Only use tox_cleanup hook when it is possible

src/tox_current_env/hooks.py

@@ -85,10 +85,15 @@ def unsupported_raise(config, venv):
         return
     regular = not (config.option.current_env or config.option.print_deps_only)
     if regular and is_current_env_link(venv):
+        if hasattr(tox.hookspecs, "tox_cleanup"):
             raise tox.exception.ConfigError(
                 "Looks like previous --current-env or --print-deps-only tox run didn't finish the cleanup. "
                 "Run tox run with --recreate (-r) or manually remove the environment in .tox."
             )
+        else:
+            raise tox.exception.ConfigError(
+                "Regular tox run after --current-env or --print-deps-only tox run is not supported without --recreate (-r)."
+            )
     elif config.option.current_env and is_proper_venv(venv):
         raise tox.exception.ConfigError(
             "--current-env after regular tox run is not supported without --recreate (-r)."
@@ -166,8 +171,10 @@ def tox_runtest(venv, redirect):
         return True
 
 
-@tox.hookimpl
+if hasattr(tox.hookspecs, "tox_cleanup"):
-def tox_cleanup(session):
+
+    @tox.hookimpl
+    def tox_cleanup(session):
         """Remove the fake virtualenv not to collide with regular tox
         Collisions can happen anyway (when tox is killed forcefully before this happens)
         Note that we don't remove real venvs, as recreating them is expensive"""

tests/test_integration.py

@@ -6,6 +6,8 @@ import subprocess
 import sys
 import textwrap
 
+from packaging import version
+
 import pytest
 
 
@@ -37,6 +39,8 @@ def is_available(python):
     return True
 
 
+TOX_VERSION = version.parse(tox("--version").stdout.split(" ")[0])
+
 needs_py3678 = pytest.mark.skipif(
     not is_available("python3.6")
     or not is_available("python3.7")
@@ -44,6 +48,10 @@ needs_py3678 = pytest.mark.skipif(
     reason="This test needs all of python3.6, python3.7 and python3.8 to be available in $PATH",
 )
 
+needs_tox38 = pytest.mark.skipif(
+    TOX_VERSION < version.parse("3.8"), reason="This test needs at least tox 3.8"
+)
+
 
 def test_native_toxenv_current_env():
     result = tox("-e", NATIVE_TOXENV, "--current-env")
@@ -195,6 +203,7 @@ def test_regular_run_native_toxenv():
         assert len(list(sitelib.glob(f"{pkg}-*.dist-info"))) == 1
 
 
+@needs_tox38
 def test_regular_after_current_is_supported():
     result = tox("-e", NATIVE_TOXENV, "--current-env")
     assert result.stdout.splitlines()[0] == NATIVE_EXECUTABLE
@@ -214,6 +223,7 @@ def test_regular_after_killed_current_is_not_supported():
     assert "--recreate" in result.stderr
 
 
+@needs_tox38
 def test_regular_after_first_deps_only_is_supported():
     result = tox("-e", NATIVE_TOXENV, "--print-deps-only")
     assert result.stdout.splitlines()[0] == "six"

tox.ini

@@ -4,6 +4,7 @@ envlist = {py36,py37,py38}-tox{35,release,master}
 [testenv]
 deps=
     pytest
+    packaging
     tox35: tox >=3.5,<3.6
     toxrelease: tox
     toxmaster: git+https://github.com/tox-dev/tox.git@master