Make ephemeral exec, venv repl and scie compatible (#3129)

Fix so that scie pex (zipapp, venv) can 1. drop into repl and 2. run in
ephemeral run mode via `-- -c`.

```shell
#!/usr/bin/env bash
set -euo pipefail

TMPDIR=$(mktemp -d)
PEX_ROOT="$TMPDIR/pex_root"
uv run python -m pex . -c pex -o "$TMPDIR/pex.pex" --scie eager --venv --pex-root "$PEX_ROOT" --runtime-pex-root "$PEX_ROOT" 2>&1 | tail -1

echo "=== Venv scie: -- -c ==="
"$TMPDIR/pex" --pex-root "$PEX_ROOT" --runtime-pex-root "$PEX_ROOT" --interpreter-constraint 'CPython>=3.12' -- -c 'import sys; print(sys.executable)' 2>&1
echo "EXIT: $?"

echo ""
echo "=== Venv scie: REPL ==="
echo 'import sys; print("REPL:", sys.executable); quit()' | "$TMPDIR/pex" --pex-root "$PEX_ROOT" --runtime-pex-root "$PEX_ROOT" --interpreter-constraint 'CPython>=3.12' -- 2>&1
echo "EXIT: $?"

echo ""
echo "=== Non-venv scie: -- -c ==="
TMPDIR2=$(mktemp -d)
uv run python -m pex . -c pex -o "$TMPDIR2/pex.pex" --scie eager 2>&1 | tail -1
"$TMPDIR2/pex" --interpreter-constraint 'CPython>=3.12' -- -c 'import sys; print(sys.executable)' 2>&1
echo "EXIT: $?"
```

This gives me on branch:
```console
(pex-dev) ➜  pex git:(fix/scie-ephemeral-run-mode) ✗ bash test_scie_ephemeral.sh
/tmp/tmp.Uj3pbC1P18/pex
=== Venv scie: -- -c ===
/tmp/tmp.Uj3pbC1P18/pex_root/scies/0/base/86232383051a0950a74a9486949c87f88da45978bafa8d826c2f268629a3f508/cpython-3.12.13+20260325-x86_64-unknown-linux-gnu-install_only.tar.gz/python/bin/python3.12
EXIT: 0

=== Venv scie: REPL ===
Pex 2.91.4 ephemeral hermetic environment with no dependencies.
Python 3.12.13 (main, Mar 24 2026, 22:49:22) [Clang 22.1.1 ] on linux
Type "help", "pex", "copyright", "credits" or "license" for more information.
>>> REPL: /tmp/tmp.Uj3pbC1P18/pex_root/scies/0/base/86232383051a0950a74a9486949c87f88da45978bafa8d826c2f268629a3f508/cpython-3.12.13+20260325-x86_64-unknown-linux-gnu-install_only.tar.gz/python/bin/python3.12
EXIT: 0

=== Non-venv scie: -- -c ===
/tmp/tmp.3ibEcHDvMR/pex
/home/tobias/.cache/nce/86232383051a0950a74a9486949c87f88da45978bafa8d826c2f268629a3f508/cpython-3.12.13+20260325-x86_64-unknown-linux-gnu-install_only.tar.gz/python/bin/python3.12
EXIT: 0
```

On main ephemeral exec and venv+repl fail in different ways:

```console
(pex-dev) ➜  pex git:(main) ✗ bash test_scie_ephemeral.sh
/tmp/tmp.tPRXEL8YLa/pex
=== Venv scie: -- -c ===
Ignoring the following environment variables in Pex venv mode:
_PEX_CLI_RUN=/tmp/tmp.tPRXEL8YLa/pex
__PEX_ENTRY_POINT__=/tmp/tmp.tPRXEL8YLa/pex_root/venvs/3/d806b12a09b8ad5281bf3d990cb91bfb694c1be1/414e2ae92b4573e3049bcfe135ed1296986a612d
Could not find script 'import sys; print(sys.executable)' in any distribution  within PEX!
EXIT: 1

=== Venv scie: REPL ===
Ignoring the following environment variables in Pex venv mode:
_PEX_CLI_RUN=/tmp/tmp.tPRXEL8YLa/pex
__PEX_ENTRY_POINT__=/tmp/tmp.tPRXEL8YLa/pex_root/venvs/3/d806b12a09b8ad5281bf3d990cb91bfb694c1be1/414e2ae92b4573e3049bcfe135ed1296986a612d
Ignoring the following environment variables in Pex venv mode:
_PEX_CLI_RUN=/tmp/tmp.tPRXEL8YLa/pex
_PEX_CLI_RUN_NO_ARGS=/tmp/tmp.tPRXEL8YLa/pex
__PEX_ENTRY_POINT__=/tmp/tmp.tPRXEL8YLa/pex_root/venvs/3/d806b12a09b8ad5281bf3d990cb91bfb694c1be1/414e2ae92b4573e3049bcfe135ed1296986a612d
... # this loops forever

=== Non-venv scie: -- -c ===
/tmp/tmp.seG0un8tmn/pex
Could not find script 'import sys; print(sys.executable)' in any distribution  within PEX!
EXIT: 1
```

