Flask view return error "View function did not return a response"

Question

I have a view that calls a function to get the response. However, it gives the error View function did not return a response. How do I fix this?

from flask import Flask
app = Flask(__name__)

def hello_world():
    return 'test'

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    hello_world()

if __name__ == '__main__':
    app.run(debug=True)

When I try to test it by adding a static value rather than calling the function, it works.

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    return "test"

Asked By: user547907

||

Source

Answer 1

The following does not return a response:

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    hello_world()

You mean to say…

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    return hello_world()

Note the addition of return in this fixed function.

Answered By: Mark Hildreth

Answer 2

No matter what code executes in a view function, the view must return a value that Flask recognizes as a response. If the function doesn’t return anything, that’s equivalent to returning None, which is not a valid response.

In addition to omitting a return statement completely, another common error is to only return a response in some cases. If your view has different behavior based on an if or a try/except, you need to ensure that every branch returns a response.

This incorrect example doesn’t return a response on GET requests, it needs a return statement after the if:

@app.route("/hello", methods=["GET", "POST"])
def hello():
    if request.method == "POST":
        return hello_world()

    # missing return statement here

This correct example returns a response on success and failure (and logs the failure for debugging):

@app.route("/hello")
def hello():
    try:
        return database_hello()
    except DatabaseError as e:
        app.logger.exception(e)
        return "Can't say hello."

Answered By: davidism

Answer 3

In this error message Flask complains that the function did not return a valid response. The emphasis on response suggests that it is not just about the function returning a value, but a valid flask.Response object which can print a message, return a status code etc. So that trivial example code could be written like this:

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    return Response(hello_world(), status=200)

Or even better if wrapped in the try-except clause:

@app.route('/hello', methods=['GET', 'POST'])
def hello():
    try:
        result = hello_world()
    except Exception as e:
        return Response('Error: {}'.format(str(e)), status=500)
    return Response(result, status=200)

Answered By: mac13k

Flask view return error "View function did not return a response"

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