Unit 2: Web Fundamentals

Displaying and Processing Forms in Flask

Notes

As always, when staring to work on a Flask application, activate an appropriate virtual environment. Here, we do:

$ source activate hello-flask

Working with forms

  • To create a form in a Python file, to be returned by a handler function in Flask, use triple-quotes """ to enclose strings that break across lines. This allows us to write large amounts of text (e.g. HTML) within a Python file.
  • If this form is a global variable (i.e. it is defined outside of any function) then it can be used by multiple handler functions.
  • By default, the action of a form is the same URL that the form is displayed at.
  • By default, the method of a form is GET.
  • The name attribute of a form element determines the key that will be used to pass the parameter to the server in the HTTP request. Thus, if an element has name='first_name' then the string 'first_name' must be used to access the value of the form element on the server.

Accessing request data

Accessing both GET and POST parameters within Flask requires the request object provided by Flask:

from flask import request

Accessing GET request parameters

A query (or GET request) parameter can be accessed via request.args:

form_value = request.args.get('param_name')

GET parameters are passed in the HTTP request as part of the URL. More specifically, they make up the query string--the portion after ?--which looked like this in the lesson:

http://localhost:5000/hello?first_name=Chris

Here, the query string is ?first_name=Chris. If there were multiple query parameters, they would be separated by the & (ampersand) character.

http://localhost:5000/hello?first_name=Chris&last_name=Bay

Accessing POST request parameters

To enable a handler function to receive POST requests, we must add a methods parameter to the @app.route decorator:

@app.route('/path', methods=['POST'])
def my_handler():
    # request handling code

A POST parameter can be accessed via request.form:

form_value = request.form['param_name']

405 - Method Not Allowed

An HTTP status of 405 - Method Not Allowed will be received if a resource/path is requested that doesn't accept requests using the given method (usually, GET or POST). This can be a common mistake when setting up a form to POST to a given path, but failing to configure the handler function to accept POST requests.

Code

View the final code from this lesson