Saturday, May 18, 2019

Scott Shawcroft: History of CircuitPython

Scott Shawcroft is a freelance software engineer working full time for Adafruit, an open source hardware company that manufactures electronics that are easy to assemble and program. Shawcroft leads development of CircuitPython, a Python interpreter for small devices.

The presentation began with a demo of Adafruit’s Circuit Playground Express, a two-inch-wide circular board with a microcontroller, ten RGB lights, a USB port, and other components. Shawcroft connected the board to his laptop with a USB cable and it appeared as a regular USB drive with a source file called He edited the source file on his laptop to dim the brightness of the board’s lights. When he saved the file, the board automatically reloaded the code and the lights dimmed. “So that's super quick,” said Shawcroft. “I just did the demo in three minutes.”

Read more 2019 Python Language Summit coverage.

CircuitPython Is Optimized For Learning Electronics

The history of CircuitPython begins with MicroPython, a Python interpreter written from scratch for embedded systems by Damien George starting in 2013. Three years later, Adafruit hired Shawcroft to port MicroPython to the SAMD21 chip they use on many of their boards. Shawcroft’s top priority was serial and USB support for Adafruit’s boards, and then to implement communication with a variety of sensors. “The more hardware you can support externally,” he said, “the more projects people can build.”

As Shawcroft worked with MicroPython’s hardware APIs, he found them ill-fitting for Adafruit’s goals. MicroPython customizes its hardware APIs for each chip family to provide speed and flexibility for hardware experts. Adafruit’s audience, however, is first-time coders. Shawcroft said, “Our goal is to focus on the first five minutes someone has ever coded.”

To build a Python for Adafruit’s needs, Shawcroft forked MicroPython and created a new project, CircuitPython. In his Language Summit talk, he emphasized it is a “friendly fork”: both projects are MIT-licensed and share improvements in both directions. In contrast to MicroPython’s hardware APIs that vary by chip, CircuitPython has one hardware API, allowing Adafruit to write one set of libraries for them all.

MicroPython has a distinct standard library that differs from CPython’s: for example, its time functions are in a module named utime with a different feature set from the standard time module. It also ships modules with features not found in CPython’s standard library, such as advanced filesystem management features. In CircuitPython, Shawcroft removed the nonstandard features and modules. This change helps new coders ramp smoothly from CircuitPython on a microcontroller to CPython on a full-size computer, and it makes Adafruit’s libraries reusable on CPython itself.

Another motive for forking was to create a separate community for CircuitPython. In the original MicroPython project’s community, Shawcroft said, “There are great folks, and there's some not-so-great folks.” The CircuitPython community welcomes beginners, publishes documentation suitable for them, and maintains standards of conduct that are safe for minors.

Audience members were curious about CircuitPython’s support for Python 3.8 and beyond. When Damien George began MicroPython he targeted Python 3.4 compliance, which CircuitPython inherits. Shawcroft said that MicroPython has added some newer Python features, and decisions about more language features rest with Damien George.

Minimal Barrier To Entry

Photo courtesy of Adafruit.

Shawcroft aims to remove all roadblocks for beginners to be productive with CircuitPython. As he demonstrated, CircuitPython auto-reloads and runs code when the user saves it; there are two more user experience improvements in the latest release. First, serial output is shown on a connected display, so a program like print("hello world") will have visible output even before the coder learns how to control LEDs or other observable effects.

Second, error messages are now translated into nine languages, and Shawcroft encourages anyone with language skills to contribute more. Guido van Rossum and A. Jesse Jiryu Davis were excited to see these translations and suggested contributing them to CPython. Shawcroft noted that the existing translations are MIT-licensed and can be ported; however, the translations do not cover all the messages yet, and CircuitPython cannot show messages in non-Latin characters such as Chinese. Chinese fonts are several megabytes of characters, so the size alone presents an unsolved problem.

