WeasyPrint/docs/tutorial.rst

Tutorial
========

As a standalone program
-----------------------

Once you have WeasyPrint :doc:`installed </install>`, you should have a
``weasyprint`` executable. Using it can be as simple as this:

.. code-block:: sh

    weasyprint http://weasyprint.org /tmp/weasyprint-website.pdf

You may see warnings on *stderr* about unsupported CSS properties.
See :ref:`command-line-api` for the details of all available options.

In particular, the ``-s`` option can add a filename for a
:ref:`user stylesheet <stylesheet-origins>`. For quick experimentation
however, you may not want to create a file. In bash or zsh, you can
use the shell’s redirection instead:

.. code-block:: sh

    weasyprint http://weasyprint.org /tmp/weasyprint-website.pdf \
        -s <(echo 'body { font-family: serif !important }')

If you have many documents to convert you may prefer using the Python API
in long-lived processes to avoid paying the start-up costs every time.


Adjusting Document Dimensions
.............................

Currently, 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: 2.5cm; /* 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,
and find an example here_.

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


As a Python library
-------------------
.. currentmodule:: weasyprint

.. attention::

    Using WeasyPrint with untrusted HTML or untrusted CSS may lead to various
    :ref:`security problems <security>`.

Quickstart
..........

The Python version of the above example goes like this:

.. code-block:: python

    from weasyprint import HTML
    HTML('http://weasyprint.org/').write_pdf('/tmp/weasyprint-website.pdf')

… or with the inline stylesheet:

.. code-block:: python

    from weasyprint import HTML, CSS
    HTML('http://weasyprint.org/').write_pdf('/tmp/weasyprint-website.pdf',
        stylesheets=[CSS(string='body { font-family: serif !important }')])


Instantiating HTML and CSS objects
..................................

If you have a file name, an absolute URL or a readable :term:`file object`,
you can just pass it to :class:`HTML` or :class:`CSS` to create an instance.
Alternatively, use a named argument so that no guessing is involved:

.. code-block:: python

    from weasyprint import HTML

    HTML('../foo.html')  # Same as …
    HTML(filename='../foo.html')

    HTML('http://weasyprint.org')  # Same as …
    HTML(url='http://weasyprint.org')

    HTML(sys.stdin)  # Same as …
    HTML(file_obj=sys.stdin)

If you have a byte string or Unicode string already in memory you can also pass
that, although the argument must be named:

.. code-block:: python

    from weasyprint import HTML, CSS

    # HTML('<h1>foo') would be filename
    HTML(string='''
        <h1>The title</h1>
        <p>Content goes here
    ''')
    CSS(string='@page { size: A3; margin: 1cm }')

If you have ``@font-face`` rules in your CSS, you have to create a
``FontConfiguration`` object:

.. code-block:: python

    from weasyprint import HTML, CSS
    from weasyprint.fonts import FontConfiguration

    font_config = FontConfiguration()
    html = HTML(string='<h1>The title</h1>')
    css = CSS(string='''
        @font-face {
            font-family: Gentium;
            src: url(http://example.com/fonts/Gentium.otf);
        }
        h1 { font-family: Gentium }''', font_config=font_config)
    html.write_pdf(
        '/tmp/example.pdf', stylesheets=[css],
        font_config=font_config)


Rendering to a single file
..........................

Once you have a :class:`HTML` object, call its :meth:`~HTML.write_pdf` method
to get the rendered document in a single PDF or PNG file.

Without arguments, these methods return a byte string in memory. If you
pass a file name or a writable :term:`file object`, they will write there
directly instead. (**Warning**: with a filename, these methods will
overwrite existing files silently.)


Individual pages, meta-data, other output formats, …
....................................................

.. currentmodule:: weasyprint.document

If you want more than a single PDF, the :meth:`~weasyprint.HTML.render` method
gives you a :class:`Document` object with access to individual :class:`Page`
objects. Thus you can get the number of pages, their size\ [#]_, the details of
hyperlinks and bookmarks, etc.  Documents also have a
:meth:`~Document.write_pdf` method, and you can get a subset of the pages with
:meth:`~Document.copy()`.  Finally, for ultimate control, :meth:`~Page.paint`
individual pages anywhere on any pydyf stream.

.. [#] Pages in the same document do not always have the same size.

