The Nix Hour #34 [python3.withPackages vs python3Packages, file sets]

The Nix Hour is an open beginner-oriented Q&A-style lecture on Nix, where you can bring your questions and we’ll answer them with more context, examples, alternatives, tradeoffs and related topics.
Files from this session: https://github.com/tweag/nix-hour/tree/master/34
Jitsi Meeting to ask questions directly: https://meet.jit.si/nix-hour
Matrix room for questions, feedback and notifications: https://matrix.to/#/%23nix-hour:matrix.org
Issue tracker for asynchronous questions: https://github.com/tweag/nix-hour/issues
Previous sessions playlist: https://www.youtube.com/playlist?list=PLyzwHTVJlRc8yjlx4VR4LU5A5O44og9in

For more information and updates, check out https://discourse.nixos.org/t/the-nix-hour-a-weekly-nix-qa-lecture-by-tweag/22767

File sets links:
Discourse post: https://discourse.nixos.org/t/easy-source-filtering-with-file-sets/29117/3
Rendered documentation: https://tweag.github.io/nixpkgs/file-sets/manual.html#sec-functions-library-fileset

00:00 Intro, no Nix Hour for the next 2 weeks
00:54 Q: what is the difference between using python.pkgs and python.withPackages
01:51 nix-shell and python3Packages
02:55 nix-shell and withPackages
03:32 venv usage works with withPackages, but not with python3Packages
04:39 Looking at PYTHONPATH with both variants
06:15 makeBinaryWrapper
09:58 difference between buildInputs and python.withPackages
10:52 buildInputs can poison derivation environment with setup-hook
12:19 python3.withPackages only provides one binary instead
13:07 Recommend using buildPythonPackage when possible
14:36 File sets: why, what and how.
15:25 Current state of things: using ./. as src for derivation
19:25 Comparing resulting derivations and their srcs
22:22 Flakes don't have this particular issue, replicating that behavior with fetchGit
25:27 Still not ideal: any change to git repo triggers rebuild
26:19 Excluding files from the src with builtins.path
31:58 Paths and strings in Nix are never equal
32:51 Filtering out object files
34:22 Filter always recurses into subdirectories
27:47 Including only one subdirectory
39:25 File sets overview
41:15 Functions: union
41:35 Functions: trace
42:49 Functions: difference
43:50 Functions: fileFilter
45:06 Functions: intersect
45:25 Nothing is imported into Nix store
47:17 Functions: toSource
49:22 Switching to file sets in our example, "unions" function
56:00 File sets overview summary
59:06 Outro, see you in 3 weeks