Common Use Cases
================


Include in Web Applications
---------------------------

Using WeasyPrint in web applications sometimes requires attention on some
details.

Security Problems
.................

First of all, rendering untrusted HTML and CSS files can lead to :ref:`security
problems <Security>`. Please be sure to carefully follow the different proposed
solutions if you allow your users to modify the source of the rendered
documents in any way.

Rights Management
.................

Another problem is rights management: you often need to render templates that
can only be accessed by authenticated users, and WeasyPrint installed on the
server doesn’t send the same cookies as the ones sent by the users. Extensions
such as Flask-WeasyPrint_ (for Flask_) or Django-WeasyPrint_ (for Django_)
solve this issue with a small amount of code. If you use another framework, you
can read these extensions and probably find an equivalent workaround.

.. _Flask-Weasyprint: https://github.com/Kozea/Flask-WeasyPrint
.. _Flask: https://flask.palletsprojects.com/
.. _Django-WeasyPrint: https://github.com/fdemmer/django-weasyprint
.. _Django: https://www.djangoproject.com/

Server Side Requests & Self-Signed SSL Certificates
...................................................

If your server is requesting data from itself, you may encounter a self-signed
certificate error, even if you have a valid certificate.

You need to add yourself as a Certificate Authority, so that your self-signed
SSL certificates can be requested.

.. code-block:: bash

   # If you have not yet created a certificate.
   sudo openssl req -x509 \
       -sha256 \
       -nodes \
       -newkey rsa:4096 \
       -days 365 \
       -keyout localhost.key \
       -out localhost.crt

   # Follow the prompts about your certificate and the domain name.
   openssl x509 -text -noout -in localhost.crt

Add your new self-signed SSL certificate to your nginx.conf, below the line
``server_name 123.123.123.123;``:

.. code-block:: bash

   ssl_certificate /etc/ssl/certs/localhost.crt;
   ssl_certificate_key /etc/ssl/private/localhost.key;

The SSL certificate will be valid when accessing your website from the
internet. However, images will not render when requesting files from the same
server.

You will need to add your new self-signed certificates as trusted:

.. code-block:: bash

   sudo cp /etc/ssl/certs/localhost.crt /usr/local/share/ca-certificates/localhost.crt
   sudo cp /etc/ssl/private/localhost.key /usr/local/share/ca-certificates/localhost.key

   # Update the certificate authority trusted certificates.
   sudo update-ca-certificates

   # Export your newly updated Certificate Authority Bundle file.
   # If using Django, it will use the newly signed certificate authority as
   # valid and images will load properly.
   sudo tee -a /etc/environment <<< 'export REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt'


Adjust Document Dimensions
--------------------------

WeasyPrint does not provide support for adjusting page size or document margins
via command-line flags. This is best accomplished with the CSS ``@page``
at-rule. Consider the following example:

.. code-block:: css

  @page {
    size: Letter; /* Change from the default size of A4 */
    margin: 3cm; /* Set margin on each page */
  }

There is much more which can be achieved with the ``@page`` at-rule,
such as page numbers, headers, etc. Read more about the page_ at-rule.

.. _page: https://developer.mozilla.org/en-US/docs/Web/CSS/@page


Improve Rendering Speed and Memory Use
--------------------------------------

WeasyPrint is often slower than other web engines. Python is the usual suspect,
but it’s not the main culprit here. :ref:`Optimization is not the main goal of
WeasyPrint <Why Python?>` and it may lead to unbearable long rendering times.

First of all: WeasyPrint’s performance gets generally better with time. You can
check WeasyPerf_ to compare time and memory needed across versions.

Some tips may help you to get better results.

- A high number of CSS properties with a high number of HTML tags can lead to a
  huge amount of time spent for the cascade. Avoiding large CSS frameworks can
  drastically reduce the rendering time.
- Tables are known to be slow, especially when they are rendered on multiple
  pages. When possible, using a common block layout instead gives much faster
  layouts.
- Optimizing images and fonts can reduce the PDF size, but increase the
  rendering time. Moreover, caching images gives the possibility to read and
  optimize images only once, and thus to save time when the same image is used
  multiple times. See :ref:`Image Cache and Optimization`.

.. _WeasyPerf: https://kozea.github.io/WeasyPerf/