TypeError: view function must be callable

TypeError: view function must be callable

$ python -c "from flask import Flask; app=Flask(__name__); app.add_url_rule('/', view_func='not-func')"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: view function must be callable

Why this happens

A view function is the function Flask calls to produce a response. Passing strings or other non-callable values as view_func raises TypeError.

Fix

Provide a proper function reference. If using class-based views, use MyView.as_view('name') which returns a callable.

Wrong code

from flask import Flask

app = Flask(__name__)

app.add_url_rule('/', view_func="home")  # ❌ string, not callable

Fixed code

from flask import Flask

app = Flask(__name__)

@app.get('/')
def home():
    return "ok"

# Or
# app.add_url_rule('/', view_func=home)

Additional notes

• For class views: app.add_url_rule('/', view_func=MyView.as_view('home')).
• Ensure decorators return a callable, not None.
• Verify endpoints don’t collide; use unique names.