Fix when there is no 4th arg passed to build_isolated

CONTRIBUTORS

@@ -9,6 +9,7 @@ Andrii Soldatenko
 Andrzej Klajnert
 Anthon van der Neuth
 Anthony Sottile
+Antoine Dechaume
 Anudit Nagar
 Ashley Whetter
 Asmund Grammeltwedt

docs/changelog/2056.bugfix.rst

@@ -0,0 +1 @@
+Fix a ``tox-conda`` isolation build bug - by :user:`AntoineD`.

src/tox/helper/build_isolated.py

@@ -29,7 +29,7 @@ def _ensure_module_in_paths(module, paths):
 dist_folder = sys.argv[1]
 backend_spec = sys.argv[2]
 backend_obj = sys.argv[3] if len(sys.argv) >= 4 else None
-backend_paths = sys.argv[4].split(os.path.pathsep) if sys.argv[4] else []
+backend_paths = sys.argv[4].split(os.path.pathsep) if (len(sys.argv) >= 5 and sys.argv[4]) else []
 
 sys.path[:0] = backend_paths
 

tests/unit/package/builder/test_package_builder_isolated.py

@@ -1,8 +1,10 @@
 import os
+import subprocess
 
 import py
 import pytest
 
+import tox.helper
 from tox.package.builder.isolated import get_build_info
 from tox.reporter import _INSTANCE
 
@@ -193,3 +195,10 @@ def test_verbose_isolated_build_in_tree(initproj, mock_venv, cmd):
     assert "running sdist" in result.out, result.out
     assert "running egg_info" in result.out, result.out
     assert "Writing example123-0.5{}setup.cfg".format(os.sep) in result.out, result.out
+
+
+def test_isolated_build_script_args(tmp_path):
+    """Verify that build_isolated.py can be called with only 2 argurments."""
+    # cannot import build_isolated because of its side effects
+    script_path = os.path.join(os.path.dirname(tox.helper.__file__), "build_isolated.py")
+    subprocess.check_call(("python", script_path, str(tmp_path), "setuptools.build_meta"))