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Chapter 05 | Building A WebApp | Getting Real

WebApp

  • Everything on the WWW is about request and response.
  • A web request is sent from a web browser to a web server, when a user does some action.
  • A web response is formulated on the web server and then sent back to the browser.
  • These are the five steps which happens in the above flow.
    • Step 1: User enters a URL on the browser, or clicks a link on the browser.
    • Step 2: The web browser converts the user action into a web request and send it over Internet.
    • Step 3: The web server, receives the request and has to decide one of the two,
      • Static Content: If the request is only for static content like, html file, images, or anything stored in web server, the web server locates its and send the request back.
      • Dynamic Content: This is the content which is generated at runtime, such as search runtime
    • Step 4: The Web server send the response back.
    • Step 5: The web browser receives the response and displays the response.

Flask

  • Flask is a popular web frameworks, and it is not part of the standard library.
  • Third party library needs to install first before use.
  • We can install flask, by using pip and give this command python -m pip --upgrade pip
  • It installs these 4 dependencies along with flask
    • Jinja2
    • MarkupSafe
    • Werkzeug
    • itsdangerous
  • flask is a micro web framework, in that it provides the minimum set of technologies needed for the task.
  • We can execute a flask application just like any other python application.
    • python hello_flask.py
  • We can see the logs where it says Running on http://127.0.0.1:5000/, so flask is serving the web page on port 5000, just click on the link and it should take you to a browser.

Hello World Flask Application

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello() -> str:
    return "Hello world from flask"


app.run()

Lets understand the above code one line at a time.

  • from flask import Flask : We are importing Flask class from flask module.
  • app = Flask(__name__) : The Flask object is created and assigned to app object.
    • __name__ : It provides the currently active module.
    • Flask class needs to know the current value of __name__ when creating new flask object.
    • Flask with this one line of code have abstracted a lot of initial setup for web development away from the programmer.
  • @app.route("/") : This introduces a new concept in python called Decorators.
    • Decorators adjust the behavior of an existing function without actually changing the function code.
    • Decorators can be applied to both function and classes, but if applied primarily applied to function.
    • The above decorators makes an arrangement that when the web application is requested of / page, it invokes the hello() function.
  • The next is the function definition.
  • app.run() : Its ask flask to run the web application.
  • 127.0.0.1 : This is the loop back URL, which is also called as localhost.
  • :5000 : The protocol port number on which the server is running.
  • The app.route(), takes an optional parameter of the methods it supports like POST.
  • app.route('/search4, methods=['GET', 'POST']') : When the URL has to support both GET and POST
  • app.run(debug=True) : When we want the application to run in debug mode, and use that for reloading the web application as soon as there is a code change.
  • Flask comes with a in built object called request that provide easy access to posted data.
  • We can redirect using the redirect function in Flask. This redirects to a particular URL.
  • We can associate multiple route to same function, in that case we can avoid the redirect

Jinja

  • We cannot put complete HTML as raw text string in python, it will become unmanageable.
  • We can always use a template engine.
  • We can define a top level template, called base template, which is then inherited by others.
  • Jinja2 is the template engine shipped with Flask.
  • We add special characters inside HTML, which is understood or replaced by Jinja
    • {{ TEXT }} : The value of TEXT will be provided by jinja before rendering.
    • {% block start %}, {% end block %}: A block of HTML code will be supplied by jinja.
  • render_template function needs to be imported from flask
  • Flask requires the web-app to be in this folder structure.
    • static : the static files like css.
    • templates : the templates are stored here.

HTTP Error Codes

  • HTTP is the protocol used to communicate between browser and servers.
  • Each HTTP request is serviced with a HTTP response and a status code.
  • 100 to 199 : These are informational messages.
  • 200 to 299 : These are success messages.
  • 300 to 399 : These are re-direction messages.
  • 400 to 499 : These are client-error messages. The request from client is not understood by the server.
  • 500 to 599 : These are server error messages.
  • There are few handful of HTTP methods, but we are focused on these 2,
    • GET : This method is used to request information from server.
    • POST : This method allows a web browser to send some data to server.

Bullet Points

  • We can install packages from PyPi using pip.
  • The __name__ value identifies the currently active name space.
  • The @ symbol before a function's name identifies it as a decorator.
    • Decorator let you change the behavior of an existing function without changing the function's code.
  • Jinja2 is a template engine.