Put the 'get_python_requires' venv cache in .tox/

Allow passing a cache directory explicitly to
`get_python_requires.py`. Subsequently, the wrapping script,
`ensure_min_python_is_tested.py` can insist on an env var which
provides the cache dir path.
Finally, `tox.ini` gets a final tweak to set this value via setenv to
the `envdir` (the virtualenv dir which also contains the tmp and log
dirs).

The choice of an env var over parsing an argument is merely that it is
marginally simpler to read an env var and fail on its absence than it
is to do the same with an argument. (At the very least, *any* argument
parsing opens the question of whether or not argparse should be used.)

scripts/ensure_min_python_is_tested.py

@@ -1,6 +1,8 @@
-#!/usr/bin/env python
-# requires that a YAML implementation is available in order to extract github
-# workflows data
+# this script can only be called via
+#    tox -e check-min-python-is-tested
+#
+# no other usages are supported
+import os
 import pathlib
 import subprocess
 import sys
@@ -17,8 +19,18 @@
 YAML = ruamel.yaml.YAML(typ="safe")
 REPO_ROOT = pathlib.Path(__file__).parent.parent
 
+try:
+    VENV_CACHE_DIR = os.environ["VENV_CACHE_DIR"]
+except KeyError:
+    raise RuntimeError(
+        "Cannot run ensure_min_python_is_tested.py without explicitly "
+        "setting VENV_CACHE dir. "
+        "Please ensure that you are invoking it with "
+        "'tox r -e check-min-python-is-tested'."
+    )
+
 proc = subprocess.run(
-    ["python", "scripts/get_python_requires.py"],
+    ["python", "scripts/get_python_requires.py", "--venv-cache-dir", VENV_CACHE_DIR],
     check=True,
     capture_output=True,
     cwd=REPO_ROOT,

scripts/get_python_requires.py

@@ -33,19 +33,26 @@
     help="How to output the python_requires data. "
     "The default ('lower-bound') mode will print only the lower bound version number.",
 )
+parser.add_argument(
+    "--venv-cache-dir",
+    default=None,
+    help="A directory in which the 'build' virtualenv will be kept for reuse.",
+)
 args = parser.parse_args()
 
-if platform.system() == "Windows":
-    win_cache_dir = os.getenv("LOCALAPPDATA", os.getenv("APPDATA"))
-    assert win_cache_dir, "cannot find APPDATA dir, cannot cache, abort!"
-    cache_dir = pathlib.Path(win_cache_dir)
+if args.venv_cache_dir:
+    VENV_CACHE_DIR = pathlib.Path(args.venv_cache_dir)
 else:
-    cache_dir = pathlib.Path.home() / ".cache"
-
-VENV_CACHE_DIR = cache_dir / "get_python_requires"
+    if platform.system() == "Windows":
+        win_cache_dir = os.getenv("LOCALAPPDATA", os.getenv("APPDATA"))
+        assert win_cache_dir, "cannot find APPDATA dir, cannot cache, abort!"
+        cache_dir = pathlib.Path(win_cache_dir)
+    else:
+        cache_dir = pathlib.Path.home() / ".cache"
+    VENV_CACHE_DIR = cache_dir / "get_python_requires"
 VENV_CACHE_DIR.mkdir(exist_ok=True)
 
-BUILD_VENV = VENV_CACHE_DIR / "venv-build"
+BUILD_VENV = VENV_CACHE_DIR / "get-python-requires-build"
 BUILD_PYTHON = str(BUILD_VENV / "bin" / "python")
 if BUILD_VENV.exists():
     if args.verbose:

tox.ini

@@ -105,6 +105,8 @@ description = Check the Requires-Python metadata against CI config
 skip_install = true
 deps = ruamel.yaml<0.18
 commands = python scripts/ensure_min_python_is_tested.py
+setenv =
+    VENV_CACHE_DIR={envdir}
 
 [testenv:prepare-release]
 skip_install = true