Python

Posts relating to the python programming language.

virtio-vsock: python examples of running the server in the guest

I’ve been using virtio-serial for communications between Linux hypervisors and guest virtual machines for ages. Lots of other people do it to — the qemu guest agent for example is implemented like this. In fact, I think that’s where I got my original thoughts on the matter from. However, virtio-serial is actually fairly terrible to write against as a programming model, because you’re left to do all the multiplexing of various requests down the channel and surely there’s something better?

Well… There is! virtio-vsock is basically the same concept, except it uses the socket interface. You can have more than one connection open and the sockets layer handles multiplexing by magic. This massively simplifies the programming model for supporting concurrent users down the channel. So that’s actually pretty cool. I should credit Kata Containers with noticing this quality of life improvement nearly a decade before I did, but I get there in the end.

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An easier to parse version of “yum history”

I got interested today in trying to come up with a solid way of determining when updates were last applied to a RHEL-derived Linux instance. Previously we’d been inferring it from the kernel version, but it turns out there is a convenient “yum history”  or “dnf history” command which will show you all the previous transactions that the package database has seen. However, the output is hard to parse in a script.

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A gotcha with the Walrus operator

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In New python syntax I was previously unaware of, I discussed some new operators I'd recently discovered. One of them is called the Walrus operator, which lets you write code like this: list = ['a', 'b', 'c'] def get_one(): if not list: return None return list.pop() while one := get_one(): print(one) See where we do the assignment inside the while? That code returns: c b a Which is as expected. However, the Walrus operator is strict about needing a None returned to end the iteration. I had code which was more like this: list = [('a', 1), ('b', 2), ('c', 3)] def get_one(): if not list: return None, None return list.pop() while one := get_one(): print(one) And the while loop never terminates. It just prints (None, None) over and over. So there you go.

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New python syntax I was previously unaware of

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This post documents the new syntax features I learned about while reading cpython internals. You can create more than one context manager on a single line. So for example Shaken Fist contains code like this: with open(path + '.new', 'w') as o: with open(path, 'r') as i: ... That can now be written like this: with open(path + '.new', 'w') as o, open(path, 'r') as i: ...   You can assign values in a while statement, but only one. Instead of this: d = f.read(8000) while f: ... d = f.read(8000) You can write this: while d := f.read(8000): ... But unfortunately this doesn't work: while a, b := thing(): ...   You can use underscores as commands in long numbers to make them easier to read. For example, you can write 1000000 or 1_000_000 and they both mean the same thing.   You can refer to positional arguments by name, but you can also disable that. I didn't realise that this was valid python: def foo(bar=None): print(bar) foo(bar='banana') You can turn it off with a forward slash in the argument list though, which should separate positional arguments from named arguments: def foo(bar, /, extra=None): print(bar) print(extra) foo('banana', extra='frog') The above example…

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cpython internals

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I have been paid money to write Python code since about 2006, so I figured it was probably time that I should understand some of the inner workings of Python. I therefore picked up two books on the topic, this one being the first of the two.

This book to be honest isn’t completely what I expected. Its very well written and quite interesting, but its more about the things you’d need to know to become a Python core developer, rather than the things you should know as a user of Python like how the Python dictionary implementation is built.

(If you want that specifically, this video is an excellent introduction).

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CPython Internals Book Cover CPython Internals
Anthony Shaw
May 5, 2021
396

Get your guided tour through the Python 3.9 interpreter: Unlock the inner workings of the Python language, compile the Python interpreter from source code, and participate in the development of CPython. Are there certain parts of Python that just seem like magic? This book explains the concepts, ideas, and technicalities of the Python interpreter in an approachable and hands-on fashion. Once you see how Python works at the interpreter level, you can optimize your applications and fully leverage the power of Python.

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All python packages require a pyproject.toml with modern pip

So last night Shaken Fist CI jobs started failing with errors like this (editted lightly for clarity): Building wheels for collected packages: shakenfist-ci Building wheel for shakenfist-ci (setup.py): started Building wheel for shakenfist-ci (setup.py): finished with status 'error' error: subprocess-exited-with-error × python setup.py bdist_wheel did not run successfully. │ exit code: 1 ╰─> [86 lines of output] ... ...setuptools/command/install.py:37: SetuptoolsDeprecationWarning: setup.py install is deprecated. Use build and pip and other standards-based tools. setuptools.SetuptoolsDeprecationWarning, installing to build/bdist.linux-x86_64/wheel running install ... warning: install_lib: byte-compiling is disabled, skipping. running install_egg_info Copying shakenfist_ci.egg-info to build/bdist.linux-x86_64/wheel/shakenfist_ci-0.0.1.dev2544-py3.7.egg-info running install_scripts error: invalid command 'bdist_wininst' [end of output] This was pretty concerning. I know that a setup.py / setup.cfg style install is a little old school, but it was unexpected that it broke entirely. At first I thought I'd have to convert to poetry to unblock this, but Chet helpfully pointed out that this is as simple as adding a pyproject.toml file to the directory which contains your setup.py and setup.cfg. The basic issue is that a modern pip doesn't assume that you're going to use setuptools, so you need to tell it that you're doing that in pyproject.toml. Then you're unblocked. So, just create a file named…

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Debian 10 buster bcrypt pip install breakage