Later this year, Shawcroft will add Bluetooth support for coders to connect their phone or tablet to an Adafruit board and enjoy the same quick edit-refresh cycle there. Touchscreens will require a different sort of code editor, perhaps more like EduBlocks. Despite the challenges, Shawcroft echoed Russell Keith-Magee’s insistence on the value of mobile platforms: “My nieces, they have tablets and phones. They do not have laptops.”

Shawcroft’s sole request for the core developers was to keep new language features simple, with few special cases. First, because each new CPython feature must be reimplemented in MicroPython and CircuitPython, and special cases make this work thorny. Second, because complex logic translates into large code size, and the space for code on microcontrollers is minuscule.

Amber Brown: Batteries Included, But They're Leaking

Amber Brown of the Twisted project shared her criticisms of the Python standard library. This proved to be the day’s most controversial talk; Guido van Rossum stormed from the room during Q & A.

Read more 2019 Python Language Summit coverage.

Applications Need More Than The Standard Library

Python claims to ship with batteries included, but according to Brown, without external packages it is only “marginally useful.” For example, asyncio requires external libraries to connect to a database or to speak HTTP. Brown asserted that there were many such dependencies from the standard library to PyPI: typing works best with mypy, the ssl module requires a monkeypatch to connect to non-ASCII domain names, datetime needs pytz, and six is non-optional for writing code for Python 2 and 3.

Other standard library modules are simply inferior to alternatives on PyPI. The http.client documentation advises readers to use Requests, and the datetime module is confusing compared to its competitors such as arrow, dateutil, and moment.

Poor Quality, Lagging Features, And Obsolete Code

“Python's batteries are leaking,” said Brown. She thinks that some bugs in the standard library will never be fixed. And even when bugs are fixed, PyPI libraries like Twisted cannot assume they run on the latest Python, so they must preserve their bug workarounds forever.

There are many modules that few applications use, but there is no method to install a subset of the standard library. Brown called out the XML parser and tkinter in particular for making the standard library larger and harder to build, burdening all programmers for the sake of a few. As Russell Keith-Magee had described earlier in the day, the size of the standard library makes it difficult for PyBee to run Python on constrained devices. Brown also noted that some standard library modules were optimized in C for Python 3, but had to be reimplemented in pure Python for PyPy to support them.

Brown identified new standard library features that were “too little, too late,” leaving users to depend on backports to use those features in Python 2. For example, socket.sendmsg was added only recently, meaning Twisted must ship its own C extension to use sendmsg in Python 2. Although Python 2 is nearly at its end of life, this only holds for the core developers, according to Brown, and for users, Red Hat and other distributors will keep Python 2 alive “until the goddam end of time.” Brown also mentioned that some itertools code is shown as examples in the documentation instead of shipped as functions in the itertools module.

Guido van Rossum, sitting at the back of the room, interrupted at this moment, “Can you keep to one topic? I'm sorry but this is just one long winding rant. What is your point?” Brown responded that her point was that there are a multitude of problems in the standard library.

Standard Library Modules Crowd Out Innovation

Brown’s most controversial opinion, in her own estimation, is that adding modules to the standard library stifles innovation, by discouraging programmers from using or contributing to competing PyPI packages. Ever since asyncio was announced she has had to explain why Twisted is still worthwhile, and now that data classes are in the standard library Hynek Schlawack must defend his attrs package. Even as standard library modules crowd out other projects, they lag behind them. According to Brown, “the standard library is where code sometimes goes to die,” because it is difficult and slow to contribute code there. She acknowledged recent improvements, from Mariatta Wijaya’s efforts in particular, but Python is still harder to contribute to than PyPI packages.

“So I know a lot of this is essentially a rant,” she concluded, “but it's fully intended to be.”


Nick Coghlan interpreted Brown’s proposal as generalizing the “ensurepip” model to ensure some packages are always available but can be upgraded separately from the standard library, and he thought this was reasonable.

