succinctly, my experience porting Mercurial and other projects to
Python 3 has significantly soured my perceptions of Python. As much as
I have historically loved Python - from the language to the welcoming
community - I am still struggling to understand how Python could manage
to inflict so much hardship on the community by choosing the transition
plan that they did.</strong> I believe Python's choices represent a terrific
example of what not to do when managing a large project or ecosystem.
Maintainers of other largely-deployed systems would benefit from taking
the time to understand and reflect on Python's missteps.</p>

<p>Python 3.0 was released on December 3, 2008. And it took the better part of
a decade for the community to embrace it. <strong>This should be universally
recognized as a failure.</strong> While hindsight is 20/20, many of the issues
with Python 3 were obvious at the time and could have been mitigated had
the language maintainers been more accommodating - and dare I say
empathetic - to its users.</p>

<p>Initially, Python 3 had a rather cavalier attitude towards backwards and
forwards compatibility. In the early years of Python 3, the attitude of
Python's maintainers was <em>Python 3 is a new, better language: you should
target it explicitly</em>. There were some tools and methods to ease the
transition. But nothing super polished, especially in the early years.
Adoption of Python 3 in the overall community was slow. Python developers
in the wild justifiably complained that the value proposition of Python 3
was too weak to justify porting effort. Not helping was that the early
advice for targeting Python 3 was to rewrite the source code to become
Python 3 native. This is in contrast with using the same source to run on both
Python 2 and 3. For library and application maintainers, this potentially
meant maintaining separate versions of your code or forcing end-users to
make a giant leap, which would realistically orphan users on an old version,
fragmenting your user base. Neither of those were great alternatives, so
you can understand why many projects didn't bite.</p>

<p>For many projects of non-trivial size, flag day transitions from Python 2 to
3 were simply not viable: the pathway to Python 3 was to make code dual
Python 2/3 compatible and gradually switch over the runtime to Python 3.
But initial versions of Python 3 made this effectively impossible! Let me
give a few specific examples.</p>

<p>In Python 2, a string literal <code>''</code> is effectively an array of bytes. In
Python 3, it is a series of Unicode code points - a fundamentally different
type! In Python 2, you could write <code>b''</code> to be explicit that a string literal
was bytes or you could write <code>u''</code> to indicate a Unicode literal, mimicking
Python 3's behavior. In Python 3, you could write <code>b''</code> to create a <code>bytes</code>
instance. But for whatever reason, Python 3 initially removed the <code>u''</code> syntax,
meaning there wasn't as easy way to explicitly denote the type of each
string literal so that it was consistent between Python 2 and 3! Python 3.3
(released September 2012) restored <code>u''</code> support, making it more viable to
write Python source code that worked on both Python 2 and 3. <strong>For nearly 4
years, Python 3 took away the consistent syntax for denoting bytes/Unicode
string literals.</strong></p>

<p>Another feature was <code>%</code> formatting of strings. Python 2 allowed use of the
<code>%</code> formatting operator on both its string types. But Python 3 initially
removed the implementation of <code>%</code> from <code>bytes</code>. Why, I have no clue. It
is perfectly reasonable to splice byte sequences into a buffer via use of
a formatting string. But the Python language maintainers insisted otherwise.
And it wasn't until the community complained about its absence loudly enough
that this feature was
<a href="https://docs.python.org/3/whatsnew/3.5.html#whatsnew-pep-461">restored in Python 3.5</a>,
which was released in September 2015. Fun fact: the lack of this feature was
once considered a blocker for Mercurial moving to Python 3 because
Mercurial uses <code>bytes</code> almost universally, which meant that nearly every use
of <code>%</code> would have to be changed to something else. And to this day, Python
3's <code>bytes</code> still doesn't have a <code>format()</code> method, so the alternative was
effectively string concatenation, which is a massive step backwards from the
expressiveness of <code>%</code> formatting.</p>

<p><strong>The initial approach of Python 3 mirrors a folly that many developers
and projects make: attempting a rewrite instead of performing incremental
evolution.</strong> For established projects, large scale rewrites often go poorly.
And Python 3 is no exception. Yes, from a code level, CPython (and likely
other Python implementations) were incremental changes over Python 2 using
the same code base. But from a language and standard library level, the
differences in Python 3 were significant enough that I - and even Python's
core maintainers - considered it a new language, and therefore a rewrite.
When your random project attempts a rewrite and fails, the blast radius of that is
often contained to that project. Maybe you don't publish a new release
as soon as you otherwise would. <strong>But when you are powering an ecosystem,
the ripple effects from a failed rewrite percolate throughout that ecosystem
and last for years and have many second order effects. We see this with
Python 3, where poor choices made in the late 2000s are inflicting significant
hardship still in 2020.</strong></p>

