python.pkgs.virtualenv: move to separate expression

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

@@ -0,0 +1,31 @@
+{ buildPythonPackage
+, fetchPypi
+, lib
+, recursivePthLoader
+}:
+
+buildPythonPackage rec {
+  pname = "virtualenv";
+  version = "15.0.3";
+  name = "${pname}-${version}";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "6d9c760d3fc5fa0894b0f99b9de82a4647e1164f0b700a7f99055034bf548b1d";
+  };
+
+  # Doubt this is needed - FRidh 2017-07-07
+  pythonPath = [ recursivePthLoader ];
+
+  patches = [ ./virtualenv-change-prefix.patch ];
+
+  # Tarball doesn't contain tests
+  doCheck = false;
+
+  meta = {
+    description = "A tool to create isolated Python environments";
+    homepage = http://www.virtualenv.org;
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [ goibhniu ];
+  };
+}

pkgs/development/python-modules/virtualenv/virtualenv-change-prefix.patch

@@ -0,0 +1,59 @@
+Without this patch `virtualenv --python=python2.7 .` fails with an
+error because it notices that the python readline.so is not in the
+same path as python2.7. I assume this is to avoid copying the wrong
+file on systems where it is possible to find incompatible libraries by
+accident. Adding "/nix/store" to the prefix fixes this problem.
+
+A sitecustomize.py is created in the virtualenv which makes libraries
+from the python specified by the --python argument available to the
+virtualenv. For example, this makes readline and sqlite3 available
+when a wrapped python is specified. If no --python argument is passed,
+it will only add the path to the python used when building
+`virtualenv`, which is the unwrapped python, so sqlite3 won't be
+available.
+
+
+diff --git a/virtualenv.py b/virtualenv.py
+index d3e76a7..cb307fa 100755
+--- a/virtualenv.py
++++ b/virtualenv.py
+@@ -1051,17 +1051,7 @@ def path_locations(home_dir):
+ 
+ 
+ def change_prefix(filename, dst_prefix):
+-    prefixes = [sys.prefix]
+-
+-    if is_darwin:
+-        prefixes.extend((
+-            os.path.join("/Library/Python", sys.version[:3], "site-packages"),
+-            os.path.join(sys.prefix, "Extras", "lib", "python"),
+-            os.path.join("~", "Library", "Python", sys.version[:3], "site-packages"),
+-            # Python 2.6 no-frameworks
+-            os.path.join("~", ".local", "lib","python", sys.version[:3], "site-packages"),
+-            # System Python 2.7 on OSX Mountain Lion
+-            os.path.join("~", "Library", "Python", sys.version[:3], "lib", "python", "site-packages")))
++    prefixes = ["/nix/store", sys.prefix]
+ 
+     if hasattr(sys, 'real_prefix'):
+         prefixes.append(sys.real_prefix)
+@@ -1078,6 +1068,8 @@ def change_prefix(filename, dst_prefix):
+             if src_prefix != os.sep: # sys.prefix == "/"
+                 assert relpath[0] == os.sep
+                 relpath = relpath[1:]
++                if src_prefix == "/nix/store":
++                    relpath = "/".join(relpath.split("/")[1:])
+             return join(dst_prefix, relpath)
+     assert False, "Filename %s does not start with any of these prefixes: %s" % \
+         (filename, prefixes)
+@@ -1190,6 +1182,11 @@ def install_python(home_dir, lib_dir, inc_dir, bin_dir, site_packages, clear, sy
+     site_filename_dst = change_prefix(site_filename, home_dir)
+     site_dir = os.path.dirname(site_filename_dst)
+     writefile(site_filename_dst, SITE_PY)
++    wrapper_path = join(prefix, "lib", py_version, "site-packages")
++    writefile(
++        join(site_dir, 'sitecustomize.py',),
++        "import sys; sys.path.append('%s')" % wrapper_path
++    )
+     writefile(join(site_dir, 'orig-prefix.txt'), prefix)
+     site_packages_filename = join(site_dir, 'no-global-site-packages.txt')
+     if not site_packages: