Forbid using a single string in tags to avoid unexpected behavior

fastapi/routing.py

@@ -466,6 +466,8 @@ class APIRouter(routing.Router):
                 "/"
             ), "A path prefix must not end with '/', as the routes will start with '/'"
         self.prefix = prefix
+        if tags:
+            assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
         self.tags: List[str] = tags or []
         self.dependencies = list(dependencies or []) or []
         self.deprecated = deprecated
@@ -515,6 +517,7 @@ class APIRouter(routing.Router):
         )
         current_tags = self.tags.copy()
         if tags:
+            assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
             current_tags.extend(tags)
         current_dependencies = self.dependencies.copy()
         if dependencies:
@@ -668,6 +671,7 @@ class APIRouter(routing.Router):
                 )
                 current_tags = []
                 if tags:
+                    assert not isinstance(tags, str), """tags should not be a string: please use ["tag"] instead of "tag"."""
                     current_tags.extend(tags)
                 if route.tags:
                     current_tags.extend(route.tags)

tests/test_tags.py

@@ -0,0 +1,27 @@
+import pytest
+from fastapi import APIRouter, FastAPI
+
+
+def test_string_is_invalid_in_router_tags():
+    with pytest.raises(AssertionError):
+        router = APIRouter(tags="test")
+
+def test_string_is_invalid_in_router_route_tags():
+    router = APIRouter()
+
+    with pytest.raises(AssertionError):
+        @router.get("", tags="test")
+        def test():
+            ...
+
+
+def test_string_is_invalid_in_include_router_tags():
+    app = FastAPI()
+    router = APIRouter()
+
+    @router.get("")
+    def test():
+        ...
+
+    with pytest.raises(AssertionError):
+        app.include_router(router, prefix="/test", tags="test")