Introduction to Flask templates

Flask templates are an essential component of Flask web development, allowing you to separate the presentation layer of your application from the logic and make it easier to maintain and update. In Flask, templates are written in HTML and can include placeholders for dynamic content, which are then populated with data from your application. The Flask template engine provides a simple syntax for including these placeholders and for adding logic to your templates, such as loops and conditional statements.

To use templates in Flask, you need to create a templates directory within your application directory and store your HTML templates there. To use a template, you will create a view function that returns a template and pass in any data that you want to include in the template. The Flask template engine will then render the template with the data and return it as a response to the client.

In this article, we will explore the basics of Flask templates and how to use them in your Flask applications. We will cover the following topics:

Basic templates and placeholders
Loops and conditional statements
Template inheritance
Custom template filters
Debugging templates
Basic templates and placeholders

The first step in using templates in Flask is to create a templates directory within your application directory. This is where you will store your HTML templates. To use a template, you will create a view function that returns a template and pass in any data that you want to include in the template. The Flask template engine will then render the template with the data and return it as a response to the client.

Here’s an example of a simple template in Flask:

<!DOCTYPE html>
<html>
  <head>
    <title>My Flask Application</title>
  </head>
  <body>
    <h1>Hello, {{ name }}!</h1>
  </body>
</html>

In this example, the {{ name }} placeholder will be replaced with the value of the name variable that is passed to the template.

To use this template in your Flask application, you would create a view function like this:

pythonCopy codefrom flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    name = "John Doe"
    return render_template("index.html", name=name)

The render_template function takes the name of the template as its first argument and a dictionary of variables as its second argument. In this example, the name variable is passed to the template and will be used to replace the {{ name }} placeholder.

You can use placeholders in your templates to display dynamic content, such as user-generated content, database data, or the results of calculations. For example, here’s a template that displays a list of users:

<!DOCTYPE html>
<html>
  <head>
    <title>My Flask Application</title>
  </head>
  <body>
    <h1>Users</h1>
    <ul>
      {% for user in users %}
        <li>{{ user.name }} ({{ user.email }})</li>
      {% endfor %}
    </ul>
  </body>
</html>

In this example, the {% for user in users %} block is used to iterate over the users list and display

Flask templates are an essential aspect of Flask web development, as they provide the means to create dynamic, user-facing pages in a clean, structured way. They allow developers to separate the presentation layer of a web application from its underlying logic, making it easier to maintain and update the look and feel of a web application without affecting its underlying functionality.

The Flask template engine is based on Jinja2, a widely used and well-documented template engine for Python. Jinja2 provides a flexible, concise syntax for creating templates, with a range of features including placeholders, loops, and conditional statements.

Creating a Template

To create a template in Flask, you simply need to create an HTML file and store it in a templates folder within your Flask application directory. The structure of the template can be as simple or as complex as you like, but at a minimum it should include the following elements:

A doctype declaration: This declares the type of document that the template represents, such as HTML5 or XHTML.
An HTML tag: This contains all of the content that makes up the template, including the head and body sections.
A head section: This contains information about the document, such as the title, which is displayed in the browser’s title bar, and any stylesheets or scripts that the template needs to load.
A body section: This contains the content that is displayed to the user, such as text, images, and links.

Here’s a simple example of a Flask template:

<!DOCTYPE html>
<html>
  <head>
    <title>My Flask Application</title>
  </head>
  <body>
    <h1>Hello, World!</h1>
  </body>
</html>

Rendering a Template

To render a template in Flask, you use the render_template function, which is provided by the Flask library. The render_template function takes the name of the template as its first argument and a dictionary of variables as its second argument.

Here’s an example of how you might use the render_template function in a Flask view:

from flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    return render_template("index.html")

In this example, the render_template function is called from the index view, which is mapped to the root URL of the Flask application. The render_template function takes the name of the template as its first argument, which in this case is “index.html”, and returns the rendered template as the response to the client.

Passing Variables to Templates

One of the key features of templates is that they allow you to pass data from your Flask application to the template, which can then be used to dynamically generate the content of the page. To pass variables to a template, you simply pass a dictionary of variables as the second argument to the render_template function.

Here’s an example of how you might pass variables to a template in Flask:

from flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    name = "John Doe"
    return render_template("index.html", name=name)