Extract venv management from test_installation

- Create and use a test.lib.helper.VirtualEnvironment class.
- Import and use venv module instead of running "python -m venv".

These changes make no significant difference in speed or clarity
for the existing test in test_installation. The reason for this
change is instead to support the use of new per-test virtual
environments in at least one other test.

test/lib/helper.py

@@ -14,6 +14,7 @@
 import textwrap
 import time
 import unittest
+import venv
 
 import gitdb
 
@@ -36,6 +37,7 @@
     "with_rw_repo",
     "with_rw_and_rw_remote_repo",
     "TestBase",
+    "VirtualEnvironment",
     "TestCase",
     "SkipTest",
     "skipIf",
@@ -390,3 +392,42 @@ def _make_file(self, rela_path, data, repo=None):
         with open(abs_path, "w") as fp:
             fp.write(data)
         return abs_path
+
+
+class VirtualEnvironment:
+    """A newly created Python virtual environment for use in a test."""
+
+    __slots__ = ("_env_dir",)
+
+    def __init__(self, env_dir, *, with_pip):
+        self._env_dir = env_dir
+        venv.create(env_dir, symlinks=(os.name != "nt"), with_pip=with_pip)
+
+    @property
+    def env_dir(self):
+        """The top-level directory of the environment."""
+        return self._env_dir
+
+    @property
+    def python(self):
+        """Path to the Python executable in the environment."""
+        return self._executable("python")
+
+    @property
+    def pip(self):
+        """Path to the pip executable in the environment, or RuntimeError if absent."""
+        return self._executable("pip")
+
+    @property
+    def sources(self):
+        """Path to a src directory in the environment, which may not exist yet."""
+        return os.path.join(self.env_dir, "src")
+
+    def _executable(self, basename):
+        if os.name == "nt":
+            path = osp.join(self.env_dir, "Scripts", basename + ".exe")
+        else:
+            path = osp.join(self.env_dir, "bin", basename)
+        if osp.isfile(path) or osp.islink(path):
+            return path
+        raise RuntimeError(f"no regular file or symlink {path!r}")

test/test_installation.py

@@ -4,31 +4,19 @@
 import ast
 import os
 import subprocess
-import sys
 
-from test.lib import TestBase
-from test.lib.helper import with_rw_directory
+from test.lib import TestBase, VirtualEnvironment, with_rw_directory
 
 
 class TestInstallation(TestBase):
-    def setUp_venv(self, rw_dir):
-        self.venv = rw_dir
-        subprocess.run([sys.executable, "-m", "venv", self.venv], stdout=subprocess.PIPE)
-        bin_name = "Scripts" if os.name == "nt" else "bin"
-        self.python = os.path.join(self.venv, bin_name, "python")
-        self.pip = os.path.join(self.venv, bin_name, "pip")
-        self.sources = os.path.join(self.venv, "src")
-        self.cwd = os.path.dirname(os.path.dirname(__file__))
-        os.symlink(self.cwd, self.sources, target_is_directory=True)
-
     @with_rw_directory
     def test_installation(self, rw_dir):
-        self.setUp_venv(rw_dir)
+        venv = self._set_up_venv(rw_dir)
 
         result = subprocess.run(
-            [self.pip, "install", "."],
+            [venv.pip, "install", "."],
             stdout=subprocess.PIPE,
-            cwd=self.sources,
+            cwd=venv.sources,
         )
         self.assertEqual(
             0,
@@ -37,9 +25,9 @@ def test_installation(self, rw_dir):
         )
 
         result = subprocess.run(
-            [self.python, "-c", "import git"],
+            [venv.python, "-c", "import git"],
             stdout=subprocess.PIPE,
-            cwd=self.sources,
+            cwd=venv.sources,
         )
         self.assertEqual(
             0,
@@ -48,9 +36,9 @@ def test_installation(self, rw_dir):
         )
 
         result = subprocess.run(
-            [self.python, "-c", "import gitdb; import smmap"],
+            [venv.python, "-c", "import gitdb; import smmap"],
             stdout=subprocess.PIPE,
-            cwd=self.sources,
+            cwd=venv.sources,
         )
         self.assertEqual(
             0,
@@ -62,9 +50,9 @@ def test_installation(self, rw_dir):
         # by inserting its location into PYTHONPATH or otherwise patched into
         # sys.path, make sure it is not wrongly inserted as the *first* entry.
         result = subprocess.run(
-            [self.python, "-c", "import sys; import git; print(sys.path)"],
+            [venv.python, "-c", "import sys; import git; print(sys.path)"],
             stdout=subprocess.PIPE,
-            cwd=self.sources,
+            cwd=venv.sources,
         )
         syspath = result.stdout.decode("utf-8").splitlines()[0]
         syspath = ast.literal_eval(syspath)
@@ -73,3 +61,13 @@ def test_installation(self, rw_dir):
             syspath[0],
             msg="Failed to follow the conventions for https://docs.python.org/3/library/sys.html#sys.path",
         )
+
+    @staticmethod
+    def _set_up_venv(rw_dir):
+        venv = VirtualEnvironment(rw_dir, with_pip=True)
+        os.symlink(
+            os.path.dirname(os.path.dirname(__file__)),
+            venv.sources,
+            target_is_directory=True,
+        )
+        return venv