{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Flask\n" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Introduction to Flask: A Python Web Development Framework\n", "\n", "Flask is a lightweight, yet powerful, web framework for Python that is designed to make getting started quick and easy, with the ability to scale up to complex applications. It was developed by Armin Ronacher, and it was initially released in April 2010. Flask provides developers with the tools and libraries they need to create web applications, and it follows the WSGI (Web Server Gateway Interface) toolkit and Jinja2 template engine.\n", "\n", "## Key Concepts of Flask\n", "\n", "### 1. Simplicity and Flexibility\n", "\n", "Flask is noted for its simplicity, flexibility, and fine-grained control. It does not include any tools or libraries that are not necessary for building a web application. This makes Flask especially suitable for developers who want to retain control over the components used in their web application.\n", "\n", "### 2. Microservice Architecture\n", "\n", "Flask follows the microservice architecture, which means it's a \"micro\" framework primarily aiming to keep the core of an application simple yet extensible. A Flask application is built around one or more 'blueprints', which can be thought of as modules or components of an application.\n", "\n", "### 3. HTTP Request Handling\n", "\n", "Flask can handle HTTP requests and it provides mechanisms for route handling. Route handlers, also known as view functions, are written as Python functions. Each view function is mapped to one or more route URLs so that Flask knows what logic to execute when a specific URL is accessed.\n", "\n", "### 4. Template Engine\n", "\n", "Flask uses a template engine called Jinja2. Jinja2 allows for dynamic web content, where elements on a webpage are changed based on user input or other factors.\n", "\n", "## Getting Started with Flask\n", "\n", "Starting a new Flask project involves setting up a new Python environment, installing Flask using pip (the Python package installer), and creating a new Python script to start the Flask development server.\n", "\n", "To install Flask, use the following command in your Python environment:\n", "\n", "```\n", "pip install flask\n", "```\n", "\n", "A minimal Flask application looks like this:\n", "\n", "```\n", "from flask import Flask\n", "app = Flask(__name__)\n", "\n", "@app.route('/')\n", "def hello_world():\n", " return 'Hello, World!'\n", "\n", "if __name__ == '__main__':\n", " app.run()\n", "```\n", "\n", "In this example, the `@app.route('/')` decorator tells Flask what URL should trigger our `hello_world` function. The 'Hello, World!' message is then returned to the user.\n", "\n", "## Understanding Blueprints in Flask\n", "\n", "Blueprints in Flask can be thought of as a way to organize your application into distinct components that can each define their own routes, templates, static files, and other utilities. By using blueprints, you can structure your application into a collection of modules, each encapsulating specific functionality.\n", "\n", "### Why Use Blueprints?\n", "\n", "Blueprints provide a number of benefits:\n", "\n", "1. **Modularity:** Blueprints allow you to break down your application into logical components (auth, admin, api, etc.), each with its own set of related routes, error handlers, and views. This makes your application more maintainable and scalable.\n", "\n", "2. **Reusability:** If you have functionality that can be used across multiple projects, you can define it in a blueprint and easily register it in any Flask application.\n", "\n", "3. **Lazy Registration:** With blueprints, you define routes in the blueprint but they are not actually registered with the application until the blueprint is registered with the application. This allows you to defer some decisions to the point of blueprint registration.\n", "\n", "### How to Use Blueprints\n", "\n", "Blueprints are a powerful feature of Flask that allow for greater modularity and organization in your Flask applications. By using blueprints, you can break down your application into distinct components, making your code more maintainable, scalable, and reusable.\n", "\n", "Creating a blueprint involves creating an instance of the `Blueprint` class. Routes, error handlers, etc., are then registered on the blueprint using the familiar decorators, but they are kept separate from the application's routes until the blueprint is registered on the application.\n", "\n", "Here's a simple example:\n", "\n", "```\n", "from flask import Blueprint, render_template\n", "\n", "# Create a Blueprint instance\n", "bp = Blueprint('main', __name__)\n", "\n", "# Define a route in the blueprint\n", "@bp.route('/')\n", "def home():\n", " return render_template('home.html')\n", "\n", "@bp.route('/about')\n", "def about():\n", " return render_template('about.html')\n", "```\n", "\n", "In the above example, we first import the `Blueprint` class and create an instance of it. Then we define two routes, `home` and `about`, in this blueprint.\n", "\n", "Now, to make use of these routes, we have to register the blueprint with a Flask application:\n", "\n", "```python\n", "from flask import Flask\n", "from . import bp # assuming bp is in __init__.py in a package\n", "\n", "app = Flask(__name__)\n", "app.register_blueprint(bp)\n", "```\n", "\n", "After registering the blueprint with the Flask application, the routes defined in the blueprint become part of the application. In this case, the URLs for the `home` and `about` views would be 'http://[hostname]/' and 'http://[hostname]/about', respectively.\n", "\n", "\n", "## HTTP Request Handling and View Functions in Flask\n", "\n", "In a Flask application, HTTP requests are handled by functions known as view functions. View functions are mapped to one or more route URLs so that Flask knows what logic to execute when a specific URL is accessed.\n", "\n", "### Understanding HTTP Request Handling\n", "\n", "HTTP, or Hypertext Transfer Protocol, is the protocol used for transferring data over the web. An HTTP request is a message that a client (usually a web browser) sends to a server requesting a specific action on a specific resource. The most common types of HTTP requests are GET (retrieve a resource), POST (send data to the server), PUT (update a resource), and DELETE (remove a resource).\n", "\n", "In Flask, you can handle different types of HTTP requests using route decorators and view functions.\n", "\n", "### Understanding View Functions\n", "\n", "A view function, or view, in Flask is a Python function that contains the logic responsible for processing a user's request and preparing the response that will be sent back to the client. A view function is mapped to a specific URL pattern through a decorator, and it is called whenever a user accesses that URL.\n", "\n", "### How to Use View Functions and Handle HTTP Requests\n", "\n", "Here's a simple example of a view function:\n", "\n", "```python\n", "from flask import Flask\n", "app = Flask(__name__)\n", "\n", "@app.route('/')\n", "def home():\n", " return 'Hello, World!'\n", "```\n", "\n", "In this example, the `@app.route('/')` decorator tells Flask what URL should trigger the `home` function. The 'Hello, World!' string is then returned to the client as a response.\n", "\n", "To handle different types of HTTP requests, you can specify the methods parameter in the route decorator. Here's an example of a view function that can handle both GET and POST requests:\n", "\n", "```python\n", "@app.route('/login', methods=['GET', 'POST'])\n", "def login():\n", " if request.method == 'POST':\n", " return 'Hello, ' + request.form['username']\n", " else:\n", " return '

