Better message when command parsing on empty input (#2807)

2 files changed, 6 insertions, 1 deletions

diff --git a/src/tox/config/loader/str_convert.py b/src/tox/config/loader/str_convert.py
index e31c034f..f07545f5 100644
--- a/src/tox/config/loader/str_convert.py
+++ b/src/tox/config/loader/str_convert.py
@@ -63,6 +63,8 @@ class StrConvert(Convert[str]):
                 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
diff --git a/src/tox/tox_env/python/pip/pip_install.py b/src/tox/tox_env/python/pip/pip_install.py
index a148ce32..2136e862 100644
--- a/src/tox/tox_env/python/pip/pip_install.py
+++ b/src/tox/tox_env/python/pip/pip_install.py
@@ -160,7 +160,10 @@ class Pip(Installer[Python]):
         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}")