tox: Regression / behaviour change: install_command with empty evaluated value

Issue

Somewhere between tox versions 3.21.4 and 4.0.8, running tox began raising an IndexError exception when using the tox.ini file provided as a minimal repro case.

Environment

• Linux
• Python 3.9
• Tox version 4.0.8

$ pip list
Package       Version
------------- -------
cachetools    5.2.0
chardet       5.1.0
colorama      0.4.6
distlib       0.3.6
filelock      3.8.2
packaging     22.0
pip           22.0.4
platformdirs  2.6.0
pluggy        1.0.0
pyproject_api 1.2.1
setuptools    58.1.0
tomli         2.0.1
tox           4.0.8
virtualenv    20.17.1

Output of running tox

Provide the output of tox -rvv:

$ tox -rvv
.pkg: 805 W remove tox env folder /home/example/.tox/.pkg [tox/tox_env/api.py:302]
py: 816 D created app data folder /home/example/.local/share/virtualenv [virtualenv/app_data/__init__.py:40]
py: 834 I find interpreter for spec PythonSpec() [virtualenv/discovery/builtin.py:56]
py: 835 D discover exe for PythonInfo(spec=CPython3.9.16.final.0-64, exe=/home/example/.venv/bin/python3, platform=linux, version='3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) in /usr/local [virtualenv/discovery/py_info.py:437]
py: 836 D filesystem is case-sensitive [virtualenv/info.py:24]
py: 840 D get interpreter info via cmd: /usr/local/bin/python3.9 /home/example/.venv/lib/python3.9/site-packages/virtualenv/discovery/py_info.py hGkkxtAd5MRNpzZcdmPHiFw51mGktBhg i6FWZ97EDRPJiSbLdkIvfQH1Ffsg7orx [virtualenv/discovery/cached_py_info.py:108]
py: 1101 D wrote python info of /usr/local/bin/python3.9 at /home/example/.local/share/virtualenv/py_info/1/397704579725e15f5c7cb49fe5f0341eb7531c82d19f2c29d197e8b64ab5776b.json [virtualenv/app_data/via_disk_folder.py:155]
py: 1103 I proposed PythonInfo(spec=CPython3.9.16.final.0-64, system=/usr/local/bin/python3.9, exe=/home/example/.venv/bin/python3, platform=linux, version='3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:63]
py: 1105 D accepted PythonInfo(spec=CPython3.9.16.final.0-64, system=/usr/local/bin/python3.9, exe=/home/example/.venv/bin/python3, platform=linux, version='3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:65]
py: 1288 I create virtual environment via CPython3Posix(dest=/home/example/.tox/py, clear=False, no_vcs_ignore=False, global=False) [virtualenv/run/session.py:48]
py: 1289 D create folder /home/example/.tox/py/bin [virtualenv/util/path/_sync.py:9]
py: 1290 D create folder /home/example/.tox/py/lib/python3.9/site-packages [virtualenv/util/path/_sync.py:9]
py: 1291 D write /home/example/.tox/py/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
py: 1291 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
py: 1291 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
py: 1291 D 	version_info = 3.9.16.final.0 [virtualenv/create/pyenv_cfg.py:34]
py: 1291 D 	virtualenv = 20.17.1 [virtualenv/create/pyenv_cfg.py:34]
py: 1292 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
py: 1292 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
py: 1292 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
py: 1292 D 	base-executable = /usr/local/bin/python3.9 [virtualenv/create/pyenv_cfg.py:34]
py: 1293 D symlink /usr/local/bin/python3.9 to /home/example/.tox/py/bin/python [virtualenv/util/path/_sync.py:28]
py: 1294 D create virtualenv import hook file /home/example/.tox/py/lib/python3.9/site-packages/_virtualenv.pth [virtualenv/create/via_global_ref/api.py:89]
py: 1295 D create /home/example/.tox/py/lib/python3.9/site-packages/_virtualenv.py [virtualenv/create/via_global_ref/api.py:92]
py: 1295 D ============================== target debug ============================== [virtualenv/run/session.py:50]
py: 1296 D debug via /home/example/.tox/py/bin/python /home/example/.venv/lib/python3.9/site-packages/virtualenv/create/debug.py [virtualenv/create/creator.py:197]
