Fix when xdist not installed or active

Fixes #246.

HISTORY.rst

@@ -2,6 +2,9 @@
 History
 -------
 
+* Fix to work when pytest-xdist is not installed or active
+  (``PluginValidationError: unknown hook 'pytest_configure_node'``).
+
 3.3.0 (2020-04-15)
 ------------------
 

src/pytest_randomly.py

@@ -10,6 +10,10 @@
 else:
     from importlib_metadata import entry_points
 
+try:
+    import xdist
+except ImportError:
+    xdist = None
 
 # factory-boy
 try:
@@ -88,6 +92,9 @@ def pytest_addoption(parser):
 
 
 def pytest_configure(config):
+    if config.pluginmanager.hasplugin("xdist"):
+        config.pluginmanager.register(XdistHooks())
+
     seed_value = config.getoption("randomly_seed")
     if seed_value == "last":
         seed = config.cache.get("randomly_seed", default_seed)
@@ -103,11 +110,12 @@ def pytest_configure(config):
     config.option.randomly_seed = seed
 
 
-def pytest_configure_node(node):
-    """
-    pytest-xdist hook. Send the selected seed through to the nodes.
-    """
-    node.workerinput["randomly_seed"] = node.config.getoption("randomly_seed")
+class XdistHooks:
+    # Hooks for xdist only, registered when needed in pytest_configure()
+    # https://docs.pytest.org/en/latest/writing_plugins.html#optionally-using-hooks-from-3rd-party-plugins
+
+    def pytest_configure_node(self, node):
+        node.workerinput["randomly_seed"] = node.config.getoption("randomly_seed")
 
 
 random_states = {}

tests/test_pytest_randomly.py

@@ -670,6 +670,11 @@ def fake_entry_points():
     testdir.runpytest("--randomly-seed=1")
 
 
+def test_works_without_xdist(simpletestdir):
+    out = simpletestdir.runpytest("-p", "no:xdist")
+    out.assert_outcomes(passed=1)
+
+
 @pytest.mark.parametrize("n", list(range(5)))
 def test_xdist(n, ourtestdir):
     """
@@ -685,7 +690,7 @@ def test_xdist(n, ourtestdir):
         test_six="def test_a(): pass",
     )
 
-    out = ourtestdir.runpytest("-n 6", "-v", "--dist=loadfile")
+    out = ourtestdir.runpytest("-n", "6", "-v", "--dist=loadfile")
     out.assert_outcomes(passed=6)
 
     # Can't make any assertion on the order, since output comes back from