Tutorial 7

This week we will learn more about:

  • Request Object

Request Object

The data from a client’s web page is sent to the server as a global request object. In order to process the request data, it should be imported from the Flask module.

Important attributes of request object are listed below:

  • Form − It is a dictionary object containing key and value pairs of form parameters and their values.
  • args − parsed contents of query string which is part of URL after question mark (?).
  • Cookies − dictionary object holding Cookie names and values.
  • files − data pertaining to uploaded file.
  • method − current request method.

Static Files

static files such as .css/.js/.png/.jpeg/…etc could be used inside Flask Template once placed in static files folder.

Example of a project_directory with static files:

  • templates
    • file1.html
    • file2.htm
  • static
    • file1.css
    • im1.png
    • im2.png
  • server.py
<img src="/static/im1.png">

Check DEMO 2

File Uploading

Handling file upload in Flask is very easy. It needs an HTML form with its enctype attribute set to ‘multipart/form-data’, posting the file to a URL. The URL handler fetches file from request.files[] object and saves it to the desired location.

Each uploaded file is first saved in a temporary location on the server, before it is actually saved to its ultimate location. Name of destination file can be hard-coded or can be obtained from filename property of request.files[file] object. However, it is recommended to obtain a secure version of it using the secure_filename() function.

It is possible to define the path of default upload folder and maximum size of uploaded file in configuration settings of Flask object.

app.config['UPLOAD_FOLDER']  = "/pth to folder" # Defines path for upload folder 
app.config['MAX_CONTENT_LENGTH'] = 5 * 1024 * 1024 # 5 MB  (Specifies maximum size of file yo be uploaded – in bytes )

Upload form example:

<html>
   <body>
      <form action = "uploader function" method = "POST" 
         enctype = "multipart/form-data">
         <input type = "file" name = "file" />
         <input type = "submit"/>
      </form>
   </body>
</html>

get uploaded file

from werkzeug import secure_filename
app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024
uploads_dir = os.path.join(app.instance_path, 'uploads')
f.save(os.path.join(uploads_dir, secure_filename(f.filename)))

Cookies

A cookie is stored on a client’s computer in the form of a text file. Its purpose is to remember and track data pertaining to a client’s usage for better visitor experience and site statistics.

A Request object contains a cookie’s attribute. It is a dictionary object of all the cookie variables and their corresponding values, a client has transmitted. In addition to it, a cookie also stores its expiry time, path and domain name of the site.

In Flask, cookies are set on response object. Use make_response() function to get response object from return value of a view function. After that, use the set_cookie() function of response object to store a cookie.

Reading back a cookie is easy. The get() method of request.cookies attribute is used to read a cookie.

resp = make_response(render_template('readcookie.html'))
resp.set_cookie('userID', user)

## to get cookie
name = request.cookies.get('userID')

Sessions

Unlike a Cookie, Session data is stored on server. Session is the time interval when a client logs into a server and logs out of it. The data, which is needed to be held across this session, is stored in a temporary directory on the server.

A session with each client is assigned a Session ID. The Session data is stored on top of cookies and the server signs them cryptographically. For this encryption, a Flask application needs a defined SECRET_KEY.

Session object is also a dictionary object containing key-value pairs of session variables and associated values.

For example, to set a ‘username’ session variable use the statement −

Session['username'] = 'admin'

# to release a session
session.pop('username', None)

Flask Deployment as external server

Deployment as external server allow any device in the same network to call it url.

  1. Run the server on ip:0.0.0.0
if __name__ == '__main__':
   app.run(host="0.0.0.0")

  1. Run the server

  2. make Hotspot from your PC and connect your phone on it

  3. Retrieve your ip address

sudo ifconfig

you should see the following: take the highlighted IP address

  1. open your mobile browser and type the retrieved IP address

HINT: When using flask tags { { , { %, % }, or } } Remove the spaces.

Demo

All demos will be available in this repository