Rich text, markup and transformations

How to store markup (such as HTML or reStructuredText) and render it with a transformation

Many content items need to allow users to provide rich text in some kind of markup, be that HTML (perhaps entered using a WYSIWYG editor), reStructuredText, Markdown or some other format. This markup typically needs to be transformed into HTML for the view template, but we also want to keep track of the original “raw” markup so that it can be edited again. Even when the input format is HTML, there is often a need for a transformation to tidy up the HTML and strip out tags that are not permitted.

It is possible to store HTML in a standard Text field. You can even get a WYSIWYG widget, by using a schema such as this:

from plone.autoform import directives as form
from plone.supermodel import model
from zope import schema
from import WysiwygFieldWidget

class ITestSchema(model.Schema):

    form.widget('body', WysiwygFieldWidget)
    body = schema.Text(title=u"Body text")

However, this approach does not allow for alternative markups or any form of content filtering. For that, we need to use a more powerful field: RichText from the package:

from import RichText
from plone.supermodel import model

class ITestSchema(model.Schema):

    body = RichText(title=u"Body text")

The RichText field constructor can take the following arguments in addition to the usual arguments for a Text field:

  • default_mime_type, a string representing the default MIME type of the input markup. This defaults to text/html.
  • output_mime_type, a string representing the default output MIME type. This defaults to text/x-html-safe, which is a Plone-specific MIME type that disallows certain tags. Use the HTML Filtering control panel in Plone to control the tags.
  • allowed_mime_types, a tuple of strings giving a vocabulary of allowed input MIME types. If this is None (the default), the allowable types will be restricted to those set in Plone’s Markup control panel.

Also note: The default field can be set to either a unicode object (in which case it will be assumed to be a string of the default MIME type) or a RichTextValue object (see below).

Below is an example of a field allow StructuredText and reStructuredText, transformed to HTML by default:

from import RichText
from plone.supermodel import model

defaultBody = """\

Please fill this in


And this

class ITestSchema(model.Schema):

    body = RichText(
        title=u"Body text",
        allowed_mime_types=('text/x-rst', 'text/structured',),

The RichTextValue

The RichText field does not store a string. Instead, it stores a RichTextValue object. This is an immutable object that has the following properties:

a unicode string with the original input markup;
the MIME type of the original markup, e.g. text/html or text/structured;
the default character encoding used when transforming the input markup. Most likely, this will be UTF-8;
the raw input encoded in the given encoding;
the MIME type of the default output, taken from the field at the time of instantiation;
a unicode object representing the transformed output. If possible, this is cached persistently until the RichTextValue is replaced with a new one (as happens when an edit form is saved, for example).

The storage of the RichTextValue object is optimised for the case where the transformed output will be read frequently (i.e. on the view screen of the content object) and the raw value will be read infrequently (i.e. on the edit screen). Because the output value is cached indefinitely, you will need to replace the RichTextValue object with a new one if any of the transformation parameters change. However, as we will see below, it is possible to apply a different transformation on demand should you need to.

The code snippet belows shows how a RichTextValue object can be constructed in code. In this case, we have a raw input string of type text/plain that will be transformed to a default output of text/html. (Note that we would normally look up the default output type from the field instance.):

from import RichTextValue

context.body = RichTextValue(u"Some input text", 'text/plain', 'text/html')

Of course, the standard widget used for a RichText field will correctly store this type of object for you, so it is rarely necessary to create one yourself.

Using rich text fields in templates

What about using the text field in a template? If you are using a DisplayForm, the display widget for the RichText field will render the transformed output markup automatically. If you are writing TAL manually, you may try something like this:

<div tal:content="structure context/body" />

This, however, will render a string like:

RichTextValue object. (Did you mean <attribute>.raw or <attribute>.output?)

The correct syntax is:

<div tal:content="structure context/body/output" />

This will render the cached, transformed output. This operation is approximately as efficient as rendering a simple Text field, since the transformation is only applied once, when the value is first saved.

Alternative transformations

Sometimes, you may want to invoke alternative transformations. Under the hood, the default implementation uses the portal_transforms tool to calculate a transform chain from the raw value’s input MIME type to the desired output MIME type. (Should you need to write your own transforms, take a look at this tutorial.) This is abstracted behind an ITransformer adapter to allow alternative implementations.

To invoke a transformation in code, you can use the following syntax:

from import ITransformer

transformer = ITransformer(context)
transformedValue = transformer(context.body, 'text/plain')

The __call__() method of the ITransformer adapter takes a RichTextValue object and an output MIME type as parameters.

If you are writing a page template, there is an even more convenient syntax:

<div tal:content="structure context/@@text-transform/body/text/plain" />

The first traversal name gives the name of the field on the context (body in this case). The second and third give the output MIME type. If the MIME type is omitted, the default output MIME type will be used.


Unlike the output property, the value is not cached, and so will be calculated each time the page is rendered.