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``keys``
========
The ``keys`` filter returns the keys of a sequence or a mapping. It is useful
when you want to iterate over the keys of a sequence or a mapping:
.. code-block:: twig
{% for key in ['a', 'b', 'c', 'd']|keys %}
{{ key }}
{% endfor %}
{# outputs: 0 1 2 3 #}
{% for key in {a: 'a_value', b: 'b_value'}|keys %}
{{ key }}
{% endfor %}
{# outputs: a b #}
.. note::
Internally, Twig uses the PHP `array_keys`_ function.
.. _`array_keys`: https://www.php.net/array_keys
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