py: 1296 D {
  "sys": {
    "executable": "/home/example/.tox/py/bin/python",
    "_base_executable": "/home/example/.tox/py/bin/python",
    "prefix": "/home/example/.tox/py",
    "base_prefix": "/usr/local",
    "real_prefix": null,
    "exec_prefix": "/home/example/.tox/py",
    "base_exec_prefix": "/usr/local",
    "path": [
      "/usr/local/lib/python39.zip",
      "/usr/local/lib/python3.9",
      "/usr/local/lib/python3.9/lib-dynload",
      "/home/example/.tox/py/lib/python3.9/site-packages"
    ],
    "meta_path": [
      "<class '_virtualenv._Finder'>",
      "<class '_frozen_importlib.BuiltinImporter'>",
      "<class '_frozen_importlib.FrozenImporter'>",
      "<class '_frozen_importlib_external.PathFinder'>"
    ],
    "fs_encoding": "utf-8",
    "io_encoding": "utf-8"
  },
  "version": "3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]",
  "makefile_filename": "/usr/local/lib/python3.9/config-3.9-x86_64-linux-gnu/Makefile",
  "os": "<module 'os' from '/usr/local/lib/python3.9/os.py'>",
  "site": "<module 'site' from '/usr/local/lib/python3.9/site.py'>",
  "datetime": "<module 'datetime' from '/usr/local/lib/python3.9/datetime.py'>",
  "math": "<module 'math' from '/usr/local/lib/python3.9/lib-dynload/math.cpython-39-x86_64-linux-gnu.so'>",
  "json": "<module 'json' from '/usr/local/lib/python3.9/json/__init__.py'>"
} [virtualenv/run/session.py:51]
py: 1410 I add seed packages via FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/example/.local/share/virtualenv) [virtualenv/run/session.py:55]
py: 1418 D install pip from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/pip-22.3.1-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
py: 1418 D install setuptools from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/setuptools-65.6.3-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
py: 1424 D build install image for setuptools-65.6.3-py3-none-any.whl to /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any [virtualenv/seed/embed/via_app_data/pip_install/base.py:45]
py: 1422 D build install image for pip-22.3.1-py3-none-any.whl to /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any [virtualenv/seed/embed/via_app_data/pip_install/base.py:45]
py: 1420 D install wheel from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/wheel-0.38.4-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
py: 1462 D build install image for wheel-0.38.4-py3-none-any.whl to /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any [virtualenv/seed/embed/via_app_data/pip_install/base.py:45]
py: 1542 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.dist-info to /home/example/.tox/py/lib/python3.9/site-packages/wheel-0.38.4.dist-info [virtualenv/util/path/_sync.py:36]
py: 1565 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel to /home/example/.tox/py/lib/python3.9/site-packages/wheel [virtualenv/util/path/_sync.py:36]
py: 1580 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.virtualenv to /home/example/.tox/py/lib/python3.9/site-packages/wheel-0.38.4.virtualenv [virtualenv/util/path/_sync.py:36]
py: 1595 D generated console scripts wheel wheel-3.9 wheel3 wheel3.9 [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
py: 1890 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools-65.6.3.virtualenv to /home/example/.tox/py/lib/python3.9/site-packages/setuptools-65.6.3.virtualenv [virtualenv/util/path/_sync.py:36]
py: 1892 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/distutils-precedence.pth to /home/example/.tox/py/lib/python3.9/site-packages/distutils-precedence.pth [virtualenv/util/path/_sync.py:36]
py: 1893 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools to /home/example/.tox/py/lib/python3.9/site-packages/setuptools [virtualenv/util/path/_sync.py:36]
