style-inline.nix

{ lib, python3Packages, stdenv, writeTextDir, substituteAll, targetPackages }:

python3Packages.buildPythonApplication rec {
  # ...
  
  crossFile = writeTextDir "cross-file.conf" ''
    [host_machine]
    system = '${targetPackages.stdenv.targetPlatform.parsed.kernel.name}'
    cpu_family = '${
      # http://mesonbuild.com/Reference-tables.html#cpu-families
      if targetPackages.stdenv.targetPlatform.isAarch32 then "arm"
      else if targetPackages.stdenv.targetPlatform.isx86_32 then "x86"
      else targetPackages.stdenv.targetPlatform.parsed.cpu.name
    }'
    cpu = '${targetPackages.stdenv.targetPlatform.parsed.cpu.name}'
    endian = ${if targetPackages.stdenv.targetPlatform.isLittleEndian then "'little'" else "'big'"}
  '';

  # ...
}

style-let.nix

{ lib, python3Packages, stdenv, writeTextDir, substituteAll, targetPackages }:

let
  # http://mesonbuild.com/Reference-tables.html#cpu-families
  cpuFamily =
    if targetPackages.stdenv.targetPlatform.isAarch32 then "arm"
    else if targetPackages.stdenv.targetPlatform.isx86_32 then "x86"
    else targetPackages.stdenv.targetPlatform.parsed.cpu.name
  ;
in
python3Packages.buildPythonApplication rec {
  # ...
  
  crossFile = writeTextDir "cross-file.conf" ''
    [host_machine]
    system = '${targetPackages.stdenv.targetPlatform.parsed.kernel.name}'
    cpu_family = '${cpuFamily}'
    cpu = '${targetPackages.stdenv.targetPlatform.parsed.cpu.name}'
    endian = ${if targetPackages.stdenv.targetPlatform.isLittleEndian then "'little'" else "'big'"}
  '';

  # ...
}