Changing request method using hidden field _method in Flask

Question

Started picking up Python and Flask as a learning exercise, and coming from PHP/Symfony2, I could add a hidden _method field to a form to override the POST method with eithe

Answer 1

As you already pointed out, your middleware makes the later request.form empty. This is because request.form is reading from a file-like object. Quoting PEP 333:

wsgi.input -- An input stream (file-like object) from which the HTTP request body can be read. (The server or gateway may perform reads on-demand as requested by the application, or it may pre- read the client's request body and buffer it in-memory or on disk, or use any other technique for providing such an input stream, according to its preference.)

Note that this paragraph doesn't tell us if this "file-like object" will provide any possibility to reset the pointer to the beginning of the file. In fact, if we try the following application:

from werkzeug.serving import run_simple

def app(environ, start_response):
    start_response('200 OK', [('Content-Type', 'text/plain')])
    yield str(dir(environ['wsgi.input']))

run_simple('localhost', 5000, app)

It doesn't show any indices that this file object has a seek method.

So, what you could do is read everything into a bytestring called data, and replace wsgi.input with BytesIO(data), which does have a seek method one can use. Doing this brings several disadvantages with it, the most obvious being that all uploaded data is guaranteed to get completely read into memory before passing it to the application. Probably there are also some dangerous edge cases that i don't know myself of, which is why i never would risk trying the following in a real application:

from werkzeug.formparser import parse_form_data
from werkzeug.wsgi import get_input_stream
from io import BytesIO

class MethodMiddleware(object):
    """Don't actually do this. The disadvantages are not worth it."""
    def __init__(self, app):
        self.app = app

    def __call__(self, environ, start_response):
        if environ['REQUEST_METHOD'].upper() == 'POST':
            environ['wsgi.input'] = stream = \
                BytesIO(get_input_stream(environ).read())
            formdata = parse_form_data(environ)[1]
            stream.seek(0)

            method = formdata.get('_method', '').upper()
            if method in ('GET', 'POST', 'PUT', 'DELETE'):
                environ['REQUEST_METHOD'] = method

        return self.app(environ, start_response)

Answer 2

You could use the MethodView from flask.views and dispatch it to the right methods. I have created a simple Flask App to demonstrate it.

from flask import Flask, jsonify, request
from flask.views import MethodView

app = Flask(__name__)

class MyView(MethodView):

    def get(self):
        return jsonify({'method': 'GET'})

    def post(self):
        method = request.form.get('_method', 'POST')
        if method == 'POST':
            return jsonify({'method':method})
        else:
            if hasattr(self, method.lower()):            
                return getattr(self, method.lower())()
            else:
                return jsonify({'method': 'UNKNOWN'})

    def put(self):
        return jsonify({'method': 'PUT'})

    def delete(self):
        return jsonify({'method': 'DELETE'})

    def create(self):
        # NOT A HTTP VERB
        return jsonify({'method': 'CREATE'})

app.add_url_rule('/', view_func=MyView.as_view('myview'))

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