Van Rossum was less convinced. He asked again, “Amber, what is your point?” Brown said her point was to move asyncio to PyPI, along with most new feature development. “We should embrace PyPI,” she exhorted. Some ecosystems such as Javascript rely too much on packages, she conceded, but there are others like Rust that have small standard libraries and high-quality package repositories. She thinks that Python should move farther in that direction.

Van Rossum argued instead that if the Twisted team wants the ecosystem to evolve, they should stop supporting older Python versions and force users to upgrade. Brown acknowledged this point, but said half of Twisted users are still on Python 2 and it is difficult to abandon them. The debate at this point became personal for Van Rossum, and he left angrily.

Nathaniel Smith commented, “I'm noticing some tension here.” He guessed that Brown and the core team were talking past each other because the core team had different concerns from other Python programmers. Brown went further adding that because few Python core developers are also major library maintainers, library authors’ complaints are devalued or ignored.

The remaining core developers continued the technical discussion. Barry Warsaw said that the core team had discussed deprecating modules in the standard library, or creating slim distributions with a subset of it, but that it required a careful design. Others objected that slimming down the standard library risked breaking downstream code, or making work for programmers in enterprises that trust the standard library but not PyPI.

Pablo Galindo Salgado was concerned that moving modules from the standard library to PyPI would create an explosion of configurations to test, but in Brown’s opinion, “We are already living that life.” Some Linux and Python distributions have selectively backported features and fixes, leading to a much more complex set of configurations than the core team realizes.

Wednesday, May 15, 2019

Paul Ganssle: Time Zones In The Standard Library

Python boasts that it comes with “batteries included,” but programmers have long been frustrated at one set of missing batteries: the standard library does not include any time zone definitions. The datetime module supports the idea of time zones, but a programmer who wants to know when Daylight Saving Time starts in Cleveland must install a third-party package. Paul Ganssle spoke to the Python Language Summit to offer a solution. Ganssle maintains the PyPI package dateutil, and contributes to the standard library datetime module. He described the state of Python time zone support and how time zone definitions could be added to the standard library.

Read more 2019 Python Language Summit coverage.

Python Comes With Limited Time Zone Support

A time zone is a function that maps a naïve time to an unambiguous Coordinated Universal Time (UTC). Individual time zones can be quite eccentric, so Python does not attempt to define time zone logic, it simply provides an abstract base class TZInfo that is subclassed by implementors. Although there could theoretically be unlimited kinds of time zones, most programmers encounter three concrete types:

1. UTC or a fixed offset from it.

2. Local time.

3. A time zone from the IANA database.

The first of these was added to the standard library in Python 3.2. Ganssle said, “Whenever I teach people about datetimes, it's really nice to be able to say, if you're using Python 3, you can just have a UTC object.” The purpose of Ganssle’s proposal was to add the second and third.

Ambiguous Times

Ganssle explained that when Eastern Daylight Time ends, clocks are set back from 2:00am to 1:00am, thus there are two UTC times that map to 1:30am local time on that day:

>>> NYC = tz.gettz("America/New_York")
>>> dt0 = datetime(2004, 10, 31, 5, 30, tzinfo=tz.UTC)
>>> print(dt0.astimezone(NYC))
2004-10-31 01:30:00-04:00
>>> print((dt0 + timedelta(hours=1)).astimezone(NYC))
2004-10-31 01:30:00-05:00

PEP 495 solved the problem of ambiguous times by adding the “fold” attribute to datetime objects. A datetime with fold=0 is the first occurrence of that local time, the second occurrence has fold=1. With this addition, standard Python provides all the prerequisites for proper time zones, so Ganssle argued they should now be added to the standard library.

How To Maintain The Time Zone Definitions?

IANA time zones are the de facto standard for time zone data, and they ship with many operating systems. Both Ganssle’s dateutil and the competing pytz package use the IANA database as their source of truth. Therefore it would be natural to include the IANA time zones in the Python standard library, but this presents a problem: the IANA database changes every time a government changes a time zone, which occurs as often as 20 times a year. Time zone changes are far more frequent than Python releases.