py: 2080 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools-65.6.3.dist-info to /home/example/.tox/py/lib/python3.9/site-packages/setuptools-65.6.3.dist-info [virtualenv/util/path/_sync.py:36]
py: 2084 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/pkg_resources to /home/example/.tox/py/lib/python3.9/site-packages/pkg_resources [virtualenv/util/path/_sync.py:36]
py: 2148 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/_distutils_hack to /home/example/.tox/py/lib/python3.9/site-packages/_distutils_hack [virtualenv/util/path/_sync.py:36]
py: 2150 D generated console scripts  [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
py: 2252 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip to /home/example/.tox/py/lib/python3.9/site-packages/pip [virtualenv/util/path/_sync.py:36]
py: 2471 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip-22.3.1.dist-info to /home/example/.tox/py/lib/python3.9/site-packages/pip-22.3.1.dist-info [virtualenv/util/path/_sync.py:36]
py: 2475 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip-22.3.1.virtualenv to /home/example/.tox/py/lib/python3.9/site-packages/pip-22.3.1.virtualenv [virtualenv/util/path/_sync.py:36]
py: 2477 D generated console scripts pip pip-3.9 pip3 pip3.9 [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
py: 2478 I add activators for Bash, CShell, Fish, Nushell, PowerShell, Python [virtualenv/run/session.py:61]
py: 2483 D write /home/example/.tox/py/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
py: 2489 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
py: 2490 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
py: 2491 D 	version_info = 3.9.16.final.0 [virtualenv/create/pyenv_cfg.py:34]
py: 2492 D 	virtualenv = 20.17.1 [virtualenv/create/pyenv_cfg.py:34]
py: 2493 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
py: 2494 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
py: 2494 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
py: 2494 D 	base-executable = /usr/local/bin/python3.9 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2508 I find interpreter for spec PythonSpec(path=/home/example/.venv/bin/python3) [virtualenv/discovery/builtin.py:56]
.pkg: 2509 D discover exe from cache /usr/local - exact False: PythonInfo({'platform': 'linux', 'implementation': 'CPython', 'version_info': VersionInfo(major=3, minor=9, micro=16, releaselevel='final', serial=0), 'architecture': 64, 'version_nodot': '39', 'version': '3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', 'os': 'posix', 'prefix': '/usr/local', 'base_prefix': '/usr/local', 'real_prefix': None, 'base_exec_prefix': '/usr/local', 'exec_prefix': '/usr/local', 'executable': '/home/example/.venv/bin/python3', 'original_executable': '/usr/local/bin/python3.9', 'system_executable': '/usr/local/bin/python3.9', 'has_venv': True, 'path': ['/home/example/.venv/lib/python3.9/site-packages/virtualenv/discovery', '/usr/local/lib/python39.zip', '/usr/local/lib/python3.9', '/usr/local/lib/python3.9/lib-dynload', '/usr/local/lib/python3.9/site-packages'], 'file_system_encoding': 'utf-8', 'stdout_encoding': 'utf-8', 'sysconfig_scheme': None, 'sysconfig_paths': {'stdlib': '{installed_base}/{platlibdir}/python{py_version_short}', 'platstdlib': '{platbase}/{platlibdir}/python{py_version_short}', 'purelib': '{base}/lib/python{py_version_short}/site-packages', 'platlib': '{platbase}/{platlibdir}/python{py_version_short}/site-packages', 'include': '{installed_base}/include/python{py_version_short}{abiflags}', 'scripts': '{base}/bin', 'data': '{base}'}, 'distutils_install': {'purelib': 'lib/python3.9/site-packages', 'platlib': 'lib/python3.9/site-packages', 'headers': 'include/python3.9/UNKNOWN', 'scripts': 'bin', 'data': ''}, 'sysconfig': {'makefile_filename': '/usr/local/lib/python3.9/config-3.9-x86_64-linux-gnu/Makefile'}, 'sysconfig_vars': {'installed_base': '/usr/local', 'base': '/usr/local', 'platlibdir': 'lib', 'platbase': '/usr/local', 'abiflags': '', 'PYTHONFRAMEWORK': '', 'py_version_short': '3.9'}, 'system_stdlib': '/usr/local/lib/python3.9', 'system_stdlib_platform': '/usr/local/lib/python3.9', 'max_size': 9223372036854775807}) [virtualenv/discovery/py_info.py:435]
