How to use Python Flask JinJa2 syntax

1. Overview

Flask is a lightweight Python web framework that supports Jinja2 template engine. Jinja2 is a popular Python template engine that can be used to create dynamic web applications using Flask.

Web pages generally require html, css and js. Maybe when you first learn python web, you may write like this:

from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello():
    return '
hello
hello world!!!
'
if __name__ == '__main__':
	app.run(host='0.0.0.0', port=8000, debug=True)

Although the above code can also be executed, it is not beautiful. Now programming is basic All above are front-end and back-end separation, and front-end code will not be embedded in the back-end proxy. In order to achieve front-end and front-end separation, the MVT design plan is used:

M is spelled out as Model, has the same function as M in MVC, and is responsible for interacting with the database and performing data processing.

V is spelled out as View, which has the same function as C in MVC. It receives requests, performs business processing, and returns responses.

T is spelled out as Template, which has the same function as V in MVC and is responsible for encapsulating and constructing the html to be returned.

2. JinJa2 syntax introduction and example explanation

JinJa2 syntax introduction and example explanation:

1) Variable

In Jinja2, use {{ }} to include a variable. In Flask, you can display variables by passing them to the template. The sample code is as follows:

# variable.py
# Flask中将变量传递给模板
from flask import Flask, render_template
app = Flask(__name__)
# 也可指定模板目录
# app = Flask(__name__, template_folder="/opt/python-projects/flask")
@app.route('/')
def hello():
    name = "Alice"
    return render_template('variable.html', name=name)
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

In the above code, the variable name is passed to the hello.html template.

    



    
hello {{ name }}!

Execution

python3 variable.py

Access

curl http://192.168.182.110:8000/

2) Control structure

In Jinja2, you can use if, Statements such as for and while are used to control the output in the template. The sample code is as follows:

# if.py
# Flask中使用if控制结构
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/')
def hello():
    user = {"name": "Alice", "age": 25}
    return render_template('if.html', user=user)
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

templates/if.html Template file

    



    {% if user %}
        {% if user.age >= 18 %}
            
Hello {{ user.name }}, you are an adult!
        {% else %}
            
Hello {{ user.name }}, you are a minor!
        {% endif %}
    {% else %}
        
Hello, anonymous user!
    {% endif %}

In the above code, use the if statement to control the output and display it differently according to the user's age news.

3) Loop structure

In Jinja2, you can use the for statement to loop the content in the output template. The sample code is as follows:

# for.py
# Flask中使用for循环结构
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/')
def hello():
    users = [{"name": "Alice", "age": 25}, {"name": "Bob", "age": 30}]
    return render_template('for.html', users=users)
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

templates/for.html Template file

    



    {% for user in users %}
        
Hello {{ user.name }}!
        
You are {{ user.age }} years old.
    {% endfor %}

In the above code, use the for loop to iterate through the user list , and output the information of each user.

4) Macro

In Jinja2, you can use macros to define a code block that can be reused. The sample code is as follows:

# great.py
# Flask中使用宏
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/')
def hello():
    users = [{"name": "Alice", "age": 25}, {"name": "Bob", "age": 30}]
    return render_template('great.html', users=users)
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

Define a macrotemplates /macros.html Template

# 定义一个宏
{% macro print_user(user) %}
    
Hello {{ user.name }}!
    
You are {{ user.age }} years old.
{% endmacro %}

In the above code, a macro named print_user is defined. The macro can be imported in the template through import , and use macros to output user information. Template templates/great.html

    



    {% for user in users %}
        {% import 'macros.html' as macros %}
        {{ macros.print_user(user) }}
    {% endfor %}

In the above code, a macro named print_user is defined to output user information.

5) Filter

In Jinja2, filters can process variables, such as formatting dates, converting case, etc. The sample code is as follows:

# filter.py
# Flask中使用过滤器
from flask import Flask, render_template
import datetime
app = Flask(__name__)
@app.route('/')
def hello():
    now = datetime.datetime.now()
    return render_template('filter.html', now=now)
# 自定义过滤器
@app.template_filter('datetimeformat')
def datetimeformat(value, format='%Y-%m-%d %H:%M:%S'):
    return value.strftime(format)
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

Template filetemplates/filter.html

    



    
The current date and time is: {{ now|datetimeformat }}

In the above code, a file named datetimeformat is defined Filter for formatting dates and times. Here are custom filters. In fact, JinJa2 also has some built-in filters. Built-in filters in Jinja2: jinja.palletsprojects.com/en/3.0.x/te…

6）继承

在Jinja2中，可以使用继承来创建一个包含共同元素的模板，并通过继承该模板来创建更具体的模板。示例代码如下：

# extend.py
# Flask中使用继承
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/')
def hello():
    return render_template('extend.html')
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

模板文件 templates/base.html

    



    {% block content %}{% endblock %}

模板文件 templates/extend.html

{% extends "base.html" %}
{% block title %}Hello{% endblock %}
{% block content %}
    
Hello World!
{% endblock %}

在上面的代码中，定义了一个名为 base.html 的模板，并在 extend.html 模板中继承了该模板。extend.html 模板中可以重写 base.html 模板中的块，并在其中添加新的内容。

7）包含

在Jinja2中，可以使用包含来将一个模板包含到另一个模板中。示例代码如下：

# contain.py
# Flask中使用包含
from flask import Flask, render_template
app = Flask(__name__)
@app.route('/')
def hello():
    return render_template('contain.html')
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000, debug=True)

模板文件 templates/contain.html

    



    {% block content %}{% endblock %}
    {% include "footer.html" %}

模板文件 templates/footer.html

在上面的代码中，定义了一个名为 footer.html 的模板，并在 contain.html 模板中使用包含将 footer.html 模板包含到页面底部。这样，可以避免在每个页面中重复添加相同的页脚。

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