How to Use If Else in Jinja2 Templates with Flask

In Flask, you use {% if condition %} ... {% else %} ... {% endif %} in Jinja2 templates to run conditional logic. This lets you show different content depending on values passed from your Flask app.

Syntax

The if else statement in Jinja2 uses tags to control what HTML is shown based on a condition.

• {% if condition %}: Starts the condition block.
• {% else %}: Defines the alternative block if the condition is false.
• {% endif %}: Ends the conditional block.

jinja2

{% if user.is_authenticated %}
  <p>Welcome, {{ user.name }}!</p>
{% else %}
  <p>Please log in.</p>
{% endif %}

Example

This example shows a simple Flask app passing a user object to a Jinja2 template. The template uses if else to greet logged-in users or ask visitors to log in.

python

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def home():
    user = {'is_authenticated': True, 'name': 'Alice'}
    return render_template('home.html', user=user)

# Template: templates/home.html
#
# {% if user.is_authenticated %}
#   <h1>Welcome, {{ user.name }}!</h1>
# {% else %}
#   <h1>Please log in.</h1>
# {% endif %}

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

Output

<h1>Welcome, Alice!</h1>

Common Pitfalls

Common mistakes include forgetting the closing {% endif %} tag, using Python-style if: syntax instead of Jinja2 tags, or mixing variable syntax inside the condition.

Always use Jinja2 tags {% %} for control flow, and {{ }} only for outputting values.

jinja2

{# Wrong: missing endif #}
{% if user.is_authenticated %}
  <p>Welcome!</p>

{# Correct: includes endif #}
{% if user.is_authenticated %}
  <p>Welcome!</p>
{% endif %}

Quick Reference

Tag	Purpose
{% if condition %}	Start a conditional block
{% elif condition %}	Add another condition (optional)
{% else %}	Fallback if all conditions fail
{% endif %}	End the conditional block

Key Takeaways

Use {% if %} ... {% else %} ... {% endif %} tags in Jinja2 templates for conditional rendering.

Pass variables from Flask to templates to control conditions dynamically.

Always close your if blocks with {% endif %} to avoid template errors.

Use {% elif %} for multiple conditions if needed.

Remember to use Jinja2 syntax, not Python syntax, inside templates.