.. -*- mode: rst; encoding: utf-8 -*-

==============
Stream Filters
==============

`Markup Streams`_ showed how to write filters and how they are applied to
markup streams. This page describes the features of the various filters that
come with Genshi itself.

.. _`Markup Streams`: streams.html

.. contents:: Contents
   :depth: 1
.. sectnum::


HTML Form Filler
================

The filter ``genshi.filters.HTMLFormFiller`` can automatically populate an HTML
form from values provided as a simple dictionary. When using thi filter, you can
basically omit any ``value``, ``selected``, or ``checked`` attributes from form
controls in your templates, and let the filter do all that work for you.

``HTMLFormFiller`` takes a dictionary of data to populate the form with, where
the keys should match the names of form elements, and the values determine the
values of those controls. For example::

  >>> from genshi.filters import HTMLFormFiller
  >>> from genshi.template import MarkupTemplate
  >>> template = MarkupTemplate("""<form>
  ...   <p>
  ...     <label>User name:
  ...       <input type="text" name="username" />
  ...     </label><br />
  ...     <label>Password:
  ...       <input type="password" name="password" />
  ...     </label><br />
  ...     <label>
  ...       <input type="checkbox" name="remember" /> Remember me
  ...     </label>
  ...   </p>
  ... </form>""")
  >>> filler = HTMLFormFiller(data=dict(username='john', remember=True))
  >>> print template.generate() | filler
  <form>
    <p>
      <label>User name:
        <input type="text" name="username" value="john"/>
      </label><br/>
      <label>Password:
        <input type="password" name="password"/>
      </label><br/>
      <label>
        <input type="checkbox" name="remember" checked="checked"/> Remember me
      </label>
    </p>
  </form>

.. note:: This processing is done without in any way reparsing the template
          output. As any stream filter it operates after the template output is
          generated but *before* that output is actually serialized.

The filter will of course also handle radio buttons as well as ``<select>`` and
``<textarea>`` elements. For radio buttons to be marked as checked, the value in
the data dictionary needs to match the ``value`` attribute of the ``<input>``
element, or evaluate to a truth value if the element has no such attribute. For
options in a ``<select>`` box to be marked as selected, the value in the data
dictionary needs to match the ``value`` attribute of the ``<option>`` element,
or the text content of the option if it has no ``value`` attribute. Password and
file input fields are not populated, as most browsers would ignore that anyway
for security reasons.

You'll want to make sure that the values in the data dictionary have already
been converted to strings. While the filter may be able to deal with non-string
data in some cases (such as check boxes), in most cases it will either not
attempt any conversion or not produce the desired results.

You can restrict the form filler to operate only on a specific ``<form>`` by
passing either the ``id`` or the ``name`` keyword argument to the initializer.
If either of those is specified, the filter will only apply to form tags with
an attribute matching the specified value.


HTML Sanitizer
==============

The filter ``genshi.filters.HTMLSanitizer`` filter can be used to clean up
user-submitted HTML markup, removing potentially dangerous constructs that could
be used for various kinds of abuse, such as cross-site scripting (XSS) attacks::

  >>> from genshi.filters import HTMLSanitizer
  >>> from genshi.input import HTML
  >>> html = HTML("""<div>
  ...   <p>Innocent looking text.</p>
  ...   <script>alert("Danger: " + document.cookie)</script>
  ... </div>""")
  >>> sanitize = HTMLSanitizer()
  >>> print html | sanitize
  <div>
    <p>Innocent looking text.</p>
  </div>

In this example, the ``<script>`` tag was removed from the output.

You can determine which tags and attributes should be allowed by initializing
the filter with corresponding sets. See the API documentation for more
information.

Inline ``style`` attributes are forbidden by default. If you allow them, the
filter will still perform sanitization on the contents any encountered inline
styles: the proprietary ``expression()`` function (supported only by Internet
Explorer) is removed, and any property using an ``url()`` which a potentially
dangerous URL scheme (such as ``javascript:``) are also stripped out::

  >>> from genshi.filters import HTMLSanitizer
  >>> from genshi.input import HTML
  >>> html = HTML("""<div>
  ...   <br style="background: url(javascript:alert(document.cookie); color: #000" />
  ... </div>""")
  >>> sanitize = HTMLSanitizer(safe_attrs=HTMLSanitizer.SAFE_ATTRS | set(['style']))
  >>> print html | sanitize
  <div>
    <br style="color: #000"/>
  </div>

.. warning:: You should probably not rely on the ``style`` filtering, as
             sanitizing mixed HTML, CSS, and Javascript is very complicated and
             suspect to various browser bugs. If you can somehow get away with
             not allowing inline styles in user-submitted content, that would
             definitely be the safer route to follow.