Merge pull request #9298 from abbradar/python-env

python: add .env for convenient nix-shell's

doc/language-support.xml

@@ -464,6 +464,27 @@ python.buildEnv.override {
     with wrapped binaries in <filename>bin/</filename>.
   </para>
 
+  <para>
+  You can also use <varname>env</varname> attribute to create local
+  environments with needed packages installed (somewhat comparable to
+  <literal>virtualenv</literal>). For example, with the following
+  <filename>shell.nix</filename>:
+
+<programlisting language="nix">
+<![CDATA[with import <nixpkgs> {};
+
+(python3.buildEnv.override {
+  extraLibs = with python3Packages;
+    [ numpy
+      requests
+    ];
+}).env]]>
+</programlisting>
+
+  Running <command>nix-shell</command> will drop you into a shell where
+  <command>python</command> will have specified packages in its path.
+  </para>
+
   <variablelist>
     <title>
       <function>python.buildEnv</function> arguments

pkgs/development/interpreters/python/wrapper.nix

@@ -6,8 +6,7 @@
 # Create a python executable that knows about additional packages.
 let
   recursivePthLoader = import ../../python-modules/recursive-pth-loader/default.nix { stdenv = stdenv; python = python; };
-in
-(buildEnv {
+  env = (buildEnv {
     name = "${python.name}-env";
     paths = stdenv.lib.filter (x : x ? pythonPath) (stdenv.lib.closePropagation extraLibs) ++ [ python recursivePthLoader ];
 
@@ -27,7 +26,20 @@ in
         makeWrapper "${python}/bin/$prg" "$out/bin/$prg" --set PYTHONHOME "$out"
       done
     '' + postBuild;
+
+    passthru.env = stdenv.mkDerivation {
+      name = "interactive-${python.name}-environment";
+      nativeBuildInputs = [ env ];
+
+      buildCommand = ''
+        echo >&2 ""
+        echo >&2 "*** Python 'env' attributes are intended for interactive nix-shell sessions, not for building! ***"
+        echo >&2 ""
+        exit 1
+      '';
+    };
   }) // {
     inherit python;
     inherit (python) meta;
-}
+  };
+in env