core/pkgs/build-support/fetchpypi/default.nix

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

let
  computeUrl = { format ? "setuptools", ... }@attrs:
    let
      computeWheelUrl = { pname, version, dist ? "py2.py3", 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/${dist}/${
          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 ? "", hash ? "", ... }@attrs:
  let url = computeUrl (builtins.removeAttrs attrs [ "sha256" "hash" ]);
  in fetchurl { inherit url sha256 hash; })