Add a "shared cache" directory (#476)

* Add a "shared cache" directory

Add an "{envdir}/.shared" directory, and create an API for accesing to it.

* Don't fail  if the ".shared" directory exists

Don't raise an exception.

* Update sessions.py

Move the "shared cache" API inside of nox.sessions.Session

* Update sessions.py

Convert the "shared cache" path into a property. Now, it returns a "pathlib.Path".

* Update test_sessions.py

Make a test for the recent changes.

* Update sessions.py

Use the "pathlib.Path" methods to reduce the variable usage.

* Update test_sessions.py

Make some modifications to the tests.

* Update test_sessions.py

Fix an import error.

* Update sessions.py

Use the parent directory to create the cache dir.

* Update test_sessions.py

Use a tempfile to test the session properties.

* Fix test indent

* Update nox/sessions.py

This avoids some unnecessary path manipulations, and using the parent directory of a virtualenv which does not necessarily exist (`PassthroughEnv` does not create a virtualenv).

Co-authored-by: Claudio Jolowicz <cjolowicz@gmail.com>

* Update sessions.py

Use ".cache" instead of ".shared".

* Update test_sessions.py

Use ".cache" instead of ".shared".

Co-authored-by: Tom Fleet <tomfleet2018@gmail.com>
Co-authored-by: Claudio Jolowicz <cjolowicz@gmail.com>

nox/sessions.py

@@ -16,6 +16,7 @@
 import enum
 import hashlib
 import os
+import pathlib
 import re
 import sys
 import unicodedata
@@ -184,6 +185,13 @@ def create_tmp(self) -> str:
         self.env["TMPDIR"] = tmpdir
         return tmpdir
 
+    @property
+    def cache_dir(self) -> pathlib.Path:
+        """Create and return a 'shared cache' directory to be used across sessions."""
+        path = pathlib.Path(self._runner.global_config.envdir).joinpath(".cache")
+        path.mkdir(exist_ok=True)
+        return path
+
     @property
     def interactive(self) -> bool:
         """Returns True if Nox is being run in an interactive session or False otherwise."""

tests/test_sessions.py

@@ -99,15 +99,20 @@ def test_create_tmp_twice(self):
 
     def test_properties(self):
         session, runner = self.make_session_and_runner()
+        with tempfile.TemporaryDirectory() as root:
+            runner.global_config.envdir = root
 
-        assert session.name is runner.friendly_name
-        assert session.env is runner.venv.env
-        assert session.posargs == runner.global_config.posargs
-        assert session.virtualenv is runner.venv
-        assert session.bin_paths is runner.venv.bin_paths
-        assert session.bin is runner.venv.bin_paths[0]
-        assert session.python is runner.func.python
-        assert session.invoked_from is runner.global_config.invoked_from
+            assert session.name is runner.friendly_name
+            assert session.env is runner.venv.env
+            assert session.posargs == runner.global_config.posargs
+            assert session.virtualenv is runner.venv
+            assert session.bin_paths is runner.venv.bin_paths
+            assert session.bin is runner.venv.bin_paths[0]
+            assert session.python is runner.func.python
+            assert session.invoked_from is runner.global_config.invoked_from
+            assert session.cache_dir == Path(runner.global_config.envdir).joinpath(
+                ".cache"
+            )
 
     def test_no_bin_paths(self):
         session, runner = self.make_session_and_runner()