Expressions

Description

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

Introduction

Expressions are part of TAL, the Template Attribute Language. They are used in Zope Page Templates (ZPT) and as part of workflow definitions, among other things.

You might want to use expressions in your own add-on product to provide user-written conditions for viewlet visibility, portlets, dynamic text, etc.

The authoritative reference is Appendix C: Zope Page Templates Reference of the Zope 2 Book

Expressions are used in:

  • the tal:condition, tal:content, tal:replace, tal:attribute, tal:define TAL directives;
  • portal_css, portal_javascript and other resource managers, to express when a 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 an optional protocol: prefix to determine the expression type.

path expression (default)

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

Path expressions use slashes for traversal (traversing), and will implicitly call callables.

Example: call the Title() method on the context object (finding it by acquisition if necessary) and return its value:

context/Title

Variables can be included using ?. Example: access a folder using the id stored in the myItemId variable, and return 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() instead if you need to use variables in your path parts.

__call__() and nocall: behavior in TAL path traversing

The TAL path expression will call Python callable objects by default.

If you try to get a hold of a helper view like this:

tal:define="commentsView context/@@comments_view"

You might get this exception:

  Module zope.tales.expressions, line 217, in __call__
  Module Products.PageTemplates.Expressions, line 155, in _eval
  Module Products.PageTemplates.Expressions, line 117, in render
  Module Products.Five.browser.metaconfigure, line 476, in __call__
AttributeError: 'coments_view' object has no attribute 'index'

It basically means that your view does not have a template assigned and the traversing logic tries to render that template.

This happens because

  1. `` context/@@comments_view`` creates a view instance

  2. then calls its __call__() method

  3. the default BrowserView.__call__() behavior to render a template by doing:

    def __call__(self):
        return self.index()
    
  4. Because your view does not have a template assigned it also lacks self.index attribute

The workaround for cases like this is to use nocall:: traversing:

tal:define="commentsView nocall:context/@@comments_view"

string: expressions

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 helper views directly by name.

Using expressions 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 in a BrowserView and expose it as a method.

Then you can call the method on a view from a 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 the view as “my_view_name”, using configure.zcml as usual.

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, depending on what kind of content the page will present.

Expression examples

Get current language

Use IPortalState context helper view.

Example how to generate a 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 a Python expression for comparison:

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

Check current language in TAL page template

For example, in case you need to generate HTML such as links conditionally, depending on the current language:

Example:

<a tal:define="language context/@@plone_portal_state/language" tal:condition="python: language == 'fi'"
       href="http://www.fi">Finnish link</a>

Example to have different footers (or something similar) for different languages:

<div tal:replace="structure context/footertext"
    tal:condition="python:context.restrictedTraverse('@@plone_portal_state').language() == 'no'" />
<div tal:replace="structure context/footertexteng"
    tal:condition="python:context.restrictedTraverse('@@plone_portal_state').language() == 'en'" />

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 a certain content type

Example:

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

Get portal description

Example:

tal:define="
        portal context/portal_url/getPortalObject;
        portal_description portal/Description"

Doing <input CHECKED> and boolean like HTML attributes in TAL

To have a value appear in TAL or not you can do:

<input type="checkbox" tal:attributes="checked python:'checked' if MYCONDITION else ''" />

We execute a Python snippet which

  • We will dynamically create a checked attribute on <input> based on Python evaluation
  • Return “checked” string if some condition we check in Python evaluates to True
  • Otherwise we return an empty string and TAL won’t output this attribute (TODO: has TAL some special support for CHECKED and SELECTED attributes)

Note

Python 2.6, Plone 4+ syntax

Through-the-web scripts

Todo

Move TTW script info to its own chapter.

The Management Interface allows one to create, edit and execute RestrictedPython sandboxed scripts directly through the web management interface. This functionality is generally discouraged nowadays in the favor of view classes.

Creating a TTW Python script in an add-on installer

Here is an example of how one can pre-seed a Python script in an add-on installer GenericSetup profile.

setuphandlers.py:

from Products.PythonScripts.PythonScript import manage_addPythonScript

DEFAULT_REDIRECT_PY_CONTENT = """
if port not in (80, 443):
    # Don't kick in HTTP/HTTPS redirects if the site
    # is directly being accessed from a Zope front-end port
    return None
"""

def runCustomInstallerCode(site):
    """ Run custom add-on product installation code to modify Plone site object and others

    Python scripts can be created by Products.PythonScripts.PythonScript.manage_addPythonScript

    http://svn.zope.org/Products.PythonScripts/trunk/src/Products/PythonScripts/PythonScript.py?rev=114513&view=auto

    @param site: Plone site
    """

    # Create the script in the site root
    id = "redirect_handler"

    # Don't override the existing installation
    if not id in site.objectIds():
        manage_addPythonScript(site, id)
        script = site[id]

        # Define the script parameters
        parameters = "url, port"

        script.ZPythonScript_edit(parameters, DEFAULT_REDIRECT_PY_CONTENT)


def setupVarious(context):
    """
    @param context: Products.GenericSetup.context.DirectoryImportContext instance
    """

    # We check from our GenericSetup context whether we are running
    # add-on installation for your product or any other proudct
    if context.readDataFile('collective.scriptedredirect.marker.txt') is None:
        # Not our add-on
        return

    portal = context.getSite()

    runCustomInstallerCode(portal)

See the full example.