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Transformations applied to HTML in Plone text fields as they are rendered

Project description


plone.outputfilters provides a framework for registering filters that get applied to text as it is rendered.

By default, these filters are wired up to occur when text is transformed from the text/html mimetype to the text/x-html-safe mimetype via the PortalTransforms machinery.

With the RichText field of, this transform is typically applied when the field value is first accessed. The result of the transform is then cached in a volatile attribute for an hour or until the value is replaced.

Included Filters

A default filter is included which provides the following features:

  • Resolving UID-based links

  • Adding captions to images

(These are implemented as one filter to avoid the overhead of parsing the HTML twice.)

These features used to be provided by similar transforms in both Products.kupu and Products.TinyMCE. New releases of these editors are being prepared which depend on the transform in plone.outputfilters, so that bugs don’t need to be fixed in multiple places.

Image captioning

Image tags with the “captioned” class and a src attribute that resolves to an image object within the site will be wrapped in a definition list (DL) tag which includes a caption based on the value of the image’s description field, if any.

For example, this image tag:

<img src="path/to/image" class="captioned"/>

might be transformed into:

<dl class="captioned">
 <dt><img src="path/to/image"/></dt>
 <dd class="image-caption">Caption text</dd>

assuming the image found at “path/to/image” has the description “Caption text”.

The captioning filter is enabled if there is at least one plone.outputfilters.filters.resolveuid_and_caption.IImageCaptioningEnabler utility whose available property returns True. This mechanism exists for compatibility with TinyMCE and kupu, which both provide their own control panel setting to enable the captioning feature.

The captioned version of an image is rendered using the @@plone.outputfilters_captioned_image view, which may be overridden to customize the caption. This view is passed the following kwargs:


The CSS class on the image.


The width attribute of the image tag.


The alt attribute of the image tag.


The path of the image, relative to the site root.


The image’s description.


The (possibly scaled) image object.


The original unscaled image object.


A full HTML tag which displays the image.


True if image is fullimage.


The width of image.

Adding a custom filter

As an example, the following filter replaces all doubled hyphens (”–”) with em dashes (“-“). (Don’t use the example verbatim, because it doesn’t parse HTML to apply itself only to text nodes, so will mangle HTML comments.)

A filter is a callable which accepts a UTF-8-encoded HTML string as input, and returns a modified UTF-8-encoded HTML string. A return value of None may be used to indicate that the input should not be modified.


import re
from zope.interface import implementer
from plone.outputfilters.interfaces import IFilter

class EmDashAdder(object):
    order = 1000

    def __init__(self, context, request):

    def is_enabled(self):
        return True

    pattern = re.compile(r'--')

    def __call__(self, data):
        return self.pattern.sub('—', data)

The order attribute may be used to affect the order in which filters are applied (higher values run later). The is_enabled method should return a boolean indicating whether the filter should be applied.

Filters are registered in ZCML as a named multi-adapter of the context and request to IFilter:

>>> from Zope2.App import zcml
>>> import Products.Five
>>> configure = """
... <configure
...     xmlns="">
...   <adapter
...     name="em_dash_adder"
...     provides="plone.outputfilters.interfaces.IFilter"
...     for="* *"
...     factory="plone.outputfilters.filters.example.EmDashAdder"
...     />
... </configure>
... """
>>> zcml.load_config("configure.zcml", Products.Five)
>>> zcml.load_string(configure)

Now when text is transformed from text/html to text/x-html-safe, the filter will be applied:

>>> app = layer['app']
>>> portal = layer['portal']
>>> str(portal.portal_transforms.convertTo('text/x-html-safe',
...     'test--test', mimetype='text/html', context=portal))

How it works

plone.outputfilters hooks into the PortalTransforms machinery by installing:

  1. a new mimetype (“text/x-plone-outputfilters-html”)

  2. a transform from text/html to text/x-plone-outputfilters-html

  3. a null transform from text/x-plone-outputfilters-html back to text/html

  4. a “transform policy” for the text/x-html-safe mimetype, which says that text being transformed to text/x-html-safe must first be transformed to text/x-plone-outputfilters-html

The filter adapters are looked up and applied during the execution of the transform from step #2.

