Skip to content

fix blueprint registering and double init#3798

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
T4rk1n merged 3 commits into from
May 29, 2026
Merged

fix blueprint registering and double init #3798

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
f02a947
fix blueprint registering and double init
T4rk1n May 27, 2026
81f148f
init app check instead of backend blueprint check
T4rk1n May 29, 2026
8cdf6fc
Merge branch 'dev' into fix/init-flask-run-command
T4rk1n May 29, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
1 change: 0 additions & 1 deletion dash/backends/_quart.py
View file
Open in desktop

@camdecostercamdecosterMay 29, 2026

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

You could revert this change and get rid of this file from the PR.

Original file line numberDiff line numberDiff line change
Expand Up@@ -122,7 +122,6 @@ def create_app(
def register_assets_blueprint(
self, blueprint_name: str, assets_url_path: str, assets_folder: str # type: ignore[name-defined]
):

bp = Blueprint(
blueprint_name,
__name__,
Expand Down
13 changes: 13 additions & 0 deletions dash/dash.py
View file
Open in desktop

@camdecostercamdecosterMay 29, 2026

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Rather than add these blueprint checks for each backend, what if you add a check in init_app to see if the same server has already been registered? If the same server is passed in, then it becomes a no-op. If it's a new server, then the init takes place with the new server.

If you go with the current per backend approach, certain calls would add duplicate handlers or attempt to register the same routes again:

self.backend.before_request(self._setup_server)
...
self._setup_routes()

Original file line numberDiff line numberDiff line change
Expand Up@@ -135,6 +135,8 @@
_ID_STORE = "_pages_store"
_ID_DUMMY = "_pages_dummy"

_UNINITIALIZED = object() # Sentinel for tracking init_app state

DASH_VERSION_URL = "https://dash-version.plotly.com:8080/current_version"

# Handles the case in a newly cloned environment where the components are not yet generated.
Expand DownExpand Up@@ -731,6 +733,17 @@ def init_app(self, app: Optional[Any] = None, **kwargs) -> None:
)
if app is not None:
self.server = app
# Also update the backend's server reference so routes are registered
# on the correct server (important when using server=False pattern)
self.backend.server = app

# Skip registration if already initialized on this server
# This prevents double registration when init_app() is called multiple times
# (e.g., with flask run pattern where __init__ calls init_app, then user does too)
if getattr(self, "_initialized_server", _UNINITIALIZED) is self.server:
return
self._initialized_server = self.server

bp_prefix = config.routes_pathname_prefix.replace("/", "_").replace(".", "_")
assets_blueprint_name = f"{bp_prefix}dash_assets"
self.backend.register_assets_blueprint(
Expand Down
40 changes: 40 additions & 0 deletions tests/unit/test_configs.py
View file
Open in desktop

@camdecostercamdecosterMay 29, 2026

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I asked Claude to write a test that would look at the idempotency when calling init twice. This might be worth adding:

deftest_init_app_idempotent_on_same_server():
external=Flask("ext")
app=Dash(__name__, server=False)
app.init_app(external)
app.init_app(external) # must not raise# request still worksclient=external.test_client()
assertclient.get("/").status_code==200

Original file line numberDiff line numberDiff line change
Expand Up@@ -524,3 +524,43 @@ def test_csrf_config_read_only():
app.config.csrf_token_name = "something_else"
with pytest.raises(AttributeError):
app.config.csrf_header_name = "something_else"


def test_init_app_with_flask_run_pattern():
"""Test that init_app works correctly with the flask run pattern.

This tests the fix for https://github.com/plotly/dash/issues/3787
where using flask run would cause a ValueError about duplicate blueprint
registration because init_app was called twice (once automatically in
__init__ with server=True, and once by the user in their create_app factory).
"""
# Simulate the flask run pattern where:
# 1. Dash app is created with server=True (default)
# 2. User's create_app factory calls init_app with their Flask server
external_server = Flask("external_test")
app = Dash(__name__)

# This should NOT raise "ValueError: The name '_dash_assets' is already registered"
app.init_app(external_server)

# Verify the backend now uses the external server
assert app.server is external_server
assert app.backend.server is external_server


def test_init_app_server_false_pattern():
"""Test that init_app works correctly when server=False is used.

This tests the fix for https://github.com/plotly/dash/issues/3787
where using server=False and then calling init_app would result in
404 errors because the backend's server reference was not updated.
"""
external_server = Flask("external_test_false")
app = Dash(__name__, server=False)

# Call init_app with the external server
app.init_app(external_server)

# Verify both the app and backend use the external server
assert app.server is external_server
assert app.backend.server is external_server
Loading