Python server programming: using Flask-Login to implement user login

Python server programming: Using Flask-Login to implement user login

With the development and popularity of web applications, user login has become an essential function for many applications. In Python server programming, Flask is a widely used web development framework. In addition, Flask also provides many third-party extensions, such as Flask-Login, which can help us quickly implement user login functions.

This article will introduce how to use the Flask-Login extension to implement user login function in Python server. Before that, we need to install Flask and Flask-Login extension. You can use the pip command to install:

pip install Flask Flask-Login

Now let’s implement our user login system.

1. Create a Flask application

First, we need to create a Flask application. Create an app.py file in the root directory and write the following code in it:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def index():
    return "Hello World!"

if __name__ == "__main__":
    app.run(debug=True)

In this example, we created a Flask application and defined a route. The root route returns "Hello World !". We also use the __name__ == "__main__" conditional statement to ensure that the Flask application only runs when we run the script.

Now, we have successfully created a simple Flask application.

2. Create a user model

In this example, we assume that the user's username and password have been saved to the database. So we need to define the user model first so that on login we can check if the user exists.

Add the following code in the app.py file:

from flask_login import UserMixin

class User(UserMixin):
    def __init__(self, id):
        super().__init__()
        self.id = id

    def __repr__(self):
        return f"<User {self.id}>"

In this example, we define a User class, which inherits from Flask-Login’s UserMixin class . UserMixin provides some useful properties and methods, including:

• is_authenticated: True if the user has been authenticated.
• is_active: True if the user is active.
• is_anonymous: True if the user is not authenticated.
• get_id: Returns the string representation of the user ID.

In the constructor of the User class, we pass in the user's ID. We also define a __repr__ method to see more information about the user object during debugging and testing.

3. Writing the Login View

Now, we need to write a login view that lets the user enter their username and password. If the user's username and password are correct, the login is successful and the user object is added to Flask-Login's user session.

Add the following code in the app.py file:

from flask_login import LoginManager, login_user, current_user

login_manager = LoginManager()
login_manager.init_app(app)

@login_manager.user_loader
def load_user(user_id):
    return User(user_id)

@app.route("/login")
def login():
    user_id = request.args.get("user_id")
    password = request.args.get("password")
    if user_id and password:
        # 省略查询数据库的代码
        user = User(user_id)
        login_user(user)
        return "Login success!"
    else:
        return "Login failed!"

@app.route("/logout")
def logout():
    logout_user()
    return "Logout success!"

In this example, we first create a LoginManager object. Then, we define a user_loader callback function that takes the user ID as a parameter and returns the user object corresponding to that ID. In this example, we create a new User object and pass the ID as a parameter.

Next, we define a login view. In the view function, we use the request.args.get method to get the user ID and password from the query string. Then we check if the user ID and password are valid. If the user's username and password are correct, a new User object is created and the object is added to Flask-Login's user session using the login_user function. If a user's username and password are incorrect, the user cannot log in.

Finally, we define a logout view, which uses the logout_user function to delete the current user object from Flask-Login's user session and returns a message indicating successful logout.

4. Protect other views

Now, we have implemented the user login system, but we are not using it to protect other views. In this example, we will protect a page called "profile" that is only accessible when the user is logged in. If the user is not logged in, they will be redirected to the login page.

Add the following code in the app.py file:

from flask_login import login_required

@app.route("/profile")
@login_required
def profile():
    return f"Welcome, {current_user.id}!"

In this example, we use the login_required decorator to protect the "profile" route. If the user is not logged in, this decorator will redirect to the login page.

When protecting the view, we also use the current_user global variable to access the information of the currently logged in user. In this example, we add a simple welcome message that contains the ID of the currently logged in user.

5. Run the application

Now, we have completed all implementations of the user login system. Let's run the application and test it.

Enter the following command in the terminal to launch the application:

python app.py

Enter the following URL in the browser to access the homepage:

http://localhost:5000/

You should see "Hello World !" string. Now try to access the following URL:

http://localhost:5000/profile

Since you are not an authenticated user, you will be redirected to the login page. Enter a valid user ID and password in the query string, for example:

http://localhost:5000/login?user_id=1&password=password123

If you entered the correct username and password, you should see the "Login success!" message. Now, access the "/profile" route again and it should display a welcome message containing the ID of the currently logged in user.

如果你在浏览器中访问以下URL，则可以注销当前会话：

http://localhost:5000/logout

这样就完成了用户登录系统的所有实现。

总结

在本文中，我们介绍了如何使用Flask-Login扩展在Python服务器中实现用户登录功能。使用Flask-Login可以方便地验证和保护Web应用程序中的用户数据。此外，Flask-Login提供了许多有用的功能和方法，如会话管理和用户保护。希望这篇文章能够帮助你更好地了解如何使用Flask-Login编写Python服务器应用程序。

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