KeyError: blueprint name collision

KeyError from duplicate blueprint names

$ flask --app app.py run
Traceback (most recent call last):
  File "/path/to/app.py", line 18, in <module>
    app.register_blueprint(bp2)
KeyError: 'api'

Why this happens

Blueprint names must be unique across the application. Reusing the same name (e.g., 'api') for multiple blueprints leads to collisions in app.blueprints. Flask raises a KeyError or an assertion depending on version.

Fix

Give each blueprint a unique name and, if needed, distinct URL prefixes. Avoid dynamically generating names; be explicit.

Wrong code

# app.py
from flask import Flask, Blueprint
app = Flask(__name__)

bp1 = Blueprint('api', __name__)
@bp1.get('/v1/ping')
def ping1():
    return {'v': 1}

bp2 = Blueprint('api', __name__)  # duplicate name
@bp2.get('/v2/ping')
def ping2():
    return {'v': 2}

app.register_blueprint(bp1)
app.register_blueprint(bp2)  # KeyError

Fixed code

# app.py
from flask import Flask, Blueprint
app = Flask(__name__)

bp1 = Blueprint('api_v1', __name__)
@bp1.get('/v1/ping')
def ping1():
    return {'v': 1}

bp2 = Blueprint('api_v2', __name__)
@bp2.get('/v2/ping')
def ping2():
    return {'v': 2}

app.register_blueprint(bp1)
app.register_blueprint(bp2)

Tips

• Choose descriptive names: admin, public, api_v1, etc.
• Register in an app factory to keep names centralized.