This should be considered an implementation detail and may change at some point in the future.


5.0.4 (2023-07-13)

Bug fixes:

  • Call registry once per filter rather than for each img tag. [gotcha] (less_call_to_registry)


  • Update configuration files. [plone devs] (7723aeaf)

5.0.3 (2023-06-16)

Bug fixes:

  • Return a 404 Not Found response if the resolveuid view is called with no uuid. @davisagli (#43)

5.0.2 (2023-04-14)


  • Update configuration files. [plone devs] (535edb14)

5.0.1 (2023-03-21)


  • Update configuration files. [plone devs] (243ca9ec)

5.0.0 (2022-11-17)

Bug fixes:

  • Use our new version of uuidToObject() from [anirudhhkashyap] (#52)

5.0.0b3 (2022-09-30)

Bug fixes:

  • Do not return prettified soup after picture variants filter. This prevents adding unneeded newlines. [petschki] (#56)

5.0.0b2 (2022-09-10)

Bug fixes:

  • isort, black, pyupgrade, manual six removal. [jensens] (#53)

5.0.0b1 (2022-06-23)

New features:

  • Add image_srcset output filter, to convert IMG tags into PICTURE tags with multiple source definitions as define in imaging control panel [MrTango] (#49)

4.1.0 (2022-02-23)

New features:

  • Resolve UIDs in SRC= attribute of of SOURCE and IFRAME elements. (#47)

4.0.2 (2020-09-28)

Bug fixes:

  • fix AttributeError: ‘NoneType’ object has no attribute ‘unwrap’ exception when a fullsize image is wrapped in an <a> tag. [flipmcf] (#39)

  • Fixed deprecation warning for html_quote. [maurits] (#3130)

4.0.1 (2020-04-21)

Bug fixes:

  • Minor packaging updates. (#1)

4.0.0 (2020-03-13)

Breaking changes:

  • Change the image caption template to use <figure> and <figcaption>. [thet]

New features:

  • Add an ImageCaptioningEnabler utility which can be enabled via the portal registry setting plone.image_captioning. [thet]

Bug fixes:

  • Don’t check for hard coded image size in test. [agitator]

  • Fixed possible package install error with Python 3.6 when no system locale is set. See coredev issue 642. [maurits]

3.1.2 (2019-03-21)

Bug fixes:

  • fix UnicodeDecodeError in Python 2 when uid-linked image has non-ascii characters in title or description [petschki]

3.1.1 (2019-01-07)

Bug fixes:

  • bugfix for KeyError caused by <a> elements without href attribute [ajung]

3.1.0 (2018-11-02)

New features:

  • remove deprecated sgmllib and move to BeautifulSoup4 [tobiasherp, petschki]

3.0.5 (2018-06-04)

Bug fixes:

  • Allow resolving of links with absolute path and host [tomgross]

  • Make plone.namedfile hard testing dependency [tomgross]

3.0.4 (2018-02-02)

Bug fixes:

  • Add Python 2 / 3 compatibility [pbauer]

3.0.3 (2017-08-04)

Bug fixes:

  • update test to reflect changes in PortalTransforms [MrTango]

3.0.2 (2017-07-03)

Bug fixes:

  • Remove unittest2 dependency [kakshay21]

3.0.1 (2017-02-05)

Bug fixes:

  • Do not transform a and img tags when inside script tag. [gotcha]

3.0.0 (2016-08-19)

Breaking changes:

  • Give up support of PortalTransforms 1.x with old style interfaces. [jensens]

Bug fixes:

  • Handle unicode errors in img attributes [vangheem]

  • Cleanup: utf8-headers, isort, pep8 [jensens]

  • Use zope.interface decorator. [gforcada]

2.1.5 (2016-06-07)

Bug fixes:

  • Make tests work with old and new safe HTML transform [tomgross]

2.1.4 (2016-05-10)


  • Explicitly exclude mailto: links from being UID-resolved. [thet]

  • Fix test isolation problem. [thet]

2.1.3 (2016-03-07)


  • Added tel: to ignored link types. [julianhandl]

2.1.2 (2015-12-15)


  • Fixed error when uid resolving if object got didn’t have absolute_url method. [Gagaro]

