Only test register_handler in a single thread

Tests/test_file_bufrstub.py

@@ -51,6 +51,9 @@ def test_save(tmp_path: Path) -> None:
 
 
 def test_handler(tmp_path: Path) -> None:
+    if BufrStubImagePlugin._handler is not None:
+        return
+
     class TestHandler(ImageFile.StubHandler):
         opened = False
         loaded = False

Tests/test_file_gribstub.py

@@ -51,6 +51,9 @@ def test_save(tmp_path: Path) -> None:
 
 
 def test_handler(tmp_path: Path) -> None:
+    if GribStubImagePlugin._handler is not None:
+        return
+
     class TestHandler(ImageFile.StubHandler):
         opened = False
         loaded = False

Tests/test_file_hdf5stub.py

@@ -53,6 +53,9 @@ def test_save() -> None:
 
 
 def test_handler(tmp_path: Path) -> None:
+    if Hdf5StubImagePlugin._handler is not None:
+        return
+
     class TestHandler(ImageFile.StubHandler):
         opened = False
         loaded = False

Tests/test_file_wmf.py

@@ -34,15 +34,19 @@ def test_load() -> None:
             assert im.load()[0, 0] == (255, 255, 255)
 
 
-def test_register_handler(tmp_path: Path) -> None:
-    class TestHandler(ImageFile.StubHandler):
-        methodCalled = False
+class TestHandler(ImageFile.StubHandler):
+    methodCalled = False
+
+    def load(self, im: ImageFile.StubImageFile) -> Image.Image:
+        return Image.new("RGB", (1, 1))
 
-        def load(self, im: ImageFile.StubImageFile) -> Image.Image:
-            return Image.new("RGB", (1, 1))
+    def save(self, im: Image.Image, fp: IO[bytes], filename: str) -> None:
+        self.methodCalled = True
 
-        def save(self, im: Image.Image, fp: IO[bytes], filename: str) -> None:
-            self.methodCalled = True
+
+def test_register_handler(tmp_path: Path) -> None:
+    if isinstance(WmfImagePlugin._handler, TestHandler):
+        return
 
     handler = TestHandler()
     original_handler = WmfImagePlugin._handler