AssertionError: View function mapping is overwriting an existing endpoint

AssertionError: View function mapping is overwriting an existing endpoint

$ python -c "from flask import Flask; app=Flask(__name__);\[email protected]('/x')\ndef a(): return 'a'\[email protected]('/y', endpoint='a')\ndef b(): return 'b'"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AssertionError: View function mapping is overwriting an existing endpoint function: a

Why this happens

Each route has an endpoint name (default: function name). Registering a route with an endpoint that already exists (same function name reused, same explicit endpoint argument, or blueprint name collisions) triggers an AssertionError to prevent ambiguous URL building.

Fix

• Use unique function names or specify unique endpoint values.
• In Blueprints, be mindful of name and automatic endpoint prefixing; avoid duplicating endpoints across modules.
• Refactor routes into separate functions or use endpoint='unique_name'.

Wrong code

from flask import Flask
app = Flask(__name__)

@app.route('/users')
def list():
    return 'list'

@app.route('/admins')
def list():  # Duplicate function name -> duplicate endpoint
    return 'admins'

Fixed code

from flask import Flask
app = Flask(__name__)

@app.route('/users')
def list_users():
    return 'list'

@app.route('/admins')
def list_admins():
    return 'admins'

Or specify endpoints:

@app.route('/users', endpoint='users_list')
def list():
    return 'list'

@app.route('/admins', endpoint='admins_list')
def list():
    return 'admins'

Blueprint tip:

bp = Blueprint('admin', __name__)

@bp.route('/dashboard')
def dashboard():
    return 'ok'

# Endpoint becomes 'admin.dashboard' avoiding conflicts with app-level 'dashboard'

Ensure every route maps to a unique endpoint to keep url_for deterministic.