Implement `--executable' option (pypa#257)

src/installer/__main__.py

@@ -31,6 +31,13 @@ def _get_main_parser() -> argparse.ArgumentParser:
         type=str,
         help="override prefix to install packages to",
     )
+    parser.add_argument(
+        '--executable',
+        metavar="path",
+        default=sys.executable,
+        type=str,
+        help="#! executable to install scripts with (default=sys.executable)",
+    )
     parser.add_argument(
         "--compile-bytecode",
         action="append",
@@ -103,7 +110,7 @@ def _main(cli_args: Sequence[str], program: Optional[str] = None) -> None:
                 source.validate_record(validate_contents=args.validate_record == "all")
             destination = SchemeDictionaryDestination(
                 scheme_dict=_get_scheme_dict(source.distribution, prefix=args.prefix),
-                interpreter=sys.executable,
+                interpreter=args.executable,
                 script_kind=get_launcher_kind(),
                 bytecode_optimization_levels=bytecode_levels,
                 destdir=args.destdir,

tests/test_main.py

@@ -57,7 +57,10 @@ def test_main_multiple_wheels(fancy_wheel, another_fancy_wheel, tmp_path):
 def test_main_prefix(fancy_wheel, tmp_path):
     destdir = tmp_path / "dest"
 
-    main([str(fancy_wheel), "-d", str(destdir), "-p", "/foo"], "python -m installer")
+    main(
+        [str(fancy_wheel), "-d", str(destdir), "-p", "/foo", "--executable", "/monty/python3.x"],
+        "python -m installer",
+    )
 
     installed_py_files = list(destdir.rglob("*.py"))
 
@@ -73,6 +76,14 @@ def test_main_prefix(fancy_wheel, tmp_path):
         "__main__",
     }
 
+    installed_scripts = destdir.rglob("bin/*")
+
+    for f in installed_scripts:
+        with f.open('rb') as fp:
+            shebang = fp.readline()
+            assert shebang == b'#!/monty/python3.x\n' or \
+                shebang == b'#! /monty/python3.x\n'
+
 
 def test_main_no_pyc(fancy_wheel, tmp_path):
     destdir = tmp_path / "dest"