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if

Show a block if the condition is true.

The {% if %} tag evaluates a variable and if the result is true (boolean true, number unequal to zero, non empty stringor a non empty list) then the contents of the if-block are output.  When the if-variable test fails then the optional {% else %} block content are output.

{% if person_list %}
  There are {{ person_list|length }} persons.
{% else %}
  There are no persons.
{% endif %}

When the person_list is not empty then the number of persons is displayed, otherwise the {% else %} block is displayed.

An {% if %} tag can have an “and” or “or” expression as argument:

{% if person_list and show_persons and full_moon %}
  There are persons that we can show during full moon.
{% endif %}

Or for example:

{% if new_moon or daytime %} Guess you can't see the moon. {% endif %}

It is also possible to mix “and” and ”or” in one expression, so the expression:

{% if full_moon or daytime and cloudy %}

is allowed.

The ”not” operator can be used to negate a boolean value:

{% if full_moon or daytime or not clearsky %}