cannot traverse an already closed generator

[ricohumme]

This behavior occurs when applying the following:

1. Filter a collection in twig and store the Generator in a variable
2. Check the Generator having iterable content by applying the filter length
3. Iterate the Generator in a for loop in twig

In some cases there is nothing to iterate over, so by checking the size of the Generator and seeing there is something to iterate over, I render some content previous to the for loop and then iterate the Generator.

However, the Generator is being closed after applying the filter length, rendering the Generator useless after that.

Example:

    {% set collection = ['property1', 'property2'] %}
    {% set generator = collection|filter(v => v is same as('property1')) %}
    {% if generator|length > 0 %}
        {% for item in generator %}
            {{ item }}
        {% endfor %}
    {% endif %}

Is this intent?

[stof]

well, computing the length of an iterator requires iterating over it.
So using a generator for the implementation of the filter filter indeed looks like a bad idea.
/cc @fabpot

[fabpot]

fixed in #3065

[ricohumme]

Thnx @fabpot

[benbor]

Hi! @stof
Displaying a stub for empty collection is a very common task.

Do you have some ideas, how to check is Generator are not empty in the twig?

{% if generator %}  {# <----- `is not empty` ? #}
        {% for item in generator %}
            {{ item }}
        {% endfor %}
{% else %}
        <div> The collection is empty</div> 
{% endif %}

[stof]

@benbor checking whether a generator is not empty is not doable in PHP without consuming it. So there is no way to do it in Twig either.

[ricohumme]

Hi @benbor
Maybe this will put you in the right direction?
https://twig.symfony.com/doc/3.x/tags/for.html#the-else-clause

[stof]

for that particular example, the else clause is indeed a solution that avoids checking separately whether the generator is empty (but it does not work if your if contains more stuff than the loop)

[benbor]

@ricohumme @stof thanks. It's exactly what I was looking for!