Control structures

Reference for OneFuse Jinja2 control structures including for loops, if statements, and variable assignment

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A control structure refers to all those things that control the flow: conditionals, for-loops, as well as assignments.

Control structures appear inside {% ... %} tags.

For

Loop over each item in a sequence.

{# List users provided in a variable called 'users' #}
{% for user in users %}
  {{ user.username | e }}
{% else %}
  {# Optional else block if no iteration took place because sequence was empty or the filtering removed all items from the sequence. #}
  No users found.
{% endfor %}

{#
Unlike other for loops, you cannot break or continue in a loop. Instead, use filters
#}

{% for user in users if not user.hidden %}
  {{ user.username |e }}
{% endfor %}

Special variables

Access special variables inside a for-loop block.

Variable	Description
`loop.index`	The current iteration of the loop (1 indexed).
`loop.index0`	The current iteration of the loop (0 indexed).
`loop.revindex`	The number of iterations from the end of the loop (1 indexed).
`loop.revindex0`	The number of iterations from the end of the loop (0 indexed).
`loop.first`	Returns `true` if first iteration.
`loop.last`	Returns `true` if last iteration.
`loop.length`	The number of items in the sequence.
`loop.cycle`	A helper function to cycle between a list of sequences.
`loop.depth`	Indicates how deep in a recursive loop the rendering currently is. Starts at level 1.
`loop.depth0`	Indicates how deep in a recursive loop the rendering currently is. Starts at level 0.
`loop.previtem`	The item from the previous iteration of the loop. Undefined during the first iteration.
`loop.nextitem`	The item from the following iteration of the loop. Undefined during the last iteration.
`loop.changed(*val)`	Returns `true` if previously called with a different value (or not called at all).

If

Test if a variable is defined, not empty and not false.

if can be used as an inline expression and for-loop filtering. You can also use more complex Expressions.

{% if foo == 1 %}
  foo is 1
{% elif foo == 2 %}
  foo is 2
{% else %}
  foo is not 1 nor 2
{% endif %}

Assignment

Assign values to variables.

{% set foo = "Hello world!" %}

Block assignments

Capture contents of a block into a variable.

{% set foo %}
Hello
world
!
block
{% endset %}

Scoping behavior

When variables are set within a block, the variable values are not scoped to show outside of the block (for example, for-loops).

{# Cannot scope variables outside of blocks #}
{% set iterated = false %}
{% for item in sequence %}
  {{ item }}
  {% set iterated = true %}
{% endfor %}

{# Print iterated variable #}
{{ iterated }}

=> false

{# Use alternative constructs like the loop-else block #}
{% for item in sequence %}
  {{ item }}
{% else %}
  Did not iterate
{% endfor %}