.pkg: 2509 I proposed PythonInfo(spec=CPython3.9.16.final.0-64, system=/usr/local/bin/python3.9, exe=/home/example/.venv/bin/python3, platform=linux, version='3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:63]
.pkg: 2510 D accepted PythonInfo(spec=CPython3.9.16.final.0-64, system=/usr/local/bin/python3.9, exe=/home/example/.venv/bin/python3, platform=linux, version='3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]', encoding_fs_io=utf-8-utf-8) [virtualenv/discovery/builtin.py:65]
.pkg: 2515 I create virtual environment via CPython3Posix(dest=/home/example/.tox/.pkg, clear=False, no_vcs_ignore=False, global=False) [virtualenv/run/session.py:48]
.pkg: 2516 D create folder /home/example/.tox/.pkg/bin [virtualenv/util/path/_sync.py:9]
.pkg: 2517 D create folder /home/example/.tox/.pkg/lib/python3.9/site-packages [virtualenv/util/path/_sync.py:9]
.pkg: 2517 D write /home/example/.tox/.pkg/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
.pkg: 2518 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2518 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2518 D 	version_info = 3.9.16.final.0 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2518 D 	virtualenv = 20.17.1 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2518 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2518 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2519 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2519 D 	base-executable = /usr/local/bin/python3.9 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 2519 D symlink /usr/local/bin/python3.9 to /home/example/.tox/.pkg/bin/python [virtualenv/util/path/_sync.py:28]
.pkg: 2520 D create virtualenv import hook file /home/example/.tox/.pkg/lib/python3.9/site-packages/_virtualenv.pth [virtualenv/create/via_global_ref/api.py:89]
.pkg: 2521 D create /home/example/.tox/.pkg/lib/python3.9/site-packages/_virtualenv.py [virtualenv/create/via_global_ref/api.py:92]
.pkg: 2522 D ============================== target debug ============================== [virtualenv/run/session.py:50]
.pkg: 2522 D debug via /home/example/.tox/.pkg/bin/python /home/example/.venv/lib/python3.9/site-packages/virtualenv/create/debug.py [virtualenv/create/creator.py:197]
.pkg: 2522 D {
  "sys": {
    "executable": "/home/example/.tox/.pkg/bin/python",
    "_base_executable": "/home/example/.tox/.pkg/bin/python",
    "prefix": "/home/example/.tox/.pkg",
    "base_prefix": "/usr/local",
    "real_prefix": null,
    "exec_prefix": "/home/example/.tox/.pkg",
    "base_exec_prefix": "/usr/local",
    "path": [
      "/usr/local/lib/python39.zip",
      "/usr/local/lib/python3.9",
      "/usr/local/lib/python3.9/lib-dynload",
      "/home/example/.tox/.pkg/lib/python3.9/site-packages"
    ],
    "meta_path": [
      "<class '_virtualenv._Finder'>",
      "<class '_frozen_importlib.BuiltinImporter'>",
      "<class '_frozen_importlib.FrozenImporter'>",
      "<class '_frozen_importlib_external.PathFinder'>"
    ],
    "fs_encoding": "utf-8",
    "io_encoding": "utf-8"
  },
  "version": "3.9.16 (main, Dec  8 2022, 02:40:11) \n[GCC 10.2.1 20210110]",
  "makefile_filename": "/usr/local/lib/python3.9/config-3.9-x86_64-linux-gnu/Makefile",
  "os": "<module 'os' from '/usr/local/lib/python3.9/os.py'>",
  "site": "<module 'site' from '/usr/local/lib/python3.9/site.py'>",
  "datetime": "<module 'datetime' from '/usr/local/lib/python3.9/datetime.py'>",
  "math": "<module 'math' from '/usr/local/lib/python3.9/lib-dynload/math.cpython-39-x86_64-linux-gnu.so'>",
  "json": "<module 'json' from '/usr/local/lib/python3.9/json/__init__.py'>"
