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Flask Basics

Installing Flask and Creating an App

Environment Setup

  1. Set up a Virtual Environment (Optional)
python -m venv myenv
source myenv/bin/activate  # Or on Windows: myenv\Scripts\activate
  1. Install Flask
pip install flask

Basic Flask App Commands

  1. Create a Flask App
mkdir myapp
cd myapp
  1. Create a Virtual Environment (Optional)
python -m venv venv
source venv/bin/activate  # Or on Windows: venv\Scripts\activate
  1. Install Flask
pip install flask
  1. Create a Python File (e.g., app.py)
from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
    return 'Hello, World!'
  1. Run the App
flask run

Understanding Flask Routes and Views

Defining Routes

  1. Define a Route
@app.route('/about')
def about():
    return 'About Us'
  1. Route with Variable
@app.route('/user/<username>')
def show_user(username):
    return f'User: {username}'

Request and Response

  1. Accessing Query Parameters
from flask import request

@app.route('/greet')
def greet():
    name = request.args.get('name', 'Guest')
    return f'Hello, {name}!'

Working with Templates in Flask

Template Setup

  1. Create a Templates Folder
mkdir templates
  1. Create HTML Template (e.g., index.html)
<!DOCTYPE html>
<html>
<head>
    <title>My Flask App</title>
</head>
<body>
    <h1>{{ title }}</h1>
    <p>{{ content }}</p>
</body>
</html>

Rendering Templates

  1. Render Template in View
from flask import render_template

@app.route('/home')
def home():
    return render_template('index.html', title='Welcome', content='This is my Flask app.')

Flask Model (No Built-in ORM)

Data Handling

  1. Use External Database Libraries (e.g., SQLAlchemy) for ORM

Data Generation

from myapp import db
from myapp.models import Book

book = Book(title="Flask for Beginners", author="John Doe", publication_date="2023-01-01")
db.session.add(book)
db.session.commit()

Data Retrieval

  1. Fetch all books from the database:
all_books = Book.query.all()

Data Filtering

  1. Filter books written by "John Doe":
flask_books = Book.query.filter_by(author="John Doe").all()

Data Modification

  1. Update data for a specific book (e.g., book with id 1):
book = Book.query.get(1)
book.title = "Flask for Professionals"
db.session.commit()

Data Deletion

  1. Delete a specific book (e.g., book with id 1):
book = Book.query.get(1)
db.session.delete(book)
db.session.commit()

Flask Design Patterns

1. Route and View

How it works

  1. Define routes in the app.py file to handle different URL paths.
  2. Create view functions or classes to handle requests and generate responses.
  3. Use decorators to associate routes with view functions.

Example

from flask import Flask, render_template
app = Flask(__name__)

@app.route('/')
def home():
   return render_template('index.html', title='Welcome', content='This is my Flask app.')

2. Template

How it works

  1. Create HTML template files in the templates directory.
  2. Use Jinja2 template engine to embed dynamic content within the template.
  3. Render templates in view functions using render_template function.

Example

<!DOCTYPE html>
<html>
<head>
   <title>{{ title }}</title>
</head>
<body>
   <h1>{{ title }}</h1>
   <p>{{ content }}</p>
</body>
</html>

3. Model (Using External Libraries)

How it works

  1. Use external database libraries like SQLAlchemy for handling models and ORM.
  2. Define model classes to represent database tables.
  3. Perform database operations using the model classes.
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()

class Book(db.Model):
   id = db.Column(db.Integer, primary_key=True)
   title = db.Column(db.String(100))
   author = db.Column(db.String(50))
   publication_date = db.Column(db.Date)

4. URL Configuration

How it works

  1. Define URL patterns in the app.py file to route requests to view functions or classes.
  2. Map URL patterns using route decorators.
  3. Flask processes requests by calling the appropriate view based on the URL pattern.
from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
   return 'Hello, World!'