2.1.1 (2015-11-25)


  • Fixed case where unicode ends up getting used when resolving img tags and (un)restrictedTraverse doesn’t work with unicode. [vangheem]

2.1 (2015-07-18)

  • Remove kupu BBB code. [gforcada]

2.0 (2015-03-13)

  • For full-size (non-scaled) images, preserve height/width specified in img tag attributes. [davisagli]

  • Convert tests to [tomgross]

1.14 (2014-04-22)

  • for plone 5, always resolveuids [vangheem]

1.13 (2014-04-13)

  • #12783 img tag referencing non existent scales leads to transform error [anthonygerrard]

1.12 (2014-01-27)

  • Nothing changed yet.

1.11.1 (2013-07-19)

  • Fix README rst. [gotcha]

1.11 (2013-07-19)

  • img unicode issue : fix resolve_image to avoid that it returns unicode [gotcha]

  • handle possibility of img tag being unicode to prevent unicode errors [vangheem]

1.10 (2013-05-23)

  • Work around bug in SGMLParser to handle singleton tags correctly. [tom_gross]

1.9 (2013-04-06)

  • If we have an image description it should go into the alt text of the img tag [ale-rt]

1.8 (2012-12-10)

  • Fix packaging issue. [esteele]

1.7 (2012-12-09)

  • When resolving images, only look upward for the full image if the image that was traversed is not a content item (i.e. is a scale). [davisagli, datakurre]

  • Also convert “resolveUid/” links (big ‘U’) that FCKeditor used to create. [hacklschorsch]

  • Also escape double quotes, fixes #13219 [maartenkling]

1.6 (2012-08-16)

  • Don’t break if an @@images scale can’t be resolved for some reason. [davisagli]

1.5 (2012-08-15)

  • Restore compatibility with Plone 4.0 when plone.outputfilters is present. [davisagli]

1.4 (2012-08-04)

  • Fix incompatibilities with plone.namedfile [do3cc]

1.3 (2012-05-25)

  • Fixed testing error by moving the part of README.rst to plone/outputfilters/README.txt. [maurits]

  • Small pep8 update [pbdiode]

1.2 - 2012-04-09

  • Prevent transformation of links to anchors on the same page. [davisagli]

  • Fixed undefined uuid variable in kupu_resolveuid_hook branch in resolveuid view. [vincentfretin]

  • Make sure links to expired objects can still be resolved by the resolveuid view. [davisagli]

  • alt/title attributes on img tags were not present if tinymce uid linking was not used [iElectric]

  • When making relative URIs absolute, use the parent as the relative root when the context is not folderish. Fixes an issue where relative URLs from Plone 3, for example, had the wrong URLs under Plone 4 when a default page was used for a folder. [rossp]

  • Fixed testing error when packaged with a missing README.rst. [maurits]

1.1 - 2011-11-21

  • Fixed resolving of protected objects for AT content [tom_gross]

  • Fixed resolving of relative ../resolveuid/… links [tom_gross]

  • Respect implementation differences in Python 2.4 and Python 2.6 sgmlparser [tom_gross]

  • Fixed resolving of images in protected folders for captioning [mj]

1.0 - 2011-05-13

  • Release 1.0 Final. [esteele]

  • Add [WouterVH]

1.0b5 - 2011-03-24

  • Make captioning and linking work with new style image scales. [elro]

  • General refactoring of link resolution. [elro]

1.0b4 - 2011-03-22

  • Add alt and title tags to images. [elro]

  • Get various image properties from the imaging view to work better with Dexterity. [elro]

  • small fix so it is possible to create object without need of REQUEST or without need of mocking it. [garbas]

1.0b3 - 2011-02-24

  • Resolve image paths beginning with a slash relative to the Plone site root. [davisagli]

  • Support image captioning for new-style image scales using the @@images view. [davisagli]

1.0b2 - 2011-01-11

  • Fix resolveuid so that uid resolution occurs after authentication. [elro]

  • Please remember to run tests before checking in! [elro]

  • Fix issue where resolving links with subpaths resulted in a reversed subpath. [elro]

1.0b1 - 2011-01-25

1.0a1 - 2011-01-03

  • Initial implementation. [davisagli]

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