} [virtualenv/run/session.py:51]
.pkg: 2635 I add seed packages via FromAppData(download=False, pip=bundle, setuptools=bundle, wheel=bundle, via=copy, app_data_dir=/home/example/.local/share/virtualenv) [virtualenv/run/session.py:55]
.pkg: 2651 D install setuptools from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/setuptools-65.6.3-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
.pkg: 2654 D install wheel from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/wheel-0.38.4-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
.pkg: 2655 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools-65.6.3.virtualenv to /home/example/.tox/.pkg/lib/python3.9/site-packages/setuptools-65.6.3.virtualenv [virtualenv/util/path/_sync.py:36]
.pkg: 2655 D install pip from wheel /home/example/.venv/lib/python3.9/site-packages/virtualenv/seed/wheels/embed/pip-22.3.1-py3-none-any.whl via CopyPipInstall [virtualenv/seed/embed/via_app_data/via_app_data.py:47]
.pkg: 2661 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip to /home/example/.tox/.pkg/lib/python3.9/site-packages/pip [virtualenv/util/path/_sync.py:36]
.pkg: 2659 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/distutils-precedence.pth to /home/example/.tox/.pkg/lib/python3.9/site-packages/distutils-precedence.pth [virtualenv/util/path/_sync.py:36]
.pkg: 2667 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools to /home/example/.tox/.pkg/lib/python3.9/site-packages/setuptools [virtualenv/util/path/_sync.py:36]
.pkg: 2675 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.dist-info to /home/example/.tox/.pkg/lib/python3.9/site-packages/wheel-0.38.4.dist-info [virtualenv/util/path/_sync.py:36]
.pkg: 2684 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel to /home/example/.tox/.pkg/lib/python3.9/site-packages/wheel [virtualenv/util/path/_sync.py:36]
.pkg: 2721 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/wheel-0.38.4-py3-none-any/wheel-0.38.4.virtualenv to /home/example/.tox/.pkg/lib/python3.9/site-packages/wheel-0.38.4.virtualenv [virtualenv/util/path/_sync.py:36]
.pkg: 2730 D generated console scripts wheel-3.9 wheel3 wheel3.9 wheel [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
.pkg: 2897 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/setuptools-65.6.3.dist-info to /home/example/.tox/.pkg/lib/python3.9/site-packages/setuptools-65.6.3.dist-info [virtualenv/util/path/_sync.py:36]
.pkg: 2905 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/pkg_resources to /home/example/.tox/.pkg/lib/python3.9/site-packages/pkg_resources [virtualenv/util/path/_sync.py:36]
.pkg: 2936 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/setuptools-65.6.3-py3-none-any/_distutils_hack to /home/example/.tox/.pkg/lib/python3.9/site-packages/_distutils_hack [virtualenv/util/path/_sync.py:36]
.pkg: 2941 D generated console scripts  [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
.pkg: 3069 D copy directory /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip-22.3.1.dist-info to /home/example/.tox/.pkg/lib/python3.9/site-packages/pip-22.3.1.dist-info [virtualenv/util/path/_sync.py:36]
.pkg: 3073 D copy /home/example/.local/share/virtualenv/wheel/3.9/image/1/CopyPipInstall/pip-22.3.1-py3-none-any/pip-22.3.1.virtualenv to /home/example/.tox/.pkg/lib/python3.9/site-packages/pip-22.3.1.virtualenv [virtualenv/util/path/_sync.py:36]
.pkg: 3076 D generated console scripts pip pip-3.9 pip3 pip3.9 [virtualenv/seed/embed/via_app_data/pip_install/base.py:41]
.pkg: 3076 I add activators for Bash, CShell, Fish, Nushell, PowerShell, Python [virtualenv/run/session.py:61]
.pkg: 3081 D write /home/example/.tox/.pkg/pyvenv.cfg [virtualenv/create/pyenv_cfg.py:30]
