I think the easiest way to do this would be to create a custom Sphinx theme in which you tell certain html elements to have this functionality. A little JQuery would go a long way here.

If, however you want to be able to specify this in your reStructuredText markup, you would need to either

• get such a thing included in Sphinx itself or
• implement it in a Sphinx/docutils extension...and then create a Sphinx theme which knew about this functionality.

This would be a bit more work, but would give you more flexibility.

You don t need a custom theme. Use the built-in directive container that allows you to add custom css-classes to blocks and override the existsting theme to add some javascript to add the show/hide-functionality.

This is _templates/page.html:

{% extends "!page.html" %}

{% block footer %}
 <script type="text/javascript">
    $(document).ready(function() {
        $(".toggle > *").hide();
        $(".toggle .header").show();
        $(".toggle .header").click(function() {
            $(this).parent().children().not(".header").toggle(400);
            $(this).parent().children(".header").toggleClass("open");
        })
    });
</script>
{% endblock %}

This is _static/custom.css:

.toggle .header {
    display: block;
    clear: both;
}

.toggle .header:after {
    content: " ▶";
}

.toggle .header.open:after {
    content: " ▼";
}

This is added to conf.py:

def setup(app):
    app.add_css_file( custom.css )

Now you can show/hide a block of code.

.. container:: toggle

    .. container:: header

        **Show/Hide Code**

    .. code-block:: xml
       :linenos:

       from plone import api
       ...

I use something very similar for exercises here: https://training.plone.org/5/mastering-plone/about_mastering.html#exercises

You can use the built-in HTML collapsible details tag by wrapping the code in two raw HTML directives

.. raw:: html

   <details>
   <summary><a>big code</a></summary>

.. code-block:: python

   lots_of_code = "this text block"

.. raw:: html

   </details>

Produces:

<details>
<summary><a>big code</a></summary>
<pre>lots_of_code = "this text block"</pre>
</details>

There is a very simplistic extension providing exactly that feature: https://github.com/scopatz/hiddencode

It works rather well for me.

The cloud sphinx theme has custom directive html-toggle that provides toggleable sections. To quote from their web page:

You can mark sections with .. rst-class:: html-toggle, which will make the section default to being collapsed under html, with a “show section” toggle link to the right of the title.

Here is a link to their test demonstration page.

sphinx-togglebutton

Looks like a new sphinx extension has been made to do just this since this question has been answered.

Run: pip install sphinx-togglebutton

Add to conf.py

extensions = [
...
 sphinx_togglebutton 
...
]

In rst source file:

.. admonition:: Show/Hide
  :class: dropdown

  hidden message

since none of the above methods seem to work for me, here s how I solved it in the end:

1. create a file substitutions.rst in your source-directory with the following content:

   .. |toggleStart| raw:: html
   
      <details>
      <summary><a> the title of the collapse-block </a></summary>
   
   .. |toggleEnd| raw:: html
   
      </details>
      <br/>
   

2. add the following line at the beginning of every file you want to use add collapsible blocks

   ..include:: substitutions.rst
   

3. now, to make a part of the code collapsible simply use:

   |toggleStart|
   
   the text you want to collapse
   ..code-block:: python 
       x=1
   
   |toggleEnd|
   

Another option is the dropdown directive in the sphinx-design extension. From the docs:

1. Install sphinx-design

pip install sphinx-design

2. Add the extension to conf.py in the extensions list

extensions = ["sphinx_design"]

3. Use the dropdown directive in your rst file:

.. dropdown::

    Dropdown content