Python Expression Reference

This is a quick reference guide for using TALES Python expressions in Zope Page Templates.

Variables and Object Access

All TALES variables, including builtin, global, and local variables, can be used in Python expressions. See TALES Reference for more information. Builtin variable uses are described below:

  • root: Get properties or sub-objects of the Zope root object.
  • here: If your template is being applied to a content object, use this to access it. You can also use this to acquire objects or properties from the current acquisition context.
  • container: Access objects in the same location as the template, without regard to where it is being used.
  • template: Get the template’s title, or use the template’s own macro definitions elsewhere in itself.
  • request: Read submitted form contents, environment information, and the RESPONSE object. Set request variables for use by other objects.
  • user: Check the user’s access rights, or get their login name.
  • modules: Call Python functions stored in filesystem modules and packages. Use subitem notation, with the full dotted module access path as the key, like this: “modules![‘package.module’]”.
  • attrs: Get the default (original) values of the statement element’s attributes. This is a dictionary, with the attribute name as the key.
  • options: Get keyword arguments passed to the template, when it is called from a Method or Script. This is a dictionary.
  • repeat: Access the iterator information for enclosing TAL repeat statements. Use subitem notation, with the repeat variable name as the key.
  • default and nothing: Return these as the value of the expression to control TAL statements.

You can also use the same builtin functions as in Python Scripts and DTML namespaces, such as ‘str()’, ‘getattr()’, and ‘DateTime()’. See “Appendix A of the Zope Book”, http://www.zope.org/Members/michel/ZB/AppendixA.dtml for more information. A few of them deserve special mention here:

  • test: This allows you to choose among two or more alternate value based on boolean expressions. This can be very useful for operations such as giving different styles to alternating lines of a table:

    tal:define="oddrow repeat/item/odd"
tal:attributes="class python:test(oddrow, 'odd', 'even')"

  • DateTime: Get the current date and time, or construct a DateTime object from a string or other data.

Path and String Expressions in Python

Each of the TALES expression types can be invoked from Python by a function with the same name. Here are some situations in which other expression types are useful inside of Python expressions:

  • path: Since the path function returns a special false value when the path cannot be traversed, boolean logic can be used to select the first of several paths which both exists and has a true value, like this:

    path('here/a') or path('here/b')

  • path: Fetch an object with a dynamic path:

    path('container/%s/index_html' % folder_name)

  • string: Fetch an object with a complex dynamic path:

    path(string('here/${options/foo}/${request/bar}'))

Formatting DateTimes

DateTime objects have dozens of methods that you can use to extract information such as the month, day of the week, etc. See the “DateTime API Reference”, http://www.zope.org/Members/michel/ZB/AppendixB.dtml for more information. The most general of these is the ‘strftime()’ method, which uses a format string with special placeholders, like this:

"%b %e, %Y" produces "Jul  4, 2001"
"%m/%d/%y" produces "07/04/01"

Formatting Numbers and Strings

Python’s formatting operator (‘%’) can be used for simple numeric formatting and string concatenation; It is very efficient. See the “Python Reference”, http://www.python.org/doc/current/lib/typesseq-strings.html for details. A simple example:

"Hello %s, you have %s tries and $%.2f left." % (
  name, attempts, dollars)

There is a library of commonly-used formatting functions in the PythonScripts Product that duplicate format options available in DTML. You can get the module with a variable definition like this:

tal:define="pss modules/Products/PythonScripts/standard"

Then you can convert newlines to HTML break tags:

tal:replace="structure python:pss.newline_to_br(lines)"

format monetary and numeric values:

tal:replace="python:pss.whole_dollars(money)"
tal:replace="python:pss.dollars_and_cents(money)"
tal:replace="python:pss.thousands_commas(n)"

quote strings for safe use in !URLs, or in SQL:

tal:replace="python:pss.url_quote(anURL)"
tal:replace="python:pss.url_quote_plus(anURL)"
tal:replace="python:pss.sql_quote(sql_text)"

or convert structured text into HTML:

tal:replace="structure python:pss.structured_text(txt)"