So, as of today by Shaken Fist CI jobs for Debian 10 are failing to install bcrypt, with an error that looks like this: Running setup.py install for bcrypt: started Running setup.py install for bcrypt: finished with status 'error' [ ... snip ... ] running build_rust =============================DEBUG ASSISTANCE============================= If you are seeing a compilation error please try the following steps to successfully install bcrypt: 1) Upgrade to the latest pip and try again. This will fix errors for most users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip 2) Ensure you have a recent Rust toolchain installed. bcrypt requires rustc >= 1.56.0. Python: 3.7.3 platform: Linux-4.19.0-21-amd64-x86_64-with-debian-10.12 pip: 18.1 setuptools: 65.2.0 setuptools_rust: 1.5.1 rustc: n/a =============================DEBUG ASSISTANCE============================= I'm not really interested in debating why installing a python package requires a rust compiler, that has been dicussed elsewhere. This specific breakage has been caused by bcrypt releasing 4.0.0, which has this in the changelog: "bcrypt is now implemented in Rust. Users building from source will need to have a Rust compiler available. Nothing will change for users downloading wheels." Unfortunately, you can't just install rustc with apt, as it is both quite big (350mb), and too old (version 1.41.1 versus the required 1.56.0 or better). I also couldn't…

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A quick summary of OpenStack release tags

I wanted a quick summary of OpenStack git release tags for a talk I am working on, and it turned out to be way more complicated than I expected. I ended up having to compile a table, and then turn that into a code snippet. In case its useful to anyone else, here it is: Or in python form for those so inclined: RELEASE_TAGS = { 'austin': {'all': '2010.1'}, 'bexar': {'all': '2011.1'}, 'cactus': {'all': '2011.2'}, 'diablo': {'all': '2011.3'}, 'essex': {'all': '2012.1.3'}, 'folsom': {'all': '2012.2.4'}, 'grizzly': {'all': '2013.1.5'}, 'havana': {'all': '2013.2.4'}, 'icehouse': {'all': '2014.1.5'}, 'juno': {'all': '2014.2.4'}, 'kilo': {'all': '2015.1.4'}, 'liberty': { 'glance': '11.0.2', 'keystone': '8.1.2', 'neutron': '7.2.0', 'nova': '12.0.6' }, 'mitaka': { 'glance': '12.0.0', 'keystone': '9.3.0', 'neutron': '8.4.0', 'nova': '13.1.4' }, 'newton': { 'glance': '13.0.0', 'keystone': '10.0.3', 'neutron': '9.4.1', 'nova': '14.1.0' }, 'ocata': { 'glance': '14.0.1', 'keystone': '11.0.4', 'neutron': '10.0.7', 'nova': '15.1.5' }, 'pike': { 'glance': '15.0.2', 'keystone': '12.0.3', 'neutron': '11.0.8', 'nova': '16.1.8' }, 'queens': { 'glance': '16.0.1', 'keystone': '13.0.4', 'neutron': '12.1.1', 'nova': '17.0.13' }, 'rocky': { 'glance': '17.0.1', 'keystone': '14.2.0', 'neutron': '13.0.7', 'nova': '18.3.0' }, 'stein': { 'glance': '18.0.1', 'keystone': '15.0.1', 'neutron': '14.4.2', 'nova': '19.3.2' }, 'train': { 'glance': '19.0.4', 'keystone': '16.0.1', 'neutron': '15.3.0', 'nova': '20.4.1' }, 'ussuri': {…

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Playing with the python prometheus query API

The last few days have been a bit icky around here, with my house apparently proudly residing in the major city with the dirtiest air in the world. So, I needed a distraction... It has also been quite hot, so I wondered how my energy usage was going. I have prometheus monitoring of my power draw, so now seemed as good a time as any to learn how to do some historical querying over the API. I ended up with a python script which can output things like this: "Yesterday had a maximum temperature of 38 and we used 28.36 kwh. The average for similar days is 25.56 kwh." The code is on github if it is of interest to others. I am sure I could push more of this processing down into the prometheus engine, but I couldn't see how to do it today. Hints welcome!

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Quick hack: extracting the contents of a Docker image to disk

Hello! Please note I've written a little python tool called Occy Strap which makes this a bit easier, and can do some fancy things around importing and exporting multiple images. You might want to read about it? For various reasons, I wanted to inspect the contents of a Docker image without starting a container. Docker makes it easy to get an image as a tar file, like this: docker save -o foo.tar image But if you extract that tar file you'll find a configuration file and manifest as JSON files, and then a series of tar files, one per image layer. You use the manifest to determine in what order you extract the tar files to build the container filesystem. That's fiddly and annoying. So I wrote this quick python hack to extract an image tarball into a directory on disk that I could inspect: #!/usr/bin/python3 # Call me like this: # docker-image-extract tarfile.tar extracted import tarfile import json import os import sys image_path = sys.argv[1] extracted_path = sys.argv[2] image = tarfile.open(image_path) manifest = json.loads(image.extractfile('manifest.json').read()) for layer in manifest[0]['Layers']: print('Found layer: %s' % layer) layer_tar = tarfile.open(fileobj=image.extractfile(layer)) for tarinfo in layer_tar: print(' ... %s' % tarinfo.name) if tarinfo.isdev(): print(' -->…

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