'\n", "```\n", "\n", "In this example, the `login` view function will respond to HTTP GET requests with a simple form that asks for a username. If the method is POST, which happens when the form is submitted, the view function will greet the user with the provided username.\n", "\n", "### Conclusion\n", "\n", "HTTP request handling and view functions are at the heart of Flask's request-response cycle. Understanding how to define view functions and handle different types of HTTP requests is essential for building web applications with Flask.\n", "\n", "## Templating and Jinja2 in Flask\n", "\n", "One of the most powerful features of Flask is its support for templates, which allow developers to generate dynamic HTML pages. Flask uses a template engine called Jinja2, which allows for the creation of templates with variable substitution and various control structures.\n", "\n", "### Understanding Templating\n", "\n", "In the context of a web application, a template is a file composed of static and dynamic parts. The static parts are HTML that define the structure and layout of the page, while the dynamic parts are placeholders for content that is generated at runtime.\n", "\n", "The main advantage of using templates is that they enable you to separate the logic of your application from its presentation. This makes your application easier to understand, maintain, and modify.\n", "\n", "### Understanding Jinja2\n", "\n", "Jinja2 is a template engine for Python that is flexible, fast, and secure. It provides useful features such as sandboxed execution and optional automatic escaping for HTML.\n", "\n", "In Jinja2, you can use variables, filters, and control structures in your templates. Variables are enclosed in double curly braces (`{{ variable }}`), while control structures such as for loops and if statements are enclosed in curly braces and percent signs (`{% for item in items %}` and `{% if condition %}`).\n", "\n", "### How to Use Templates and Jinja2 in Flask\n", "\n", "To render a template, you can use the `render_template()` function. You need to pass the name of the template and the variables you want to pass to the template engine as keyword arguments.\n", "\n", "Here's an example of how to use a template in Flask:\n", "\n", "```python\n", "from flask import Flask, render_template\n", "app = Flask(__name__)\n", "\n", "@app.route('/hello/')\n", "def hello(name):\n", " return render_template('hello.html', name=name)\n", "```\n", "\n", "In this example, the `hello` view function takes a parameter from the URL and passes it to the template.\n", "\n", "The corresponding `hello.html` template could look like this:\n", "\n", "```html\n", "\n", "\n", " \n", " Hello from Flask\n", " \n", " \n", "

Hello, {{ name }}!

\n", " \n", "\n", "```\n", "\n", "In this template, `{{ name }}` is a placeholder for a variable that gets replaced with its value when the template is rendered.\n", "\n", "## Resources for Learning Flask\n", "* [Official Flask Documentation](https://flask.palletsprojects.com/en/2.3.x/)\n", "* [Flask Tutorial](https://flask.palletsprojects.com/en/2.3.x/tutorial/)\n" ] }, { "cell_type": "markdown", "metadata": {}, "source": [] } ], "metadata": { "kernelspec": { "display_name": "Python 3 (ipykernel)", "language": "python", "name": "python3" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3.11.3" } }, "nbformat": 4, "nbformat_minor": 1 }