Impossible to override libxml2 and libxslt with pythonSupport = false

[veprbl]

Describe the bug
It appears that there is no simple way to override libxml2 and libxslt with pythonSupport = false

To Reproduce

with import <nixpkgs> {
  overlays = [
    (self: super: {
      libxslt = super.libxslt.override { pythonSupport = false; };
      libxml2 = super.libxml2.override { pythonSupport = false; };
    })
  ];
};

libxslt

# nix-instantiate test.nix --show-trace
error: while evaluating the attribute 'libxslt.override' at nixpkgs/pkgs/top-level/all-packages.nix:12864:3:
while evaluating 'callPackageWith' at nixpkgs/lib/customisation.nix:117:35, called from /home/veprbl/nixpkgs/pkgs/top-level/all-packages.nix:12864:13:
while evaluating 'makeOverridable' at nixpkgs/lib/customisation.nix:67:24, called from /home/veprbl/nixpkgs/lib/customisation.nix:121:8:
while evaluating anonymous function at nixpkgs/pkgs/development/libraries/libxslt/default.nix:1:1, called from /home/veprbl/nixpkgs/lib/customisation.nix:69:16:
assertion failed at nixpkgs/pkgs/development/libraries/libxslt/default.nix:7:1

Expected behavior
Straightforward override should work

Metadata

• version: nix-env (Nix) 2.3
• channels(veprbl): "nixpkgs-20.03pre194957.bef773ed53f"

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
  - libxml2
  - libxslt
# a list of nixos modules affected by the problem
module:

[FRidh]

For some reason

libxslt = super.libxslt.override { pythonSupport = false; };

does not seem to function. Pass in a string e.g., and see what happens. I am puzzled by this.

[veprbl]

@FRidh
Well we get an assertion when evaluating libxslt before we can access .override. And it seems like the problem was there from the beginning:

with import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/be31980848dee782c4504b88c57edcfbb72c6f6b.tar.gz") {
  config = {
    packageOverrides = pkgs: {
      libxslt = pkgs.libxslt.override { pythonSupport = false; };
      libxml2 = pkgs.libxml2.override { supportPython = false; };
    };
  };
};

libxslt

Maybe we should just remove the assertion?

[FRidh]

Right, so this is #53487.

[jtojnar]

Could we default to pythonSupport = false as before, and only enable it in pythonPackages?

[FRidh]

@jtojnar do you have any specific reason to disable it by default? Often we enable options and split into multiple outputs to reduce the amount of building that is needed.