ValueError: View Function Did Not Return a Response

The Problem

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1478, in full_dispatch_request
    response = self.make_response(rv)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1566, in make_response
    raise ValueError('View function did not return a response')
ValueError: View function did not return a response

The Solution

All Flask views must return a value, usually a Response object. If the flask view throwing the error accepts GET and POST methods, check if you are neglecting return a value for one of the cases.

@app.route('/', methods=['GET', 'POST'])
def index():
    if request.method == 'POST':
        model.save()
        # Failure to return a redirect or render_template
    else:
        return render_template('index.html')

Further Reading

If you’re looking to get a deeper understanding of how Flask application monitoring works, take a look at the following articles:

• Building a SaaS application block by block
• How slow is slow?
• Getting started with the Flask SDK (docs)