.pkg: 3081 D 	home = /usr/local/bin [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3082 D 	implementation = CPython [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3083 D 	version_info = 3.9.16.final.0 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3083 D 	virtualenv = 20.17.1 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3083 D 	include-system-site-packages = false [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3084 D 	base-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3084 D 	base-exec-prefix = /usr/local [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3085 D 	base-executable = /usr/local/bin/python3.9 [virtualenv/create/pyenv_cfg.py:34]
.pkg: 3090 W install_requires> python -I -m pip install setuptools [tox/tox_env/api.py:408]
Requirement already satisfied: setuptools in ./.tox/.pkg/lib/python3.9/site-packages (65.6.3)
.pkg: 7266 I exit 0 (4.17 seconds) /home/example> python -I -m pip install setuptools pid=477 [tox/execute/api.py:275]
.pkg: 7269 W _optional_hooks> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta [tox/tox_env/api.py:408]
Backend: run command _optional_hooks with args {}
Backend: Wrote response {'return': {'get_requires_for_build_sdist': True, 'prepare_metadata_for_build_wheel': True, 'get_requires_for_build_wheel': True, 'build_editable': True, 'get_requires_for_build_editable': True, 'prepare_metadata_for_build_editable': True}} to /tmp/pep517__optional_hooks-5rldeusq.json
.pkg: 8943 I exit None (1.67 seconds) /home/example> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta pid=485 [tox/execute/api.py:275]
.pkg: 8946 W get_requires_for_build_sdist> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta [tox/tox_env/api.py:408]
Backend: run command get_requires_for_build_sdist with args {'config_settings': None}
running egg_info
creating UNKNOWN.egg-info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
Backend: Wrote response {'return': []} to /tmp/pep517_get_requires_for_build_sdist-b4kog6hw.json
.pkg: 9098 I exit None (0.15 seconds) /home/example> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta pid=485 [tox/execute/api.py:275]
.pkg: 9102 W prepare_metadata_for_build_wheel> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta [tox/tox_env/api.py:408]
Backend: run command prepare_metadata_for_build_wheel with args {'metadata_directory': '/home/example/.tox/.pkg/.meta', 'config_settings': {'--build-option': []}}
running dist_info
creating /home/example/.tox/.pkg/.meta/UNKNOWN.egg-info
writing /home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/PKG-INFO
writing dependency_links to /home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/dependency_links.txt
writing top-level names to /home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/top_level.txt
writing manifest file '/home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/SOURCES.txt'
reading manifest file '/home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/SOURCES.txt'
writing manifest file '/home/example/.tox/.pkg/.meta/UNKNOWN.egg-info/SOURCES.txt'
creating '/home/example/.tox/.pkg/.meta/UNKNOWN-0.0.0.dist-info'
Backend: Wrote response {'return': 'UNKNOWN-0.0.0.dist-info'} to /tmp/pep517_prepare_metadata_for_build_wheel-li5bj951.json
.pkg: 9314 I exit None (0.21 seconds) /home/example> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta pid=485 [tox/execute/api.py:275]
.pkg: 9319 W build_sdist> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta [tox/tox_env/api.py:408]
Backend: run command build_sdist with args {'sdist_directory': '/home/example/.tox/.pkg/dist', 'config_settings': None}
running sdist
running egg_info
writing UNKNOWN.egg-info/PKG-INFO
writing dependency_links to UNKNOWN.egg-info/dependency_links.txt
writing top-level names to UNKNOWN.egg-info/top_level.txt
reading manifest file 'UNKNOWN.egg-info/SOURCES.txt'
writing manifest file 'UNKNOWN.egg-info/SOURCES.txt'
warning: sdist: standard file not found: should have one of README, README.rst, README.txt, README.md

