tests: Use sys.exec_prefix, not sys.executable, to determine the Python run

Virtualenv 20.0 follows symlinks when populating sys.executable,
so we can't use it. (AFAIUI it's done to better handle cases where the
system executable is changed after a virtualenv is created.)
See: https://github.com/pypa/virtualenv/pull/1522

sys.exec_prefix still points to inside the virtualenv, so use that
in the tests to determine what's being run.

tests/fixtures/tox.ini

@@ -6,4 +6,4 @@ deps =
     six
     py
 commands =
-    python -c 'import os, sys; print(os.path.realpath(sys.executable))'
+    python -c 'import os, sys; print(os.path.realpath(sys.exec_prefix), "is the exec_prefix")'

tests/test_integration.py

@@ -13,6 +13,8 @@ import pytest
 
 NATIVE_TOXENV = f"py{sys.version_info[0]}{sys.version_info[1]}"
 NATIVE_EXECUTABLE = str(pathlib.Path(sys.executable).resolve())
+NATIVE_EXEC_PREFIX = str(pathlib.Path(sys.exec_prefix).resolve())
+NATIVE_EXEC_PREFIX_MSG = f"{NATIVE_EXEC_PREFIX} is the exec_prefix"
 FIXTURES_DIR = pathlib.Path(__file__).parent / "fixtures"
 DOT_TOX = pathlib.Path("./.tox")
 
@@ -62,14 +64,14 @@ needs_tox38 = pytest.mark.skipif(
 
 def test_native_toxenv_current_env():
     result = tox("-e", NATIVE_TOXENV, "--current-env")
-    assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
+    assert result.stdout.splitlines()[0] == NATIVE_EXEC_PREFIX_MSG
     assert not (DOT_TOX / NATIVE_TOXENV / "lib").is_dir()
 
 
 @needs_py3678
 def test_all_toxenv_current_env():
     result = tox("--current-env", check=False)
-    assert NATIVE_EXECUTABLE in result.stdout.splitlines()
+    assert NATIVE_EXEC_PREFIX_MSG in result.stdout.splitlines()
     assert result.stdout.count("InterpreterMismatch:") >= 2
     assert result.returncode > 0
 
@@ -186,9 +188,9 @@ def test_print_deps_only_print_deps_to_file_are_mutually_exclusive():
 def test_regular_run():
     result = tox()
     lines = sorted(result.stdout.splitlines()[:3])
-    assert "/.tox/py36/bin/python" in lines[0]
+    assert "/.tox/py36 is the exec_prefix" in lines[0]
-    assert "/.tox/py37/bin/python" in lines[1]
+    assert "/.tox/py37 is the exec_prefix" in lines[1]
-    assert "/.tox/py38/bin/python" in lines[2]
+    assert "/.tox/py38 is the exec_prefix" in lines[2]
     assert "congratulations" in result.stdout
     for y in 6, 7, 8:
         for pkg in "py", "six", "test":
@@ -200,7 +202,7 @@ def test_regular_run():
 def test_regular_run_native_toxenv():
     result = tox("-e", NATIVE_TOXENV)
     lines = sorted(result.stdout.splitlines()[:1])
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in lines[0]
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in lines[0]
     assert "congratulations" in result.stdout
     for pkg in "py", "six", "test":
         sitelib = (
@@ -213,9 +215,9 @@ def test_regular_run_native_toxenv():
 @needs_tox38
 def test_regular_after_current_is_supported():
     result = tox("-e", NATIVE_TOXENV, "--current-env")
-    assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
+    assert result.stdout.splitlines()[0] == NATIVE_EXEC_PREFIX_MSG
     result = tox("-e", NATIVE_TOXENV)
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in result.stdout
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in result.stdout
     assert "--recreate" not in result.stderr
 
 
@@ -237,7 +239,7 @@ def test_regular_after_first_deps_only_is_supported():
     result = tox("-e", NATIVE_TOXENV)
     lines = sorted(result.stdout.splitlines()[:1])
     assert "--recreate" not in result.stderr
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in lines[0]
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in lines[0]
 
     # check that "test" was not installed to current environment
     shutil.rmtree("./test.egg-info")
@@ -252,16 +254,16 @@ def test_regular_after_first_deps_only_is_supported():
 
 def test_regular_recreate_after_current():
     result = tox("-e", NATIVE_TOXENV, "--current-env")
-    assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
+    assert result.stdout.splitlines()[0] == NATIVE_EXEC_PREFIX_MSG
     result = tox("-re", NATIVE_TOXENV)
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in result.stdout
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in result.stdout
     assert "not supported" not in result.stderr
     assert "--recreate" not in result.stderr
 
 
 def test_current_after_regular_is_not_supported():
     result = tox("-e", NATIVE_TOXENV)
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in result.stdout
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in result.stdout
     result = tox("-e", NATIVE_TOXENV, "--current-env", check=False)
     assert result.returncode > 0
     assert "not supported" in result.stderr
@@ -269,9 +271,9 @@ def test_current_after_regular_is_not_supported():
 
 def test_current_recreate_after_regular():
     result = tox("-e", NATIVE_TOXENV)
-    assert f"/.tox/{NATIVE_TOXENV}/bin/python" in result.stdout
+    assert f"/.tox/{NATIVE_TOXENV} is the exec_prefix" in result.stdout
     result = tox("-re", NATIVE_TOXENV, "--current-env")
-    assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
+    assert result.stdout.splitlines()[0] == NATIVE_EXEC_PREFIX_MSG
 
 
 def test_current_after_deps_only():
@@ -281,7 +283,7 @@ def test_current_after_deps_only():
         assert "bin/python" not in result.stdout
         assert "six" in result.stdout
         result = tox("-re", NATIVE_TOXENV, "--current-env")
-        assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
+        assert result.stdout.splitlines()[0] == NATIVE_EXEC_PREFIX_MSG
 
 
 def test_regular_recreate_after_deps_only():
@@ -290,7 +292,7 @@ def test_regular_recreate_after_deps_only():
     assert "six" in result.stdout
 
     result = tox("-re", NATIVE_TOXENV)
-    assert result.stdout.splitlines()[0] != NATIVE_EXECUTABLE
+    assert result.stdout.splitlines()[0] != NATIVE_EXEC_PREFIX_MSG
     sitelib = DOT_TOX / f"{NATIVE_TOXENV}/lib/python3.{NATIVE_TOXENV[-1]}/site-packages"
     assert sitelib.is_dir()
     assert len(list(sitelib.glob("test-*.dist-info"))) == 1