Ganssle offered two solutions for updating time zone data, and then offered a compromise between them as his actual proposal. The first solution is to rely on the operating system’s time zone database. Python could rely on the system update mechanism to refresh this data, and it would use the same time zone definitions as most other applications. System time zone data is not officially supported on Windows, however, and is not always installed on Linux.

The second solution is to publish IANA time zone definitions as a PyPI package. It could be updated frequently, but the core team would have to invent some way to notify users when it is time to update their time zone data. Plus, it would be risky for Python to use different time zones than the rest of the system.

Ganssle proposed a hybrid: the Python standard library should use the system’s time zone data if possible, otherwise fall back to a PyPI package which would be installed conveniently, analogous to installing pip with “ensurepip” today.

The Local Time Zone

Naïve times in Python are sometimes treated as times in the local time zone, sometimes not. Ganssle showed an example demonstrating that if a programmer converts a naïve time to UTC, Python assumes its original time zone is local:

>>> dt = datetime(2020, 1, 1, 12)
>>> dt.astimezone(timezone.utc)
2020-01-01 17:00:00+00:00

However, adding a naïve time to a UTC time is prohibited:

>>> datetime(2020, 1, 1, 12) - datetime(2020, 1, 1, tzinfo=timezone.utc)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't subtract offset-naive and offset-aware datetimes

Ganssle’s dateutil package offers a more thorough implementation of “local time zone”, and he thinks Python programmers would appreciate local times in the standard library. To add them, however, the core team must first handle the astonishing behavior of local times when the system time zone changes. The first surprise is that changing the system time zone has no effect until the Python program calls time.tzset(). (And on Windows, time.tzset() is not available.) The second surprise is that changing system time and then calling time.tzset() changes the UTC offset of existing times created before the change.

Ganssle proposed several ways the standard library could act in this scenario. It could ignore changes to the system time zone while a Python program is running, or it could detect time zone changes but avoid mutating the offsets of existing time objects. He had no opinion about the best outcome.


Ned Deily wondered what Ganssle’s proposal would solve that which pytz does not. Ganssle responded that pytz’s author has stopped maintaining the package because he believes time zones should move to the standard library. Full time zone is a basic feature that should always be available. In Ganssle’s view, however, his own dateutil is a better package to emulate than pytz. “I would take dateutil, clean up some of the rough edges, and propose it as some of the batteries that would be included.”

Łukasz Langa said that he planned, as Python 3.8’s release manager, to issue monthly patch releases, and he thought that should be frequent enough to keep users’ time zone data updated. Russell Keith-Magee said no, North Korea once announced a time zone change with three days’ notice. Other audience members thought this scenario was obscure, and the PEP should not be required to handle such emergencies.

At the end of his talk Ganssle summarized his proposal. He believes that the standard library should support IANA time zones, using the operating system as the source of time zone data or falling back to a PyPI package. There are several options for handling local time zone changes at runtime. The design should be formalized in at least an informational PEP, “if not one where it's contentious and we all hate each other at the end of it.”

Russell Keith-Magee: Python On Other Platforms

Russell Keith-Magee spoke in his capacity as the founder and “Benevolent Dictator For Now” of the BeeWare Project. The project’s slogan is “Write once. Deploy everywhere.” The goal of the BeeWare Project is to run Python applications on Android, iOS, in the browser, even on smart watches, and to distribute Python applications using platform-specific channels like app stores. Keith-Magee described a number of obstacles to this goal, and expressed his hope that the core team would consider these problems when they plan Python’s future.

Read more 2019 Python Language Summit coverage.

Cross-Compilation Is Supported And Tested Poorly

On the server side, x86-64 dominates, but mobile architectures are varied and rapidly changing. iOS alone has encompassed six architectures in recent memory: i386, x86-64, arm6, arm7, arm7s and arm64.

