pythonPackages.purepng: fix for py3.7, enable tests

the pypi package includes pre-cythonized versions of some components,
however these were cythonized with a py3.7-unaware cython which is what
caused the main breakage. fetching the equivalent version from github
allows us to offer either the pure-python version or one we cythonize
ourselves if the cython package is provided.

the added bonus of using a github checkout is that the tests are included,
so enable these.

these tests revealed that for full py3.7 compatibility an additional patch
is needed from an upstream PR.

1 files changed, 30 insertions, 4 deletions

diff --git a/pkgs/development/python-modules/purepng/default.nix b/pkgs/development/python-modules/purepng/default.nix
index 49e4d93a5342f..cecd7cc576d96 100644
--- a/pkgs/development/python-modules/purepng/default.nix
+++ b/pkgs/development/python-modules/purepng/default.nix
@@ -1,17 +1,43 @@
 { stdenv
 , buildPythonPackage
-, fetchPypi
+, python
+, fetchFromGitHub
+, fetchpatch
+, cython ? null
+, numpy ? null
 }:
 
 buildPythonPackage rec {
   pname = "purepng";
   version = "0.2.0";
 
-  src = fetchPypi {
-    inherit pname version;
-    sha256 = "1kcl7a6d7d59360fbz2jwfk6ha6pmqgn396962p4s62j893d2r0d";
+  src = fetchFromGitHub {
+    owner = "Scondo";
+    repo = "purepng";
+    rev = "449aa00e97a8d7b8a200eb9048056d4da600a345";
+    sha256 = "105p7sxn2f21icfnqpah69mnd74r31szj330swbpz53k7gr6nlsv";
   };
 
+  patches = [
+    (fetchpatch {
+      name = "fix-py37-stopiteration-in-generators.patch";
+      url = "https://github.com/Scondo/purepng/pull/28/commits/62d71dfc2be9ffdc4b3e5f642af0281a8ce8f946.patch";
+      sha256 = "1ag0pji3p012hmj8kadcd0vydv9702188c0isizsi964qcl4va6m";
+    })
+  ];
+  patchFlags = "-p1 -d code";
+
+  # cython is optional - if not supplied, the "pure python" implementation will be used
+  nativeBuildInputs = [ cython ];
+
+  # numpy is optional - if not supplied, tests simply have less coverage
+  checkInputs = [ numpy ];
+  # checkPhase begins by deleting source dir to force test execution against installed version
+  checkPhase = ''
+    rm -r code/png
+    ${python.interpreter} code/test_png.py
+  '';
+
   meta = with stdenv.lib; {
     description = "Pure Python library for PNG image encoding/decoding";
     homepage    = https://github.com/scondo/purepng;