How to intentionally cause a 400 Bad Request in Python/Flask?

Also, You can use jsonify

from flask import jsonify

class SomeView(MethodView):
    def post(self, *args, **kwargs):
        if "csv_file" not in request.files:
            return jsonify({'errors': 'No csv_file key in request.files.'}), 400

You can also use abort with custom message error:

from flask import abort
abort(400, 'My custom message')

See https://flask-restplus.readthedocs.io/en/stable/errors.html

you can return the status code as a second parameter of the return, see example below

@app.route('/my400')
def my400():
    code = 400
    msg = 'my message'
    return msg, code

You can use abort to raise an HTTP error by status code.

from flask import abort
@app.route('/badrequest400')
def bad_request():
    abort(400)