To make matters worse, anyone deploying Python to these architectures must cross-compile, using a desktop or server machine to compile Python and its C extensions, but designating a mobile architecture as the target. “The good news,” said Keith-Magee, is that CPython uses Autoconf, and “Autoconf has really good support for cross compilation baked in. The bad news is that CPython doesn't so much.” Cross compilation is not tested continuously, and so it is broken occasionally in CPython’s build configuration. There is a vendored copy of libffi in the Python source tree, apparently only for supporting the ctypes module on PowerPC, which makes compilation for iOS even more difficult.

Distutils and pip present a tougher challenge. They do not support cross-compilation at all, and assume that the machine on which they run is the target architecture of the C extensions they compile. Neither do they support the “fat binary” format required to support multiple iOS architectures in a single extension module.

OS Differences Break The Test Suite

Python’s assumptions about system calls are violated, in a variety of ways, on mobile and web. The most dangerous pitfalls are fork and spawn — they are provided on iOS but any program that calls them will hang. Large amounts of code in the Python standard library assume the existence of fork and spawn, and even more in the test suite.

Some years ago Keith-Magee provided a patch to fix or disable tests as needed for iOS, but as he recalled, core developer Ned Deily found them “too invasive.” Instead of changing the test suite so pervasively, a new approach might be to skip large chunks of the test suite based on platform limitations.

Python Applications Must Include a Python Installation

Each Python application on a phone or table OS must ship an entire Python installation with it, since Python is not available as a system library, and applications cannot install it in a shared location. “If you've got ten Python apps on your phone,” says Keith-Magee, “you are going to have ten installs of Python.” Python for iOS is a fat binary that supports multiple architectures, weighing around 100 MB in total. Python’s size handicaps browser applications, too, which cannot begin running until the Python runtime has been loaded.

The Javascript community uses “tree shaking”: Javascript applications are distributed with the minimum subset of all their dependencies that are actually required. Keith-Magee proposes a tool that would automatically apply this technique to the Python standard library. Python builtins or portions of the interpreter itself could be jettisoned, too. For example, if an application is distributed as bytecode, then the parser and compiler could be removed. Keith-Magee said, “While I'm sure that someone has written code that uses Python's complex number data type, I am not that person, and in all my apps, the code for complex number handling is dead code.” To write a tool that would remove such components, however, would require some public API for editing the runtime and standard library.

asyncio Does Not Integrate With Mobile GUI Events

Mobile applications are GUIs, and GUI programming is event-based. Python’s asyncio is a natural framework for GUI programming, but it must be adapted to run atop the GUI environment’s own event loop. According to Keith-Magee this is relatively easy on Unix platforms like Linux, macOS, and iOS, with poll-based APIs. He estimates it requires a few hundred lines of code. On Android, Windows, and the web, however, the event model is entirely different, and asyncio integration has been an open bug for years. Keith-Magee said he does not know asyncio internals well enough to propose a solution but welcomes any suggestions.

Only CPython Can Use C Standard Library Modules

Keith-Magee has been experimenting with alternatives to CPython for mobile and web. Replacing the core language implementation is “relatively achievable,” he said, “but the standard library’s another thing.” The C modules in the standard library are only compatible with CPython. Some, like the decimal module, have recently been ported from Python to C, which accelerates them but presents more hindrances for non-CPython interpreters.

Keith-Magee asked the core developers to maintain pure-Python implementations of all standard library modules, for the sake of alternative interpreters. But recognizing how burdensome that would be, he requested as a fallback that the C interface for each extension module be clearly defined. Cannot Build App Store Bundles

An application packaged for a mobile app store requires various metadata, such as a bundle identifier, that is not currently expressible in Keith-Magee described his confusion about how to proceed, given his confusion about the direction of Python packaging in general. Should the metadata be in the pyproject.toml file specified by PEP 518? Should he adapt pip or use a custom build tool? He felt that if the core team has any clear vision for the future of Python packaging, “I can tell you from down in the trenches that message isn't getting through.” He requested more information and more opinions about packaging from the core team.

