Expressions

Description

Expressions are one line long string templates or Python expressions which are used in various places on Plone for templates, action conditions and URL generation.

Introduction

Expressions are user or site-administration through-the-web-written conditions which can be evaluated. You might want to use expressions in your own add-on product to provide user written conditions for viewlet visibility, portlets, dynamic text, etc. Expressions are also used on TAL template language.

Expressions are used in

• tal:condition, tal:content, tal:replace, tal:attribute, tal:define TAL template language directive content
• portal_css, portal_javascript and other resource manages to express when the resource should be included or not
• portal_actions to define when content, site and user actions are visible

Expression types

There are three main categories of expressions.

Expression can contain optional protocol: prefix to determine the expression type.

path expression (default)

Unless you specify expression type using python: or string notation a path expression is assumed.

Do traversing using slashes.

Example - Call Title() function of context object and return its value:

context/Title

Variables can be used with ? - enter folder subject by its id and get its title:

context/?myItemId/Title

Note

With this kind of usage, if the variable you're dereferencing isn't sanitized, there could be security ramifications. Use python: restrictedTraverse() traversing instead if you need to use variables in your path parts.

string: expression

Do string replace operation.

Example:

string:${context/portal_url}/@@my_view_name

python: expression

Evaluate as Python code.

Example:

python:object.myFunction() == False

Expression variables

Available expression variables are defined in CMFCore/Expressions.py:

data = {
    'object_url':   object_url,
    'folder_url':   folder.absolute_url(),
    'portal_url':   portal.absolute_url(),
    'object':       object,
    'folder':       folder,
    'portal':       portal,
    'nothing':      None,
    'request':      getattr(portal, 'REQUEST', None),
    'modules':      SecureModuleImporter,
    'member':       member,
    'here':         object,
    }

You can also access :doc`helper views </misc/context>` directly by name.

Using expression in your own code

Expressions are persistent objects. You usually want to attach them to something, but this is not necessary.

Example:

from Products.CMFCore.Expression import Expression, getExprContext

# Create a sample expression - usually this is taken from
# the user input
expression = Expression("python:context.Title() == 'foo')

expression_context = getExprContext(self.context)

# Evaluate expression by calling
# Expression.__call__(). This
# will return whatever value expression evaluation gives
value = expression(expression_context)

if value.strip() == "":
        # Usually empty expression field means that
        # expression should be True
        value = True

if value:
        # Expression succeeded
        pass
else:
        pass

Custom expression using a helper view

If you need to add complex Python code to your expression conditions it is best to put this code to BrowserView and expose it as a method.

Then you can call view from TALES expression:

object/@@my_view_name/my_method

Your view code would look like:

class MyViewName(BrowserView):
    """ Exposes methods for expression conditions """

    def my_method(self):
        """ Funky condition

        self.context = object for which this view was traversed
        """
        if self.context.Title().startswith("a"):
            return True
        else:
            return False

Register view normally using configure.zcml as "my_view_name".

You can use context interfaces like

• Products.CMFCore.interfaces.IContentish
• zope.interface.Interface (or "*")

to make sure that this view is available on all content objects, as TALES will be evaluated on every page, regarding on what kind of content the page wil present.

Expression examples

Get current language

Use IPortalState context helper view.

Example how to generate multilingual aware RSS feed link:

string:${object/@@plone_portal_state/portal_url}/site-feed/RSS?set_language=${object/@@plone_portal_state/language}

...or you can use Python expression for comparison:

python:object.restrictedTraverse('@@plone_portal_state').language() == 'fi'

Check if object implements an interface

Example:

python:context.restrictedTraverse('@@plone_interface_info').provides('Products.CMFCore.interfaces.IFolderish')

Returns True or False. Useful for actions.

Check if a certain hostname was used for HTTP request

Example:

python:"localhost" in request.environ.get("HTTP_HOST", "")

Check if the object is certain content type

Example:

python:getattr(object, "portal_type", "") == "Custom GeoLocation"