diff --git a/pkgs/top-level/python-packages.nix b/pkgs/top-level/python-packages.nix
index eccf1047cc4..a896e7a4e9e 100644
--- a/pkgs/top-level/python-packages.nix
+++ b/pkgs/top-level/python-packages.nix
@@ -53,15 +53,11 @@ let
         let
           url = "https://files.pythonhosted.org/packages/${python}/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}-${python}-${abi}-${platform}.whl";
         in pkgs.fetchurl {inherit url sha256;};
-
-      fetchSource = {pname, version, sha256}:
+      fetchSource = {pname, version, sha256, extension ? "tar.gz"}:
       # Fetch a source tarball.
         let
-          urls = [
-            "mirror://pypi/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}.tar.gz"
-            "mirror://pypi/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}.zip"
-          ];
-        in pkgs.fetchurl {inherit urls sha256;};
+          url = "mirror://pypi/${builtins.substring 0 1 pname}/${pname}/${pname}-${version}.${extension}";
+        in pkgs.fetchurl {inherit url sha256;};
       fetcher = (if format == "wheel" then fetchWheel
         else if format == "setuptools" then fetchSource
         else throw "Unsupported kind ${kind}");