Google Summer of Code 2020

In the summer of 2020, I worked with the contributors of pip, trying to improve the networking performance of the package manager. Admittedly, at the end of the internship period, the benchmark said otherwise; though I really hope the clean-up and minor fixes I happened to be doing to the codebase over the summer, in addition to the implementation of parallel utils and lazy wheel, might actually help the project.

Personally, I learned a lot: not just about Python packaging and networking stuff, but also on how to work with others. I am really grateful to @pradyunsg (my mentor), @chrahunt, @uranusjr, @pfmoore, @brainwane, @sbidoul, @xavfernandez, @webknjaz, @jaraco, @deveshks, @gutsytechster, @dholth, @dstufft, @cosmicexplorer and @ofek. While this feels like a long shout-out list, it really isn’t. These people are the maintainers, the contributors of pip and/or other Python packaging projects, and more importantly, they have been more than helpful, encouraging and patient to me throughout my every activities, showing me the way when I was lost, fixing me when I was wrong, putting up with my carelessness and showing me support across different social media.

To best serve the community, below I have tried my best to document what I have done, how I’ve done it and why I’ve done it for over the last three months. At the time of writing, some work is still in progress, so these also serve as a reference point for myself and others to reason about decisions in relevant topics.

  1. The Main Story
    1. Act One: Parallelization Utilities
    2. Act Two: Lazy Wheels
    3. Act Three: Late Downloading
    4. Act Four: Batch Downloading in Parallel
  2. The Side Quests
  3. The Plot Summary

The Main Story

The storyline can be divided into the following four main acts.

Act One: Parallelization Utilities

In this first act, I ensured the portibility of parallelization measures for later use in the final act. Multithreading and multiprocessing map were properly fellback on platforms without full support.

  • GH-8320: Add utilities for parallelization (close GH-8169)
  • GH-8538: Make utils.parallel tests tear down properly
  • GH-8504: Parallelize pip list --outdated and --uptodate (using GH-8320)

Act Two: Lazy Wheels

As proposed by @cosmicexplorer in GH-7819, it is possible to only download a portion of a wheel to obtain metadata during dependency resolution. Not only that this would reduce the total amount of data to be transmitted over the network in case the resolver needs to perform heavy backtracking, but also it would create a synchronization point at the end of the resolution progress where parallel downloading can be applied to the needed wheels (some wheels solely serve their metadata during dependency backtracking and are not needed by the users).

  • GH-8467: Add utitlity to lazily acquire wheel metadata over HTTP
  • GH-8584: Revise lazy wheel and its tests
  • GH-8681: Make range requests closer to chunk size (help GH-8670)
  • GH-8716 and GH-8730: Disable caching for range requests

Act Three: Late Downloading

During this act, the main works were refactoring to integrate the lazy wheel into pip’s codebase and clean up the way for download parallelization.

  • GH-8411: Refactor operations.prepare.prepare_linked_requirement
  • GH-8629: Abstract away AbstractDistribution in higher-level resolver code
  • GH-8442, GH-8532 and GH-8588 (later reworked by @chrahunt in GH-8685): Use lazy wheel to obtain dependency information for the new resolver
  • GH-8743: Test hash checking for fast-deps
  • GH-8804: Check download directory before making range requests

Act Four: Batch Downloading in Parallel

The final act is mostly about the UI of the parallel download. My work involved around how the progress should be displayed and how other relevant information should be reported to the users.

  • GH-8710: Revise method fetching metadata using lazy wheels
  • GH-8722: Dedent late download logs (fix GH-8721)
  • GH-8737: Add a hook for batch downloading
  • GH-8771: Parallelize wheel download

The Side Quests

In order to keep the wheel turning (no pun intended) and avoid wasting time waiting for the pull requests above to be reviewed, I decided to create even more PRs (as I am typing this, many of the patches listed below are nowhere near being merged).

  • GH-7878: Fail early when install path is not writable
  • GH-7928: Fix rst syntax in Getting Started guide
  • GH-7988: Fix tabulate col size in case of empty cell
  • GH-8137: Add subcommand alias mechanism
  • GH-8143: Make mypy happy with beta release automation
  • GH-8248: Fix typo and simplify ireq call
  • GH-8332: Add license requirement to _vendor/README.rst
  • GH-8423: Nitpick logging calls
  • GH-8435: Use str.format style in logging calls
  • GH-8456: Lint src/pip/_vendor/README.rst
  • GH-8568: Declare constants in configuration.py as such
  • GH-8571: Clean up Configuration.unset_value and nit __init__
  • GH-8578: Allow verbose/quiet level to be specified via config files and environment variables
  • GH-8599: Replace tabs by spaces for consistency
  • GH-8614: Use monkeypatch.setenv to mock environment variables
  • GH-8674: Fix tests/functional/test_install_check.py, when run with new resolver
  • GH-8692: Make assertion failure give better message
  • GH-8709: List downloaded distributions before exiting (fix GH-8696)
  • GH-8759: Allow py2 deprecation warning from setuptools
  • GH-8766: Use the new resolver for test requirements
  • GH-8790: Mark tests using remote svn and hg as xfail
  • GH-8795: Reformat a few spots in user guide

The Plot Summary

Every Monday throughout the Summer of Code, I summarized what I had done in the week before in the form of either a short blog or an (even shorter) check-in. These write-ups often contain handfuls of popular culture references and was originally hosted on Python GSoC.

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—Nguyễn Gia Phong, 2020-08-31

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