How can I check the size of a collection within a Django template?

See https://docs.djangoproject.com/en/stable/ref/templates/builtins/#if : just use, to reproduce their example:

{% if athlete_list %}
    Number of athletes: {{ athlete_list|length }}
{% else %}
    No athletes.
{% endif %}

If you're using a recent Django, changelist 9530 introduced an {% empty %} block, allowing you to write

{% for athlete in athlete_list %}
  ...
{% empty %}
  No athletes
{% endfor %}

Useful when the something that you want to do involves special treatment for lists that might be empty.

A list is considered to be False if it has no elements, so you can do something like this:

{% if mylist %}
    <p>I have a list!</p>
{% else %}
    <p>I don't have a list!</p>
{% endif %}

If you tried myList|length and myList|length_is and its not getting desired results, then you should use myList.count

You can try with:

{% if theList.object_list.count > 0 %}
    blah, blah...
{% else %}
    blah, blah....
{% endif %} 

This works:

{% if myList|length %}
    Do something!
{% endif %}

Why there's so many answers here and why there's so much confusion is that this didn't always work. I think at one point template filters couldn't be used on arguments to the if statement and this was later added. It's also now possible to do things such as {% if myList|length >= 3 %}. The filter should do the equivalent of len(myList) so any type of object that can handle that would also be able to handle the |length filter.

Collection.count no bracket

{% if request.user.is_authenticated %}
{{wishlists.count}}
{% else %}0{% endif %}

I need the collection length to decide whether I should render table <thead></thead>

but don't know why @Django 2.1.7 the chosen answer will fail(empty) my forloop afterward.

I got to use {% if forloop.first %} {% endif %} to overcome:

<table>
    {% for record in service_list %}
        {% if forloop.first %}
            <thead>
            <tr>
                <th>日期</th>
            </tr>
            </thead>
        {% endif %}
        <tbody>
        <tr>
            <td>{{ record.date }}</td>
        </tr>
    {% endfor %}
    </tbody>
</table>