Emit warning for unknown marks

src/_pytest/mark/__init__.py

@@ -146,8 +146,7 @@ def pytest_collection_modifyitems(items, config):
 
 def pytest_configure(config):
     config._old_mark_config = MARK_GEN._config
-    if config.option.strict:
-        MARK_GEN._config = config
+    MARK_GEN._config = config
 
     empty_parameterset = config.getini(EMPTY_PARAMETERSET_OPTION)
 

src/_pytest/mark/structures.py

@@ -11,6 +11,7 @@
 from ..compat import MappingMixin
 from ..compat import NOTSET
 from _pytest.outcomes import fail
+from _pytest.warning_types import PytestWarning
 
 EMPTY_PARAMETERSET_OPTION = "empty_parameter_set_mark"
 
@@ -283,28 +284,37 @@ def test_function():
     on the ``test_function`` object. """
 
     _config = None
+    _markers = set()
 
     def __getattr__(self, name):
         if name[0] == "_":
             raise AttributeError("Marker name must NOT start with underscore")
+
         if self._config is not None:
-            self._check(name)
+            self._update_markers(name)
+            if name not in self._markers:
+                warnings.warn(
+                    "Unknown mark %r.  You can register custom marks to avoid this "
+                    "warning, without risking typos that silently break your tests. "
+                    "See https://docs.pytest.org/en/latest/mark.html for more detail.",
+                    PytestWarning,
+                )
+                if self._config.option.strict:
+                    fail("{!r} not a registered marker".format(name), pytrace=False)
+
         return MarkDecorator(Mark(name, (), {}))
 
-    def _check(self, name):
-        try:
-            if name in self._markers:
-                return
-        except AttributeError:
-            pass
-        self._markers = values = set()
-        for line in self._config.getini("markers"):
-            marker = line.split(":", 1)[0]
-            marker = marker.rstrip()
-            x = marker.split("(", 1)[0]
-            values.add(x)
+    def _update_markers(self, name):
+        # We store a set of registered markers as a performance optimisation,
+        # but more could be added to `self._config` by other plugins at runtime.
+        # If we see an unknown marker, we therefore update the set and try again!
         if name not in self._markers:
-            fail("{!r} not a registered marker".format(name), pytrace=False)
+            for line in self._config.getini("markers"):
+                # example lines: "skipif(condition): skip the given test if..."
+                # or "hypothesis: tests which use Hypothesis", so to get the
+                # marker name we we split on both `:` and `(`.
+                marker = line.split(":")[0].split("(")[0].strip()
+                self._markers.add(marker)
 
 
 MARK_GEN = MarkGenerator()