Wish List

The talk concluded with a wish list for adapting Python to mobile and web:

1. Host/target separation testing in CI

2. Host/target separation in distutils/pip

3. Feature gating (especially in the test suite)

4. Unvendoring libffi for macOS

5. Tree (and/or root) shaking

6. AsyncIO support for other eventing styles

7. Reference implementation of modules (or a clear native interface)

8. Clearer communications on packaging

Keith-Magee claims that Python faces an “existential risk” if mobile and web support does not improve. His son, who uses only an iPad for school, asked him, “When can I learn to program like you?” Unless students like him can program Python on their devices, Python risks being left behind by the next generation of programmers.

The 2019 Python Language Summit

The Python Language Summit is a small gathering of Python language implementers, both the core developers of CPython and alternative Pythons, held on the first day of PyCon. The summit features short presentations from Python developers and community members, followed by longer discussions. The 2019 summit is the first held since Guido van Rossum stepped down as Benevolent Dictator for Life, replaced by a five-member Steering Council. covered the proceedings from 2015 to 2018; this year the PSF has chosen to feature summit coverage on its own blog, written by A. Jesse Jiryu Davis.

Individual writeups will be linked from this page as they are posted.


Lightning Talks, Round 1 (pre-selected lightning talks)

Writing Stdlib C Extension Modules in Python? Jukka Lehtosalo and Michael Sullivan

Async REPL and Async-Exec, Mattias Bussonnier

The Night’s Watch Is Fixing the CIs in the Darkness for You, Pablo Galindo Salgado

Asyncio and the Case for Re-Entrancy, Jason Fried

Optimising CPython, or Not, Mark Shannon

The Night’s Watch Is Fixing the CIs in the Darkness for You, Pablo Galindo Salgado

Black Under, Łukasz Langa

Half-Hour Sessions

Python on Other Platforms, Russell Keith-Magee

History of CircuitPython, Scott Shawcroft

Dynamic Extension Module Objects, Petr Viktorin

PEP 581 / PEP 588, Mariatta Wijaya

Feedback on Mentoring Python Contributors, Victor Stinner

Lightning Talks, Round 2 (on-site signup)

SSL Module Updates, Christian Heimes

Let’s Argue About Clinic, Larry Hastings

The C-API, Eric Snow

Python in the Windows Store, Steve Dower

Bors: How the Rust Team Avoids Pablo’s Problems, Nathaniel Smith

Mypyc for Stdlib: Extended Discussion, Michael Sullivan

Python Core Sprints at Bloomberg in September, Pablo Galindo

Status of the Stable ABI, Victor Stinner

Cognitive Encapsulation: Anchor of Working Together, Yarko Tymciurak

Thanks to Bloomberg Engineering for sponsoring the summit.

Wednesday, May 01, 2019

Building the PSF: the Q2 2019 Fundraiser

Thank you to everyone who has donated to our past fundraisers! Donations, memberships, and sponsorships support sprints, meetups, community events, Python documentation, fiscal sponsorships, software development, and community projects.

We can’t do any of this without your financial contributions. We’ve just launched a new fundraiser for 2019! Please donate today and help us meet our goal of $60,000 by May 22nd!

Your donations have IMPACT!

  • The PSF awarded $118,543 in financial aid to 143 PyCon attendees in 2018
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We understand the need for transparency and hope to help our community and stakeholders find necessary information about the PSF in a single place. We’re proud to launch our first ever Annual Report. 2018 was a year of growth for the PSF while still focussing on sustainability for our staff and community. We’re excited to share these data points with you!

Something new this year - the PSF and Jet Brains!

This year we're trying something new. In addition to our regular donation drive, we're partnering with JetBrains to help raise money for the PSF. JetBrains PyCharm & the PSF is happy to announce a 30% discount with all proceeds going to the Python Software Foundation general fund!

Please consider becoming a PSF Supporting Member today. Or simply make a one-time or recurring donation.

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The PSF Team