nixpkgs/pkgs/development/interpreters/python/fetchpypi.nix

# `fetchPypi` function for fetching artifacts from PyPI.
{ fetchurl
, makeOverridable
}:

let
  computeUrl = {format ? "setuptools", ... } @attrs: let
    computeWheelUrl = {pname, version, python ? "py2.py3", abi ? "none", platform ? "any"}:
    # Fetch a wheel. By default we fetch an universal wheel.
    # See https://www.python.org/dev/peps/pep-0427/#file-name-convention for details regarding the optional arguments.
      "https://files.pythonhosted.org/packages/${python}/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}-${python}-${abi}-${platform}.whl";

    computeSourceUrl = {pname, version, extension ? "tar.gz"}:
    # Fetch a source tarball.
      "mirror://pypi/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}.${extension}";

    compute = (if format == "wheel" then computeWheelUrl
      else if format == "setuptools" then computeSourceUrl
      else throw "Unsupported format ${format}");

  in compute (builtins.removeAttrs attrs ["format"]);

in makeOverridable( {format ? "setuptools", sha256, ... } @attrs:
  let
    url = computeUrl (builtins.removeAttrs attrs ["sha256"]) ;
  in fetchurl {
    inherit url sha256;
  })