---------

Co-authored-by: John Sirois <john.sirois@gmail.com>

Author: tobni

CHANGES.md

@@ -1,5 +1,11 @@
 # Release Notes
 
+## 2.91.5
+
+This release fixes Pex PEX scie behavior to match Pex.
+
+* Make ephemeral exec, venv repl and scie compatible (#3129)
+
 ## 2.91.4
 
 This release brings 2 performance fixes from @tobni:

pex/bin/pex.py

@@ -1289,6 +1289,11 @@ def main(args=None):
 
     try:
         with global_environment(options) as env:
+            # These are set if we're running from within a Pex PEX scie and already installed.
+            # We clear them to allow recursive use of Pex / PEX.
+            env.pop("__PEX_EXE__", None)
+            env.pop("__PEX_ENTRY_POINT__", None)
+
             try:
                 resolver_configuration = resolver_options.configure(
                     options,

pex/pex_boot.py

@@ -229,12 +229,11 @@ def boot(
             if os.path.isfile(pex_exe):
                 sys.argv[0] = pex_exe
 
-    overridden_entry_point = os.environ.get("__PEX_ENTRY_POINT__", None)
+    overridden_pex = os.environ.get("__PEX_EXE__", None)
     sys.path[0] = os.path.abspath(sys.path[0])
-    sys.path.insert(
-        0, os.path.abspath(os.path.join(overridden_entry_point or entry_point, bootstrap_dir))
-    )
+    sys.path.insert(0, os.path.abspath(os.path.join(overridden_pex or entry_point, bootstrap_dir)))
 
+    overridden_entry_point = os.environ.get("__PEX_ENTRY_POINT__", None)
     if overridden_entry_point and overridden_entry_point != entry_point:
         # This PEX has already been installed out of band; so we short-circuit to execute the
         # pre-installed PEX.

pex/version.py

@@ -1,4 +1,4 @@
 # Copyright 2015 Pex project contributors.
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-__version__ = "2.91.4"
+__version__ = "2.91.5"

tests/integration/cli/commands/test_lock_subset.py

@@ -139,7 +139,7 @@ def test_lock_subset_miss(lock):
     # type: (str) -> None
 
     _, original_locked_reqs = index(lock)
-    requests_version = original_locked_reqs[ProjectName("requests")].pin.version
+    requests_version = original_locked_reqs[ProjectName("requests")].pin.version.raw
     run_pex3(
         "lock", "subset", "--lock", lock, "requests!={version}".format(version=requests_version)
     ).assert_failure(

tests/integration/scie/test_scie_ephemeral_run.py

@@ -0,0 +1,68 @@
+# Copyright 2026 Pex project contributors.
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import absolute_import
+
+import subprocess
+import sys
+
+import pytest
+
+from pex.typing import TYPE_CHECKING
+from testing import IS_PYPY, make_env, run_pex_command
+from testing.pytest_utils.tmp import Tempdir
+from testing.scie import skip_if_no_provider
+
+if TYPE_CHECKING:
+    from typing import List
+
+
+@pytest.mark.parametrize(
+    "execution_mode_args",
+    [
+        pytest.param([], id="ZIPAPP"),
+        pytest.param(["--venv"], id="VENV"),
+    ],
+)
+@skip_if_no_provider
+def test_scie_ephemeral_run(
+    tmpdir,  # type: Tempdir
+    pex_wheel,  # type: str
+    execution_mode_args,  # type: List[str]
+):
+    # type: (...) -> None
+
+    pex_scie = tmpdir.join("pex")
+    run_pex_command(
+        args=[pex_wheel, "-c", "pex", "-o", pex_scie, "--scie", "eager"] + execution_mode_args
+    ).assert_success()
+
+    ic = "{impl}=={major}.{minor}.*".format(
+        impl="PyPy" if IS_PYPY else "CPython", major=sys.version_info[0], minor=sys.version_info[1]
+    )
+
+    # Verify the scie can perform an ephemeral run with `-- -c`.
+    output = subprocess.check_output(
+        args=[
+            pex_scie,
+            "--interpreter-constraint",
+            ic,
+            "--",
+            "-c",
+            "import sys; print(sys.executable)",
+        ],
+        env=make_env(PATH=None),
+    )
+    assert output.decode("utf-8").strip()
+
+    # Verify the scie can drop into a REPL via ephemeral run.
+    process = subprocess.Popen(
+        args=[pex_scie, "--interpreter-constraint", ic, "--"],
+        stdin=subprocess.PIPE,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        env=make_env(PATH=None),
+    )
+    stdout, stderr = process.communicate(input=b"import sys; print(sys.executable)\nquit()\n")
+    assert process.returncode == 0, stderr.decode("utf-8")
+    assert b">>>" in stdout or b">>>" in stderr