Conditional statements and logical operators

Introduction

In the context of Jinja templating, conditional statements and logical operators are vital tools for creating dynamic and responsive templates. This article provides an in-depth exploration of if, elif, and else statements, as well as the logical operators and, or, and not within the framework of Jinja.

Understand if, elif, and else statements in Jinja

In Jinja templating, if, elif, and else statements function similarly to their counterparts in traditional programming languages. They enable you to control the flow of your templates based on specific conditions.

• if is a foundational conditional statement in Jinja. When the specified condition is true, the associated template code block within the if statement is executed.

• elif stands for "else if" and is utilized to check additional conditions after the initial if statement. elif is evaluated only if the preceding if statement is false.

• else is a fallback option. It executes a template code block when none of the preceding conditions (if and elif, if present) are true.

Example 1: Use if and else statements in Jinja

{%- set dogs = 1 -%}
{%- set cats = 0 -%}
{%- if dogs -%}
    {{- "we have dogs!\n" -}}
{%- else -%}
    {{- "sorry no dogs today\n" -}}
{%- endif -%}

{%- if not cats -%}
    {{- "sorry, no cats today.\n" -}}
{%- else -%}
    {{- "we have cats!\n" -}}
{%- endif -%}

Example 2: Use if, elif, and else ladders in Jinja

{%- set value = 1 -%}
{%- if value == 2 -%}
    {{- "value is 2" -}}
{%- elif value == 3 -%}
    {{- "value is 3" -}}
{%- elif value == 4 -%}
    {{- "value is 4" -}}
{%- elif value == 5 -%}
    {{- "value is 5" -}}
{%- else -%}
    {{- "value is "+value | string -}}
{% endif %}

While using multiple elif statements can be an option, it's generally not the best practice for handling numerous conditions in Jinja templates. Instead, consider using a dictionary switch for improved readability and efficiency when dealing with extensive branching logic.

Logical operators: and, or, and not in Jinja

Logical operators in Jinja allow you to combine multiple conditions or negate a condition.

• and requires all specified conditions to be true for the entire expression to be true.

• or requires at least one of the specified conditions to be true for the entire expression to be true.

• not negates a condition, making it true if the original condition is false and vice versa.

Example: Use logical operators in Jinja

{%- set dogs = 1 -%}
{%- set cats = 0 -%}
{%- set lizards = 1 -%}
{%- set chickens = 0 -%}

{%- if dogs and lizards -%}
    {{- "we have both dogs and lizards.\n" -}}
{%- endif -%}

{%- if not cats -%}
    {{- "we have no cats.\n" -}}
{%- endif -%}

{%- if dogs or chickens -%}
    {{- "we have either dogs or chickens, or both, but not necessarily both.\n" -}}
{%- endif -%}

{%- if dogs and not chickens -%}
    {{- "we have dogs but not chickens.\n" -}}
{%- endif -%}

{%- if (dogs or chickens) and lizards -%}
    {{- "we have lizards and either dogs or chickens but not necessarily both.\n" -}}
{%- endif -%}

{%- if chickens < dogs -%}
    {{- "we have more dogs than chickens.\n" -}}
{%- endif -%}

{%- if lizards == dogs -%}
    {{- "we have as many lizards as dogs.\n" -}}
{%- endif -%}

{%- if lizards > chickens -%}
    {{- "we have more lizards than chickens.\n" -}}
{%- endif -%}