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

Understanding TypeError: 'bool' object is not callable in Flask Views

When debugging a view in Flask that returns a 500 status with the error "TypeError: 'bool' object is not callable," it's crucial to understand the expected return values of a view function.

In Flask, views should return one of the following:

• A string
• A Response object (or subclass)
• A tuple of (string, status, headers) or (string, status)
• A valid WSGI application

The issue arises when a view returns a boolean value, such as True or False, which is mistaken for a WSGI application. Flask checks for the first three options and assumes the fourth if none match.

To resolve this error, ensure that your view function returns one of the valid types listed above. In the provided code example:

@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 flask.redirect(flask.url_for('home'))

    return flask.render_template('login.html')

The login view now returns a proper Response object when login is successful (redirecting to the home page) or renders a template when login fails. By adhering to the correct return types, you can avoid this error and ensure proper behavior of your Flask views.

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