ZPT Basics (part 2) - Comparing Apples and Oranges
( Page 6 of 9 )

The example you've just seen was very rudimentary. To really add some punch, you need to know how to construct what the geeks call a conditional statement. And the very basis of a conditional statement is comparison - for example, "if this is equal to that, do thus and such".

Since Zope is built around Python, and Python comes with a bunch of useful operators designed specifically for use in conditional statements, you can use Python operators to build conditional statements in your ZPT. Here's a list:

Assume delta = 12 and omega = 9

Operator   What It Means                  Expression         Evaluates
To
------------------------------------------------------------------------
--

==         is equal to                    delta == omega     False

!=         is not equal to                delta != omega     True

>          is greater than                delta > omega      True

<          is less than                   delta < omega      False

=          is greater than or equal to	  delta >= omega     True

<=         is less than or equal to       delta <= omega     False

These comparison operators can be used for both strings and numbers. A positive result returns true, while a negative result returns false.

Here's an example which illustrates how these operators can be combined with the "condition" attribute discussed on the previous page to set up multiple decision branches within a template.

<span tal:define="global name string:max"></span>

<span tal:condition="python:name == 'neo'">
<font face="Arial" size="2">Welcome to the Matrix, Neo. Access
granted.</font> </span>

<span tal:condition="python:name != 'neo'">
<font face="Arial" size="2">I wonder if you've heard of Shakespeare,
<span
    tal:content="name">name here</span>.
<p>He postulated that a rose by any other name would smell just as
sweet.</p> <p>Unfortunately for you, I disagree. Access
denied.</p></font> </span> 

Here's what the output looks like:

Welcome to the Matrix, Neo. Access granted.

Now, try changing the value of the "name" variable in the first line of the template,

<span tal:define="global name string:joe"></span>

and look what happens:

I wonder if you've heard of Shakespeare, joe.

He postulated that a rose by any other name would smell just as sweet.

Unfortunately for you, I disagree. Access denied.

Nothing too complicated here. A quick glance at the code will show you that I've used the TAL "condition" attribute twice, in two different tests. The first time around, the first conditional test will evaluate to true and the second will evaluate to false, because the value of the variable is set to "neo"; consequently, the contents of the first <span> block will be displayed, while the contents of the second will be omitted.

<span tal:condition="python:name == 'neo'">
<font face="Arial" size="2">Welcome to the Matrix, Neo. Access granted.
</font></span>

The second time around, since the value of the variable is no longer "neo", the second test will evaluate to true and the contents of the second <span> block will be displayed.

<span tal:condition="python:name != 'neo'">
<font face="Arial" size="2">I wonder if you've heard of Shakespeare,
<span
    tal:content="name">name here</span>.
<p>He postulated that a rose by any other name would smell just as
sweet.</p> <p>Unfortunately for you, I disagree. Access
denied.</p></font> </span> 

I know what you're thinking - this would have been much easier to implement with an "if-else" conditional statement. Unfortunately, TAL doesn't have one of those yet; the only way to obtain that functionality is to simulate it via multiple "condition" attributes, as in the example above.

>>> More Zope Articles          >>> More By Harish Kamath, (c) Melonfire