<p>From the initial restrained adoption of Python 3, it is obvious that the
Python ecosystem overwhelmingly rejected the initial boil the oceans approach
of Python 3. Python's maintainers eventually got the message and started
restoring features like <code>u''</code> and <code>bytes</code> <code>%</code> formatting back into the
language to placate the community. All the while Python 3 had been accumulating
new features and the cumulative sum of those features was compelling enough
to win over users.</p>

<p>For many projects (including Mercurial), Python 3.4/3.5 was the first viable
porting target for Python 3. Python 3.5 was released in September 2015, almost
7 years after Python 3.0 was released in December 2008. <strong>Seven. Years.</strong>
An ecosystem that falters for that long is generally not healthy. What may have
saved Python from total collapse here is that Python 2 was still going strong and
people were generally happy with it. I really do think Python dodged a bullet
here, because there was a massive window where the language could have
hemorrhaged a critical amount of its user base and been relegated to an
afterthought. One could draw an analogy to Perl, which lost out to PHP,
Python, and Ruby, and whose fall from grace aligned with a lengthy
transition from Perl 5 to 6.</p>

<p>If you look back at the early history of Python 3, <strong>I think you are forced
to conclude that Python effectively kneecapped itself for 5-7 years
through questionable implementation choices that prevented users from
incurring incremental transitions between the major language versions. 2008
to 2013-2015 should be known as the <em>lost years of Python</em> because so much
opportunity and energy was squandered.</strong> Yes, Python is still healthy today
and Python 3 is (finally) being adopted at scale. But had earlier versions
of Python 3 been more <em>empathetic</em> towards Python 2 users porting to it,
Python and Python 3 in 2020 would be even stronger than it is. The community
was artificially hindered for years. And we won't know until 2023-2025 what
things could have looked like in 2020 had the Python core language team
spent more time paving a smoother road between the major language versions.</p>

<p>To be clear, I do think Python 3 is generally a better language than Python 2.
It has fewer warts, more compelling features, and better performance (except
for startup time, which is still slower than Python 2). I am ecstatic the
community is finally rallying around Python 3! For my Python coding, it has
reached the point where I curse under my breath when I need to support
Python 2 or even older versions of Python 3, like 3.5 or 3.6: I just wish
the world would move on and adopt the future already!</p>

<p>But I would be remiss if I failed to mention some of my gripes with Python
3 beyond the transition shenanigans.</p>

<p>Perhaps my least favorite <em>feature</em> of Python 3 is its insistence that the
world is Unicode. In Python 2, the default string type was backed by
bytes. In Python 3, the default string type is backed by Unicode code
points. As part of that transition, large parts of the standard library
now operate in the Unicode space instead of the domain of bytes. I understand
why Python does this: they want <em>strings</em> to be Unicode and don't want
users to have to spend that much energy thinking about when to use
<code>str</code> versus <code>bytes</code>. This approach is admirable and somewhat defensible
because it takes a stand on a solution that is arguably <em>good enough</em> for
most users. However, <strong>the approach of assuming the world is Unicode is
flat out wrong and has significant implications for systems level
applications</strong> (like version control tools).</p>

<p>There are a myriad of places in Python's standard library where Python
insists on using the Unicode-backed <code>str</code> type and rejects <code>bytes</code>. For
example, various networking modules refuse to accept <code>bytes</code> for hostnames
or URLs. HTTP libraries won't accept <code>bytes</code> for HTTP header names or values.
Functions that are proxies to POSIX-defined functions won't accept <code>bytes</code>
even though the POSIX function it calls into is using <code>char *</code> and isn't
Unicode aware. Then there's filename handling, where Python assumes the
existence of a global encoding for filenames and uses this encoding to convert
between <code>str</code> and <code>bytes</code>. And it does this despite POSIX filesystem paths
being a bag of bytes where the only rules are that <code>\0</code> terminates the
filename and <code>/</code> is special.</p>

<p>In cases like Python refusing to accept <code>bytes</code> for things like HTTP
header names (which will just be spit out over the wire as bytes), Python's
pendulum has swung too far towards Unicode only. In my opinion, Python needs
to be more accommodating and allow <code>bytes</code> when it makes sense. I hope the
pendulum knocks some sense into people when it swings back towards a more
reasonable solution that better acknowledges the realities of the world we
live in.</p>

<p>For areas like filename handling, the world is more complicated. Python
is effectively an abstraction layer over the operating system APIs exposing
this functionality. And there is often an impedance mismatch between operating
systems. For example, POSIX (Linux) tends to use <code>char *</code> for everything
and doesn't care about encoding and Windows tends to use 16 bit character
types where the encoding is... a can of worms.</p>

<p><strong>The reality here is that it is impossible to abstract over differences
between operating system behavior without compromises that can result in data
loss, outright wrong behavior, or loss of functionality. But Python 3 attempts
to do it anyway, making Python 3 unsuitable (or at least highly undesirable) for
certain systems level applications that rely on it</strong> (like a version control
tool).</p>

