Python Jinja
last modified January 29, 2024
Jinja tutorial shows how to create templates in Python with Jinja module.
Python Jinja module
Jinja is a template engine for Python. It is similar to the Django template engine.
A template engine or template processor is a library designed to combine templates with a data model to produce documents. Template engines are often used to generate large amounts of emails, in source code preprocessing, or producing dynamic HTML pages.
We create a template engine, where we define static parts and dynamic parts. The dynamic parts are later replaced with data. The rendering function later combines the templates with data.
Jinja installation
$ sudo pip3 install jinja2
We use the pip3 tool to install Jinja.
Jinja delimiters
Jinja uses various delimiters in the template strings.
{% %}- statements{{ }}- expressions to print to the template output{# #}- comments which are not included in the template output# ##- line statements
Jinja simple example
In the first example, we create a very simple template.
#!/usr/bin/python
from jinja2 import Template
name = input("Enter your name: ")
tm = Template("Hello {{ name }}")
msg = tm.render(name=name)
print(msg)
The example asks for a user name and generates a message string, which
is printed to the user. The template engine is similar to the Python
format method; but template engines are more powerful
and have many more features.
from jinja2 import Template
We import the Template object from the jinja2
module. Template is the central template object. It
represents a compiled template and is used to evaluate it.
tm = Template("Hello {{ name }}")
In our template, we have the {{ }} syntax which is used
to print the variable. The variable is passed in the render
method.
msg = tm.render(name=name)
With the render method, we generate the final output.
The method joins the template string with the data passed as argument.
The variable that is passed to the render method
is called the context variable.
$ ./simple.py Enter your name: Paul Hello Paul
In the next example, we use two variables.
#!/usr/bin/python
from jinja2 import Template
name = 'Peter'
age = 34
tm = Template("My name is {{ name }} and I am {{ age }}")
msg = tm.render(name=name, age=age)
print(msg)
The template string renders two variables: name and age. This time the variables are hard-coded.
$ ./simple2.py My name is Peter and I am 34
Jinja objects
We can work with objects in our template strings.
#!/usr/bin/python
from jinja2 import Template
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def getAge(self):
return self.age
def getName(self):
return self.name
person = Person('Peter', 34)
tm = Template("My name is {{ per.getName() }} and I am {{ per.getAge() }}")
msg = tm.render(per=person)
print(msg)
In the example, we define a Person object. We get the name
and age via the two getters.
Dictionaries
Jinja allows a convenient dot notation to access data in Python dictionaries.
#!/usr/bin/python
from jinja2 import Template
person = { 'name': 'Person', 'age': 34 }
tm = Template("My name is {{ per.name }} and I am {{ per.age }}")
# tm = Template("My name is {{ per['name'] }} and I am {{ per['age'] }}")
msg = tm.render(per=person)
print(msg)
We have a person dictionary. We access the dictionary keys with a dot operator.
tm = Template("My name is {{ per.name }} and I am {{ per.age }}")
# tm = Template("My name is {{ per['name'] }} and I am {{ per['age'] }}")
Both the active and the commented way are valid. The dot notation is more convenient.
Jinja raw data
We can use raw, endraw markers to escape
Jinja delimiters.
#!/usr/bin/python
from jinja2 import Template
data = '''
{% raw %}
His name is {{ name }}
{% endraw %}
'''
tm = Template(data)
msg = tm.render(name='Peter')
print(msg)
By using the raw, endraw block, we escape
the Jinja {{ }} syntax. It is printed in its literal meaning.
Jinja escape data
To escape data such as < or > characters,
we can use a filter or the escape function.
#!/usr/bin/python
from jinja2 import Template, escape
data = '<a>Today is a sunny day</a>'
tm = Template("{{ data | e}}")
msg = tm.render(data=data)
print(msg)
print(escape(data))
The example escapes < and > characters.
tm = Template("{{ data | e}}")
Using the e filter, the data is escaped. Filters are applied
with the | character.
print(escape(data))
The escape function does the same.
Jinja for expressions
The for expression is used to iterate over a data collection in a template.
Now we do not use a simple string template anymore. We use a text file
which is loaded with FileSystemLoader.
#!/usr/bin/python
from jinja2 import Environment, FileSystemLoader
persons = [
{'name': 'Andrej', 'age': 34},
{'name': 'Mark', 'age': 17},
{'name': 'Thomas', 'age': 44},
{'name': 'Lucy', 'age': 14},
{'name': 'Robert', 'age': 23},
{'name': 'Dragomir', 'age': 54}
]
file_loader = FileSystemLoader('templates')
env = Environment(loader=file_loader)
template = env.get_template('showpersons.txt')
output = template.render(persons=persons)
print(output)
In this example, the template is the showpersons.txt file.
The file is located in the templates directory.
persons = [
{'name': 'Andrej', 'age': 34},
{'name': 'Mark', 'age': 17},
{'name': 'Thomas', 'age': 44},
{'name': 'Lucy', 'age': 14},
{'name': 'Robert', 'age': 23},
{'name': 'Dragomir', 'age': 54}
]
The data is a list of dictionaries.
file_loader = FileSystemLoader('templates')
env = Environment(loader=file_loader)
We define a FileSystemLoader. The template is retrieved
from the templates directory.
template = env.get_template('showpersons.txt')
We get the template with the get_template method.
