python.pkgs.altair: move to separate expression

pkgs/development/python-modules/altair/default.nix

@@ -0,0 +1,30 @@
+{ stdenv, buildPythonPackage, fetchPypi
+, pytest, vega, pandas, ipython, traitlets }:
+
+buildPythonPackage rec {
+  pname = "altair";
+  version = "1.2.0";
+  name = "${pname}-${version}";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "05c47dm20p7m0017p2h38il721rxag1q0457dj7whp0k8rc7qd1n";
+  };
+
+  buildInputs = [ pytest ];
+
+  checkPhase = ''
+    export LANG=en_US.UTF-8
+    py.test altair --doctest-modules
+  '';
+
+  propagatedBuildInputs = [ vega pandas ipython traitlets ];
+
+  meta = with stdenv.lib; {
+    description = "A declarative statistical visualization library for Python.";
+    homepage = https://github.com/altair-viz/altair;
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ teh ];
+    platforms = platforms.linux;
+  };
+}

pkgs/top-level/python-packages.nix

@@ -264,28 +264,7 @@ in {
 
   aafigure = callPackage ../development/python-modules/aafigure { };
 
-  altair = buildPythonPackage rec {
+  altair = callPackage ../development/python-modules/altair { };
-    name = "altair-1.2.0";
-
-    src = pkgs.fetchurl {
-      url = "mirror://pypi/a/altair/${name}.tar.gz";
-      sha256 = "05c47dm20p7m0017p2h38il721rxag1q0457dj7whp0k8rc7qd1n";
-    };
-    buildInputs = [ self.pytest ];
-
-    checkPhase = ''
-      export LANG=en_US.UTF-8
-      py.test altair --doctest-modules
-    '';
-    propagatedBuildInputs = with self; [ vega pandas ipython traitlets ];
-    meta = {
-      description = "A declarative statistical visualization library for Python.";
-      homepage = https://github.com/altair-viz/altair;
-      license = licenses.bsd3;
-      platforms = platforms.linux;
-      maintainers = with maintainers; [ teh ];
-    };
-  };
   vega = buildPythonPackage rec {
     name = "vega-0.4.4";