nnJinja’s if expression, also known as a ternary operator, offers a concise way to handle conditional logic on a single line. Instead of using a multi-line if statement block, you can use an if expression to choose between two values based on a condition. This is particularly useful for tasks like setting a variable, extending a template, or conditionally rendering a small piece of text. It follows a simple, readable syntax: value_if_true if condition else value_if_false.nn

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Basic Syntax and Structure

nThe fundamental structure of an if expression is straightforward:n

{{ expression_if_true if condition else expression_if_false }}

nJinja evaluates the condition first.n

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• If the condition is “truthy” (e.g., true, a non-empty string, a non-zero number), the expression returns expression_if_true.

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• If the condition is “falsy” (e.g., false, an empty string, 0, none), the expression returns expression_if_false.

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nThis is a single-line equivalent to a full if-else block:n

{% if condition %}n    {{ expression_if_true }}n{% else %}n    {{ expression_if_false }}n{% endif %}

nThe if expression is a cleaner and more compact way to handle simple choices, making your templates more readable.nn

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Practical Applications

nThe if expression is a versatile tool for various templating tasks.n

1. Conditional Template Inheritance

nOne of the most powerful uses is in the extends tag. You can dynamically choose which parent template to extend based on a variable.n

{% extends layout_template if layout_template is defined else 'default.html' %}

nIn this example, Jinja checks if the variable layout_template has been defined. If it has, the current template extends that layout. If not, it falls back to a default layout, 'default.html'. This prevents errors if a specific layout isn’t provided and ensures a consistent base for your pages.n

2. Dynamic Variable Assignment

nYou can use an if expression to set a variable to one of two values.n

{% set button_class = 'btn-primary' if user.is_logged_in else 'btn-secondary' %}n<a href="#" class="{{ button_class }}">Click here</a>

nThis snippet checks if the user is logged in. The button_class is set to btn-primary if they are, and btn-secondary if they aren’t. This allows you to apply different CSS styles based on a condition with minimal code.n

3. Inline Text Rendering

nThe if expression is excellent for rendering a small piece of text or a simple value based on a condition, right within a larger string.n

<p>You have {{ cart.item_count }} item{{ 's' if cart.item_count != 1 }} in your cart.</p>

nThis example correctly pluralizes the word “item.” If cart.item_count is anything other than 1, it adds an “s” to the end of the word.nn

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The Optional else Part

nThe else part of the if expression is optional. When you omit it, and the condition is false, the expression will implicitly evaluate to an Undefined object.n

{{ "[{}]".format(page.title) if page.title }}

nIn this case, if page.title is a truthy value (e.g., "My Page"), the output will be [My Page]. However, if page.title is falsy (e.g., an empty string or none), the expression returns Undefined. When Undefined is printed, it typically results in an empty string, so no output is rendered. This is a common pattern for conditionally including a value without needing to specify a fallback.nn

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Conclusion

nJinja’s if expression is a powerful and concise tool for embedding conditional logic directly into your templates. By using the if expression, you can write more readable and compact code for tasks like dynamic inheritance, variable assignment, and inline text modifications. While it’s best suited for simple conditions, its ability to streamline your template logic makes it an indispensable part of your Jinja toolkit. For complex or multi-step logic, a full if/elif/else block is still the better choice, but for quick, one-off decisions, the if expression is the ideal solution.nn