running check
warning: check: missing required meta-data: name

creating UNKNOWN-0.0.0
creating UNKNOWN-0.0.0/UNKNOWN.egg-info
copying files to UNKNOWN-0.0.0...
copying pyproject.toml -> UNKNOWN-0.0.0
copying UNKNOWN.egg-info/PKG-INFO -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/SOURCES.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/dependency_links.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
copying UNKNOWN.egg-info/top_level.txt -> UNKNOWN-0.0.0/UNKNOWN.egg-info
Writing UNKNOWN-0.0.0/setup.cfg
Creating tar archive
removing 'UNKNOWN-0.0.0' (and everything under it)
Backend: Wrote response {'return': 'UNKNOWN-0.0.0.tar.gz'} to /tmp/pep517_build_sdist-7nqesrzb.json
.pkg: 9455 I exit None (0.13 seconds) /home/example> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta pid=485 [tox/execute/api.py:275]
.pkg: 9457 D package .tmp/package/1/UNKNOWN-0.0.0.tar.gz links to .pkg/dist/UNKNOWN-0.0.0.tar.gz (/home/example/.tox) [tox/util/file_view.py:36]
py: 9460 E internal error [tox/session/cmd/run/single.py:57]
Traceback (most recent call last):
  File "/home/example/.venv/lib/python3.9/site-packages/tox/session/cmd/run/single.py", line 45, in _evaluate
    tox_env.setup()
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/api.py", line 237, in setup
    self._setup_with_env()
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/runner.py", line 149, in _setup_with_env
    self._setup_pkg()
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/runner.py", line 173, in _setup_pkg
    self.installer.install(self._packages, RunToxEnv.__name__, "package")
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/python/pip/pip_install.py", line 85, in install
    self._install_list_of_deps(arguments, section, of_type)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/python/pip/pip_install.py", line 157, in _install_list_of_deps
    self._execute_installer(install_args + groups["pkg"], of_type)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/python/pip/pip_install.py", line 164, in _execute_installer
    cmd = self.build_install_cmd(deps)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/tox_env/python/pip/pip_install.py", line 169, in build_install_cmd
    cmd: Command = self._env.conf["install_command"]
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/sets.py", line 114, in __getitem__
    return self.load(item)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/sets.py", line 125, in load
    return config_definition.__call__(self._conf, self.loaders, ConfigLoadArgs(chain, self.name, self.env_name))
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/of_type.py", line 102, in __call__
    value = loader.load(key, self.of_type, self.factory, conf, args)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/loader/api.py", line 127, in load
    converted = self.to(prepared, of_type, factory)
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/loader/convert.py", line 44, in to
    return self.to_command(raw)  # type: ignore[return-value]
  File "/home/example/.venv/lib/python3.9/site-packages/tox/config/loader/str_convert.py", line 66, in to_command
    if args[0] != "-" and args[0].startswith("-"):
IndexError: list index out of range
.pkg: 9467 W _exit> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta [tox/tox_env/api.py:408]
Backend: run command _exit with args {}
Backend: Wrote response {'return': 0} to /tmp/pep517__exit-mluwuv97.json
.pkg: 9485 I exit None (0.02 seconds) /home/example> python /home/example/.venv/lib/python3.9/site-packages/pyproject_api/_backend.py True setuptools.build_meta pid=485 [tox/execute/api.py:275]
.pkg: 9595 D delete package /home/example/.tox/.tmp/package/1/UNKNOWN-0.0.0.tar.gz [tox/tox_env/python/virtual_env/package/pyproject.py:171]
  py: FAIL code 2 (8.79 seconds)
  evaluation failed :( (9.12 seconds)

Minimal example

If possible, provide a minimal reproducer for the issue:

tox.ini

[tox]
isolated_build = true

[testenv]
install_command =
    foo: true

(note: a valid pyproject.toml / setup.py file is required alongside this – I’ve been using the example pyproject.toml provided below)

pyproject.toml

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"