AssertionError: blueprint already registered

AssertionError: blueprint already registered

$ python - <<'PY'
from flask import Flask, Blueprint
bp = Blueprint('main', __name__)
app = Flask(__name__)
app.register_blueprint(bp)
app.register_blueprint(bp)
PY
Traceback (most recent call last):
  File "<stdin>", line 6, in <module>
AssertionError: A blueprint named 'main' is already registered

Why this happens

Flask prevents duplicate Blueprint registrations under the same name to keep routes, templates, and static paths consistent. Double registration usually occurs due to import side effects: importing a module that registers the blueprint at import-time, and then registering it again in create_app. Another common case is creating multiple Blueprint instances with the same name.

Fix

• Register blueprints exactly once, typically inside your create_app function.
• Avoid registration at import-time in module globals; expose the blueprint as a symbol and let the application factory register it.
• Use unique name values per Blueprint, and ensure url_prefix is consistent.

Wrong code

# blog/routes.py
from flask import Blueprint
bp = Blueprint('blog', __name__)
# Side-effect registration (bad)
from app import app
app.register_blueprint(bp)

Fixed code

# blog/routes.py
from flask import Blueprint
bp = Blueprint('blog', __name__)

# app/__init__.py
from flask import Flask
from blog.routes import bp

def create_app():
    app = Flask(__name__)
    app.register_blueprint(bp)
    return app

Keep registration inside the application factory pattern and avoid import-time side effects to prevent assertion failures.