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ZPT Basics (part 3)
Learn about TAL loops, dynamically-generated attributes and error handlers

| Playing The Numbers |

Before we get started, let's get the jargon straight - what's a sequence anyway?

As traditionally understood, a "sequence" is a series of items, usually connected to each other by a logical thread. For example,

0, 1, 2, 3, 4, 5...

p, q, r, s, t...

huey, dewey, louie...

are all valid sequences

In the Zope context, the definition of a sequence can be scoped down a little further - a sequence here is usually a set of objects. For example, if you create a folder in Zope, the objects stored within that folder can be considered a sequence. Or, if you retrieve a set of records from a database, the resultset returned can also be considered a sequence of data items.

A sequence, however, is just one part of the jigsaw. In order to access the elements that make up a sequence, you usually need a programming structure that will iterate, or loop, through the sequence, processing each element in turn. This loop can be combined with conditional statements to perform specific actions or execute specific commands while processing the elements in a sequence.

In order to demonstrate this, let's create a sequence in Zope and write a loop to process it. Fire up Zope, log into the Zope management interface, and create an instance of the Page Template object. Name it "CountingDown"

Output image

and fill it with the following code:

<li tal:repeat="countdown python:10,9,8,7,6,5,4,3,2,1,0">
<span tal:replace="countdown" />

When you view the output, here's what you'll see:

''.preg_replace(array('/  /', '/ /'), array('  ', '   '), '
* 10
* 9
* 8
* 7
* 6
* 5
* 4
* 3
* 2
* 1
* 0

First, I've created a simple sequence by brute force - it contains numbers from 0 to 10, in reverse order. Note the use of the "python" keyword - this is used to indicate that what follows is valid Python code.

The TAL "repeat" attribute is used to loop through the sequence, in a manner similar to that used in the "for" loops PHP and Perl programmers are familiar with. This "repeat" attribute takes two arguments - a variable name and an expression, which must evaluate to a sequence. For each sequence iteration, the value of the variable specified is set to the current element of the sequence. This variable value is then used with the TAL "replace" attribute to print the contents of each element in the sequence. From the output, it's obvious that this loop iterates eleven times to display the elements in the sequence.

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