<p>In fairness to Python, it isn't the only programming language that gets
this wrong. The only language I've seen <em>properly</em> implement higher-order
abstractions on top of operating system facilities is Rust, whose approach can
be generalized as <em>use Python 3's solution of normalizing to Unicode/UTF-8 by
default</em>, but expose <em>escape hatches</em> which allow access to the raw underlying
types and APIs used by the operating system for the advanced consumers who
require it. For example, Rust's <code>Path</code> type which represents a filesystem path
<a href="https://doc.rust-lang.org/std/path/struct.Path.html#method.as_os_str">allows access</a>
to the raw <a href="https://doc.rust-lang.org/std/ffi/struct.OsStr.html">OsStr</a> value
used by the operating system, not a normalization of it to bytes or Unicode,
which may be lossy. This allows consumers to e.g. create and retrieve
OS-native filesystem paths without data loss. This functionality is critical
in some domains. Python 3's awareness/insistence that the world is
Unicode (which it isn't universally) reduces Python's applicability in these
domains.</p>

<p>Speaking of Rust, at the Mercurial developer meetup in October 2019, we were
discussing the use of Rust in Mercurial and one of the core maintainers blurted
out something along the lines of <em>if Rust were at its current state 5 years ago,
Mercurial would have likely ported from Python 2 to Rust instead of Python 3</em>.
As crazy as it initially sounded, I think I agree with that assessment. With the
benefit of hindsight, having been a key player in the Python 3 porting effort,
seeing all the complications and headaches Python 3 is introducing, and
having learned Rust and witnessed its benefits for performance, control,
and correctness firsthand, porting to Rust would likely have been the correct
move for the project at that point in time. 2020 is not 2014, however, and I'm
not sure if I would opt for a rewrite in Rust today. (Most rewrites are follies
after all.) But I know one thing: I certainly wouldn't implement a new version
control tool in Python 3 and I would probably choose Rust as an implementation
language for most new projects in the systems level space or with an expected
shelf life of 10+ years. (I really should blog about how awesome Rust is.)</p>

<p>Back to the topic of Python itself, <strong>I'm really soured on Python at this
point in time. The effort required to port to Python 3 was staggering. For
Mercurial, Python 3 introduces a ton of problems and doesn't really solve
many. We effectively sludged through mud for several years only to wind
up in a state that feels strictly worse than where we started. I'm sure it will
be strictly better in a few years. But at that point, we're talking about a
5+ year transition. To call the Python 3 transition disruptive and
distracting for the project would be an understatement. As a project maintainer,
it's natural to ask what we could have accomplished if we weren't forced
to carry out this sideshow.</strong></p>

<p>I can't shake the feeling that a lot of the pain afflicted by the Python 3
transition could have been avoided had Python's language leadership made
a different set of decisions and more highly prioritized the transition
experience. (Like not initially removing features like <code>u''</code> and <code>bytes %</code>
and not introducing gratuitous backwards compatibility breaks, like with
<code>items()/iteritems()</code>. I would have also liked to see a feature like
<code>from __future__</code> - maybe <code>from __past__</code> - that would make it easier for
Python 3 code to target semantics in earlier versions in order to provide
a more turnkey on-ramp onto new versions.) I simultaneously see Python 3
losing its position as a justifiable tool in some domains (like systems
level tooling) due to ongoing design decisions and poor implementation (like
startup overhead problems). (In contrast, I see Rust excelling where Python
is faltering and find Rust code surprisingly expressive to write and maintain
given how low-level it is and therefore feel that Rust is a compelling
alternative to Python in a surprisingly large number of domains.)</p>

<p>Look, I know it is easy for me to armchair quarterback and critique with the
benefit of hindsight/ignorance. I'm sure there is a lot of nuance here. I'm
sure there was disagreement within the Python community over a lot of these
issues. Maintaining a large and successful programming language and community
like Python's is hard and you aren't going to please all the people all the
time. And speaking as a maintainer, I have mad respect for the people leading
such a large community. But niceties aside, everyone knows the Python 3
transition was rough and could have gone better. It should not have taken 11
years to get to where we are today.</p>

<p><strong>I'd like to encourage the Python Project to conduct a thorough postmortem on
the transition to Python 3.</strong> Identify what went well, what could have gone
better, and what should be done next time such a large language change is wanted.
Speaking as a Python user, a maintainer of a Python project, and as someone in
industry who is now skeptical about use of Python at work due to risks of
potentially company crippling high-effort migrations in the future, a postmortem
would help restore my confidence that Python's maintainers learned from the
various missteps on the road to Python 3 and these potentially ecosystem
crippling mistakes won't be made again.</p>

<p>Python had a wildly successful past few decades. And it can continue to
thrive for several more. But the Python 3 migration was painful for all
involved. And as much as we need to move on and leave Python 2 behind us,
there are some important lessons to be learned. I hope the Python community
takes the opportunity to reflect and am confident it will grow stronger by
taking the time to do so.</p>
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