Why Does My Flask View Return 'TypeError: 'bool' object is not callable'?

"TypeError: 'bool' object is not callable" in Flask Views: Troubleshooting

Introduction

When debugging a Flask view that triggers a 500 status, developers may encounter the enigmatic error "TypeError: 'bool' object is not callable." This article delves into the cause of this error and provides a solution.

Understanding the Error

Flask views can return various types, including strings, Flask Response objects, tuples of (string, status, headers), and WSGI applications. However, if the returned value does not match any of the expected types, Flask interprets it as a WSGI application.

Cause of the Error

In the example provided, the view returns True to indicate a successful login. However, the bool value True is not a valid WSGI application. Consequently, Flask assumes it is a WSGI application and attempts to call it, resulting in the error "TypeError: 'bool' object is not callable."

Solution

To resolve this issue, the view must return one of the valid response types as specified in the Flask documentation: About Responses. In this case, returning a Response object with a status code of 200 and a message indicating successful login would be appropriate.

@app.route('/login', methods=['POST'])
def login():
    username = request.form['username']
    user = User.query.filter_by(username=username).first()

    if user:
        login_user(user)
        return Response("Login successful", status=200)

    return Response("Login failed", status=401)

By ensuring that the view returns a valid response type, developers can prevent the "TypeError: 'bool' object is not callable" error and improve the reliability of their Flask applications.

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