Expressions

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.

Plone has built-in support for TALES expression. They are internally used at least in Javascript and CSS machinery to pick the files, actions to determine whether the action is visible or not.

Expression types

There are three main categories of expressions.

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

Default TALES expression

Do traversing using slashes.

Example:

context/getTitle

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,
    }

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.

Example:

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