{% for person in persons -%}
{{ person.name }} {{ person.age }}
{% endfor %}
In the template file, we use the for expression to iterate over the collection. We show the person's name and age. The dash character next to the % characters is used to control white space.
Jinja conditionals
Conditionals are expressions that are evaluated when a certain condition is met.
#!/usr/bin/python
from jinja2 import Environment, FileSystemLoader
persons = [
{'name': 'Andrej', 'age': 34},
{'name': 'Mark', 'age': 17},
{'name': 'Thomas', 'age': 44},
{'name': 'Lucy', 'age': 14},
{'name': 'Robert', 'age': 23},
{'name': 'Dragomir', 'age': 54},
]
file_loader = FileSystemLoader('templates')
env = Environment(loader=file_loader)
env.trim_blocks = True
env.lstrip_blocks = True
env.rstrip_blocks = True
template = env.get_template('showminors.txt')
output = template.render(persons=persons)
print(output)
The example prints only minor persons; a minor is someone younger than 18.
env.trim_blocks = True env.lstrip_blocks = True env.rstrip_blocks = True
White space in output can be controlled with environment attributes.
{% for person in persons %}
{% if person.age < 18 %}
{{- person.name }}
{% endif %}
{%- endfor %}
In the template, we output only persons younger than 18 using if expression.
$ ./conditionals.py Mark Lucy
Jinja sum filter
Filters can be applied to data to modify them. For instance, the sum filter can sum data, escape filter escapes them, and sort filter sorts them.
#!/usr/bin/python
from jinja2 import Environment, FileSystemLoader
cars = [
{'name': 'Audi', 'price': 23000},
{'name': 'Skoda', 'price': 17300},
{'name': 'Volvo', 'price': 44300},
{'name': 'Volkswagen', 'price': 21300}
]
file_loader = FileSystemLoader('templates')
env = Environment(loader=file_loader)
template = env.get_template('sumprices.txt')
output = template.render(cars=cars)
print(output)
In the example, we use the sum filter to calculate
the sum of all car prices.
cars = [
{'name': 'Audi', 'price': 23000},
{'name': 'Skoda', 'price': 17300},
{'name': 'Volvo', 'price': 44300},
{'name': 'Volkswagen', 'price': 21300}
]
We have a list of car dictionaries. Each dictionary has a price key. It will be used to calculate the sum.
The sum of car prices is {{ cars | sum(attribute='price') }}
In the template file, we apply the filter on the cars collection
object. The sum is calculated from the price attribute.
$ ./sum_filter.py The sum of car prices is 105900
Jinja template inheritance
Template inheritance is a powerful feature that reduces code duplication and improves code organization. We define a base template from which we inherit in other template files. These template files overwrite specific blocks of the base template file.
#!/usr/bin/python
from jinja2 import Environment, FileSystemLoader
content = 'This is about page'
file_loader = FileSystemLoader('templates')
env = Environment(loader=file_loader)
template = env.get_template('about.html')
output = template.render(content=content)
print(output)
We render the about.html file. It inherits from the
base.html file.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>{% block title %}{% endblock %}</title>
</head>
<body>
{% block content%}
{% endblock %}
</body>
</html>
In the base.html file, we declare two blocks: title and
content. These blocks are going to be filled with specific tags and text
in the child templates.
{% extends 'base.html' %}
{% block title%}About page{% endblock %}
{% block content %}
<h1>About page</h1>
<p>
This is about page
</p>
{% endblock %}
The about.html template file inherits from base.html.
It adds data specific to this page. We avoid code repetition; we
do not repeat tags that are same for both pages, such as body and html
and meta tags.
{% extends 'base.html' %}
The inheritance is done with the extends directive.
{% block title%}About page{% endblock %}
We define a title.
{% block content %}
<h1>About page</h1>
<p>
This is about page
</p>
{% endblock %}
And we define content.
Jinja Flask example
In the next example, we create a simple Flask application that uses Jinja.
#!/usr/bin/python
from flask import Flask, render_template, request
app = Flask(__name__)
@app.route("/greet")
def greet():
username = request.args.get('name')
return render_template('index.html', name=username)
if __name__ == "__main__":
app.run()
In this Flask application, we get the name of a user and pass it as a
parameter to the render_template method. The greet
function reacts to the /greet path.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Greeting</title>
</head>
<body>
<p>
Hello {{ name }}
</p>
</body>
</html>
This is the template file, located in the templates
directory. We add the name of the user to the template file with
{{ name }} syntax.
$ python3 app.py * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
We start the server.
$ curl http://127.0.0.1:5000/greet?name=Peter
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Greeting</title>
</head>
<body>
<p>
Hello Peter
</p>
</body>
</html>
We connect to the application with the curl tool. We
add a name parameter.
Source
In this article we have covered Python Jinja module.
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