jinja if else with code examples

Jinja is a template engine for Python that allows developers to create dynamic templates for web applications. One of the most powerful features of Jinja is the ability to use control structures such as if-else statements in templates. This allows for more complex logic to be implemented in the template, rather than in the application code.

To use an if-else statement in Jinja, you use the {% if %} and {% else %} tags. The basic syntax of an if-else statement in Jinja is as follows:

{% if condition %}
    <!-- code to execute if condition is true -->
{% else %}
    <!-- code to execute if condition is false -->
{% endif %}

The condition in the if statement can be any valid Python expression that evaluates to either True or False. Here is an example of an if-else statement in Jinja that checks if a variable is_published is True or False:

{% if is_published %}
    <p>This article has been published.</p>
{% else %}
    <p>This article has not been published yet.</p>
{% endif %}

It is also possible to use elif in Jinja, which is equivalent to the else if statement in Python. The syntax for an elif statement is as follows:

{% if condition1 %}
    <!-- code to execute if condition1 is true -->
{% elif condition2 %}
    <!-- code to execute if condition1 is false and condition2 is true -->
{% else %}
    <!-- code to execute if condition1 and condition2 are both false -->
{% endif %}

Here is an example of an if-elif-else statement that checks if a variable status is "approved", "pending" or "rejected"

{% if status == "approved" %}
    <p>This article has been Approved.</p>
{% elif status == "pending" %}
    <p>This article is pending.</p>
{% else %}
    <p>This article has been rejected.</p>
{% endif %}

It is also possible to use comparison operators like ==, !=, <, >, <= and >= in conditions.

You can also use not and and and or to combine multiple conditions.

{% if status == "approved" and author == "John" %}
    <p>This article has been Approved by John.</p>
{% else %}
    <p>This article has been approved by someone else.</p>
{% endif %}

You can also use loop statements like for and while inside an if statement.

{% if articles|length > 0 %}
    <ul>
    {% for article in articles %}
        <li>{{ article.title }}</li>
    {% endfor %}
    </ul>
{% else %}
    <p>No articles found.</p>
{% endif %}

In this way, you can use Jinja if-
Jinja also provides several built-in functions that can be used in conditions, such as defined, undefined, equalto, and sameas. The defined function returns True if a variable is defined and False if it is not. The undefined function is the opposite of defined, and returns True if a variable is not defined and False if it is. The equalto function is used to check if two variables are equal, and the sameas function is used to check if two variables refer to the same object.

Here is an example of using the defined function to check if a variable user is defined:

{% if user is defined %}
    <p>Welcome, {{ user.name }}!</p>
{% else %}
    <p>Welcome, guest!</p>
{% endif %}

Jinja also provides several built-in filters that can be used to manipulate variables. These filters can be applied to a variable using the | operator. Some common filters include length, lower, upper, sort, reverse, number, date, default, and join.

For example, you can use the length filter to get the length of a list, and the lower filter to convert a string to lowercase.

<p>Number of articles: {{ articles|length }}</p>
<p>Lowercase: {{ name|lower }}</p>

Additionally, you can also define your own custom filters in Jinja. To define a custom filter, you need to use the @app.template_filter decorator in Flask or jinja2.filters.FILTERS in jinja2, and then use it in the template.

from jinja2 import filters

def my_custom_filter(value):
    return value + " World!"

filters.FILTERS["hello"] = my_custom_filter

You can use your custom filter like this:

<p>{{ "Hello"|hello }}</p>

In this way, Jinja's if-else statements, built-in functions and filters, and custom filters can be used to add complex logic and functionality to your templates. This allows for more efficient and maintainable code, as well as a more dynamic and interactive user experience.

Popular questions

1. What is Jinja and what is it used for?

• Jinja is a template engine for Python that allows developers to create dynamic templates for web applications.

2. How do you use if-else statements in Jinja templates?

• To use an if-else statement in Jinja, you use the {% if %} and {% else %} tags. The basic syntax of an if-else statement in Jinja is as follows:

{% if condition %}
    <!-- code to execute if condition is true -->
{% else %}
    <!-- code to execute if condition is false -->
{% endif %}

3. Can you use comparison operators like ==, !=, <, >, <= and >= in Jinja conditions?

• Yes, you can use comparison operators like ==, !=, <, >, <= and >= in conditions.

4. Can you use 'and' and 'or' to combine multiple conditions in Jinja?

• Yes, you can use not and and and or to combine multiple conditions in Jinja.

5. How do you define custom filters in Jinja?

• To define a custom filter in Jinja, you need to use the @app.template_filter decorator in Flask or jinja2.filters.FILTERS in jinja2, and then use it in the template.

from jinja2 import filters

def my_custom_filter(value):
    return value + " World!"

filters.FILTERS["hello"] = my_custom_filter

You can use your custom filter like this:

<p>{{ "Hello"|hello }}</p>

Tag

Templating