See the :ref:`python-api` for details. A few random examples:

.. code-block:: python

    # Write odd and even pages separately:
    #   Lists count from 0 but page numbers usually from 1
    #   [::2] is a slice of even list indexes but odd-numbered pages.
    document.copy(document.pages[::2]).write_pdf('odd_pages.pdf')
    document.copy(document.pages[1::2]).write_pdf('even_pages.pdf')

.. code-block:: python

    # Print the outline of the document.
    # Output on http://www.w3.org/TR/CSS21/intro.html
    #     1. Introduction to CSS 2.1 (page 2)
    #       1. A brief CSS 2.1 tutorial for HTML (page 2)
    #       2. A brief CSS 2.1 tutorial for XML (page 5)
    #       3. The CSS 2.1 processing model (page 6)
    #         1. The canvas (page 7)
    #         2. CSS 2.1 addressing model (page 7)
    #       4. CSS design principles (page 8)
    def print_outline(bookmarks, indent=0):
        for i, bookmark in enumerate(bookmarks, 1):
            page = bookmark.destination[0]
            print('%s%d. %s (page %d)' % (
                ' ' * indent, i, bookmark.label.lstrip('0123456789. '), page))
            print_outline(bookmark.children, indent + 2)
    print_outline(document.make_bookmark_tree())


.. _url-fetchers:

URL fetchers
............

WeasyPrint goes through a *URL fetcher* to fetch external resources such as
images or CSS stylesheets. The default fetcher can natively open file and
HTTP URLs, but the HTTP client does not support advanced features like cookies
or authentication. This can be worked-around by passing a custom
``url_fetcher`` callable to the :class:`HTML` or :class:`CSS` classes.
It must have the same signature as :func:`~weasyprint.default_url_fetcher`.

Custom fetchers can choose to handle some URLs and defer others
to the default fetcher:

.. code-block:: python

    from weasyprint import default_url_fetcher, HTML

    def my_fetcher(url):
        if url.startswith('graph:'):
            graph_data = map(float, url[6:].split(','))
            return dict(string=generate_graph(graph_data),
                        mime_type='image/png')
        return default_url_fetcher(url)

    source = '<img src="graph:42,10.3,87">'
    HTML(string=source, url_fetcher=my_fetcher).write_pdf('out.pdf')

Flask-WeasyPrint_ for Flask_ and Django-Weasyprint_ for Django_ both make
use of a custom URL fetcher to integrate WeasyPrint and use the filesystem
instead of a network call for static and media files.

A custom fetcher should be returning a :obj:`dict` with

* One of ``string`` (a :obj:`bytestring <bytes>`) or ``file_obj`` (a
  :term:`file object`).
* Optionally: ``mime_type``, a MIME type extracted e.g. from a *Content-Type*
  header. If not provided, the type is guessed from the file extension in the
  URL.
* Optionally: ``encoding``, a character encoding extracted e.g. from a
  *charset* parameter in a *Content-Type* header
* Optionally: ``redirected_url``, the actual URL of the resource if there were
  e.g. HTTP redirects.
* Optionally: ``filename``, the filename of the resource. Usually derived from
  the *filename* parameter in a *Content-Disposition* header

If a ``file_obj`` is given, the resource will be closed automatically by
the function internally used by WeasyPrint to retreive data.

.. _Flask-WeasyPrint: http://packages.python.org/Flask-WeasyPrint/
.. _Flask: http://flask.pocoo.org/
.. _Django-WeasyPrint: https://pypi.org/project/django-weasyprint/
.. _Django: https://www.djangoproject.com/


Logging
.......

Most errors (unsupported CSS property, missing image, ...)
are not fatal and will not prevent a document from being rendered.

WeasyPrint uses the :mod:`logging` module from the Python standard library to
log these errors and let you know about them. When WeasyPrint is launched in a
terminal, logged messaged will go to *stderr* by default. You can change that
by configuring the ``weasyprint`` logger object:

.. code-block:: python

    import logging
    logger = logging.getLogger('weasyprint')
    logger.addHandler(logging.FileHandler('/path/to/weasyprint.log'))

The ``weasyprint.progress`` logger is used to report the rendering progress. It
is useful to get feedback when WeasyPrint is launched in a terminal (using the
``--verbose`` or ``--debug`` option), or to give this feedback to end users
when used as a library.

See the documentation of the :mod:`logging` module for details.


.. _security:

Security
--------

When used with untrusted HTML or untrusted CSS, WeasyPrint can meet security
problems. You will need extra configuration in your Python application to avoid
high memory use, endless renderings or local files leaks.

*This section has been added thanks to the very useful reports and advice from
Raz Becker.*

.. _long-renderings:

Long renderings
...............

WeasyPrint is pretty slow and can take a long time to render long documents or
specially crafted HTML pages.

When WeasyPrint used on a server with HTML or CSS files from untrusted sources,
this problem can lead to very long time renderings, with processes with high
CPU and memory use. Even small documents may lead to really long rendering
times, restricting HTML document size is not enough.

