Better message when command parsing on empty input

docs/changelog/2695.bugfix.rst

@@ -0,0 +1 @@
+Fail more gracefully when pip :ref:`install_command` is empty - by :user:`jayaddison`.

src/tox/config/loader/str_convert.py

@@ -63,6 +63,8 @@ def to_command(value: str) -> Command:
                 pos = splitter.instream.tell()
         except ValueError:
             args.append(value[pos:])
+        if len(args) == 0:
+            raise ValueError(f"attempting to parse {value!r} into a command failed")
         if args[0] != "-" and args[0].startswith("-"):
             args[0] = args[0][1:]
             args = ["-"] + args

src/tox/tox_env/python/pip/pip_install.py

@@ -160,7 +160,10 @@ def _execute_installer(self, deps: Sequence[Any], of_type: str) -> None:
         outcome.assert_success()
 
     def build_install_cmd(self, args: Sequence[str]) -> list[str]:
-        cmd: Command = self._env.conf["install_command"]
+        try:
+            cmd: Command = self._env.conf["install_command"]
+        except ValueError as exc:
+            raise Fail(f"unable to determine pip install command: {str(exc)}") from exc
         install_command = cmd.args
         try:
             opts_at = install_command.index("{packages}")

tests/config/loader/test_str_convert.py

@@ -62,6 +62,7 @@ def test_str_convert_ok(raw: str, value: Any, of_type: type[Any]) -> None:
         ("a", TypeVar, TypeError, r"a cannot cast to .*typing.TypeVar.*"),
         ("3", Literal["1", "2"], ValueError, r"3 must be one of \('1', '2'\)"),
         ("3", Union[str, int], TypeError, r"3 cannot cast to typing.Union\[str, int\]"),
+        ("", Command, ValueError, r"attempting to parse '' into a command failed"),
     ],
 )
 def test_str_convert_nok(raw: str, of_type: type[Any], msg: str, exc_type: type[Exception]) -> None:

tests/session/cmd/test_show_config.py

@@ -99,6 +99,14 @@ def test_show_config_exception(tox_project: ToxProjectCreator) -> None:
     assert txt in outcome.out
 
 
+def test_show_config_empty_install_command_exception(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({"tox.ini": "[testenv:a]\ninstall_command="})
+    outcome = project.run("c", "-e", "a", "-k", "install_command")
+    outcome.assert_success()
+    txt = "\ninstall_command = # Exception: " "ValueError(\"attempting to parse '' into a command failed\")"
+    assert txt in outcome.out
+
+
 @pytest.mark.parametrize("stdout_is_atty", [True, False])
 def test_pass_env_config_default(tox_project: ToxProjectCreator, stdout_is_atty: bool, mocker: MockerFixture) -> None:
     mocker.patch("sys.stdout.isatty", return_value=stdout_is_atty)

tests/tox_env/python/pip/test_pip_install.py

@@ -6,8 +6,10 @@
 from unittest.mock import Mock
 
 import pytest
+from packaging.requirements import Requirement
 
 from tox.pytest import CaptureFixture, ToxProjectCreator
+from tox.tox_env.errors import Fail
 
 
 @pytest.mark.parametrize("arg", [object, [object]])
@@ -35,6 +37,15 @@ def test_pip_install_empty_list(tox_project: ToxProjectCreator) -> None:
     assert execute_calls.call_count == 0
 
 
+def test_pip_install_empty_command_error(tox_project: ToxProjectCreator) -> None:
+    proj = tox_project({"tox.ini": "[testenv]\ninstall_command="})
+    result = proj.run("l")
+    pip = result.state.envs["py"].installer
+
+    with pytest.raises(Fail, match="unable to determine pip install command"):
+        pip.install([Requirement("name")], "section", "type")
+
+
 def test_pip_install_flags_only_error(tox_project: ToxProjectCreator) -> None:
     proj = tox_project({"tox.ini": "[testenv:py]\ndeps=-i a"})
     result = proj.run("r")