[flake8-bugbear] Remove hidden global state to import only once

Author: Pierre-Sassoulas

pyproject.toml

@@ -141,7 +141,6 @@ select = [
 ignore = [
     # bugbear ignore
     "B004", # Using `hasattr(x, "__call__")` to test if x is callable is unreliable.
-    "B006", # Do not use mutable data structures for argument defaults
     "B007", # Loop control variable `i` not used within loop body
     "B009", # Do not call `getattr` with a constant attribute value
     "B010", # [*] Do not call `setattr` with a constant attribute value.

src/_pytest/unittest.py

@@ -361,14 +361,21 @@ def pytest_runtest_makereport(item: Item, call: CallInfo[None]) -> None:
 
 
 # Twisted trial support.
+classImplements_has_run = False
 
 
 @hookimpl(wrapper=True)
 def pytest_runtest_protocol(item: Item) -> Generator[None, object, object]:
     if isinstance(item, TestCaseFunction) and "twisted.trial.unittest" in sys.modules:
         ut: Any = sys.modules["twisted.python.failure"]
+        global classImplements_has_run
         Failure__init__ = ut.Failure.__init__
-        check_testcase_implements_trial_reporter()
+        if not classImplements_has_run:
+            from twisted.trial.itrial import IReporter
+            from zope.interface import classImplements
+
+            classImplements(TestCaseFunction, IReporter)
+            classImplements_has_run = True
 
         def excstore(
             self, exc_value=None, exc_type=None, exc_tb=None, captureVars=None
@@ -396,16 +403,6 @@ def excstore(
     return res
 
 
-def check_testcase_implements_trial_reporter(done: List[int] = []) -> None:
-    if done:
-        return
-    from twisted.trial.itrial import IReporter
-    from zope.interface import classImplements
-
-    classImplements(TestCaseFunction, IReporter)
-    done.append(1)
-
-
 def _is_skipped(obj) -> bool:
     """Return True if the given object has been marked with @unittest.skip."""
     return bool(getattr(obj, "__unittest_skip__", False))