If you use WeasyPrint on a server with HTML or CSS samples coming from
untrusted users, you should:

- limit rendering time and memory use of your process, for example using
  ``evil-reload-on-as`` and ``harakiri`` options if you use uWSGI,
- limit memory use at the OS level, for example with ``ulimit`` on Linux,
- automatically kill the process when it uses too much memory or when the
  rendering time is too high, by regularly launching a script to do so if no
  better option is available,
- truncate and sanitize HTML and CSS input to avoid very long documents and
  access to external URLs.

Infinite requests
.................

WeasyPrint can reach files on the network, for example using ``http://``
URIs. For various reasons, HTTP requests may take a long time and lead to
problems similar to :ref:`long-renderings`.

WeasyPrint has a default timeout of 10 seconds for HTTP, HTTPS and FTP
resources. This timeout has no effect with other protocols, including access to
``file://`` URIs.

If you use WeasyPrint on a server with HTML or CSS samples coming from
untrusted users, or need to reach network resources, you should:

- use a custom `URL fetcher <url-fetchers>`_,
- follow solutions listed in :ref:`long-renderings`.

Infinite loops
..............

WeasyPrint has been hit by a large number of bugs, including infinite
loops. Specially crafted HTML and CSS files can quite easily lead to infinite
loops and infinite rendering times.

If you use WeasyPrint on a server with HTML or CSS samples coming from
untrusted users, you should:

- follow solutions listed in :ref:`long-renderings`.

Huge values
...........

WeasyPrint doesn't restrict integer and float values used in CSS. Using huge
values for some properties (page sizes, font sizes, block sizes) can lead to
various problems, including infinite rendering times, huge PDF files, high
memory use and crashes.

This problem is really hard to avoid. Even parsing CSS stylesheets and
searching for huge values is not enough, as it is quite easy to trick CSS
pre-processors using relative units (``em`` and ``%`` for example).

If you use WeasyPrint on a server with HTML or CSS samples coming from
untrusted users, you should:

- follow solutions listed in :ref:`long-renderings`.

Access to local files
.....................

As any web renderer, WeasyPrint can reach files on the local filesystem using
``file://`` URIs. These files can be shown in ``img`` or ``embed`` tags for
example.

When WeasyPrint used on a server with HTML or CSS files from untrusted sources,
this feature may be used to know if files are present on the server filesystem,
and to embed them in generated documents.

Unix-like systems also have special local files with infinite size, like
``/dev/urandom``. Referencing these files in HTML or CSS files obviously lead
to infinite time renderings.

If you use WeasyPrint on a server with HTML or CSS samples coming from
untrusted users, you should:

- restrict your process access to trusted files using sandboxing solutions,
- use a custom `URL fetcher <url-fetchers>`_ that doesn't allow ``file://``
  URLs or filters access depending on given paths.
- follow solutions listed in :ref:`long-renderings`.

System information leaks
........................

WeasyPrint relies on many libraries that can leak hardware and software
information. Even when this information looks useless, it can be used by
attackers to exploit other security breaches.

Leaks can include (but are not restricted to):

- locally installed fonts (using ``font-family`` and ``@font-face``),
- network configuration (IPv4 and IPv6 support, IP addressing, firewall
  configuration, using ``http://`` URIs and tracking time used to render
  documents),
- Python, Pango and other libraries versions (implementation details
  lead to different renderings).

SVG images
..........

Rendering SVG images more or less suffers from the same problems as the ones
listed here for WeasyPrint.

Security advices apply for untrusted SVG files as they apply for untrusted HTML
and CSS documents.

Note that WeasyPrint’s URL fetcher is used to render SVG files.


Errors
------

If you get an exception during rendering, it is probably a bug in WeasyPrint.
Please copy the full traceback and report it on our `issue tracker`_.

.. _issue tracker: https://github.com/Kozea/WeasyPrint/issues


.. _stylesheet-origins:

Stylesheet origins
------------------

HTML documents are rendered with stylesheets from three *origins*:

* The HTML5 `user agent stylesheet`_ (defines the default appearance
  of HTML elements);
* Author stylesheets embedded in the document in ``<style>`` elements
  or linked by ``<link rel=stylesheet>`` elements;
* User stylesheets provided in the API.

Keep in mind that *user* stylesheets have a lower priority than *author*
stylesheets in the cascade_, unless you use `!important`_ in declarations
to raise their priority.

.. _user agent stylesheet: https://github.com/Kozea/WeasyPrint/blob/master/weasyprint/css/html5_ua.css
.. _cascade: http://www.w3.org/TR/CSS21/cascade.html#cascading-order
.. _!important: http://www.w3.org/TR/CSS21/cascade.html#important-rules