# Chapter 9. Advanced Page Templates

In Chapter 5, "Using Zope Page Templates" you learned the basics about Page Templates. In this chapter you'll learn about advanced techniques including new types of expressions and macros.

## Advanced TAL

You've already learned about a few TAL statements. In this section we'll go over all TAL statements and their various options. Note, this material is covered more concisely in Appendix C, "Zope Page Templates Reference".

### Advanced Content Insertion

You've already seen how `tal:content` and `tal:replace` work in Chapter 5, "Using Zope Page Templates". In this section you'll learn some advanced tricks for inserting content.

#### Inserting Structure

Normally, the `tal:replace` and `tal:content` statements escape HTML tags and entities in the text that they insert, converting `<` to `&lt;`, for instance. If you actually want to insert the unquoted text, you need to precede the expression with the `structure` keyword. For example:

              <p replace="structure here/story">
                the <b>story</b>
              </p>

This feature is useful when you are inserting a fragment of HTML or XML that is stored in a property or generated by another Zope object. For instance, you may have news items that contain simple HTML markup such as bold and italic text when they are rendered, and you want to preserve this when inserting them into a "Top News" page. In this case, you might write:

              <p tal:repeat="newsItem here/topNews"
                 tal:content="structure newsItem">
                A news item with<code>HTML</code> markup.
              </p>

This will insert the news items including their HTML markup into paragraphs.

#### Dummy Elements

You can include page elements that are visible in the template but not in generated text by using the built-in variable `nothing`, like this:

              <tr tal:replace="nothing">
                <td>10213</td><td>Example Item</td><td>$15.34</td>
              </tr>

This can be useful for filling out parts of the page that will be populated with dynamic content. For instance, a table that usually has ten rows will only have one row in the template. By adding nine dummy rows, the template's layout will look more like the final result.

It's not always necessary to use the `tal:replace="nothing"` trick to get dummy content into your Page Template. For example, you've already seen that anything inside a `tal:content` or `tal:replace` element is normally removed when the template is rendered. In these cases you don't have do anything special to make sure that dummy content is removed.

#### Default Content

You can leave the contents of a tag along by using the `default` expression with `tal:content` or `tal:replace`. For example:

              <p tal:content="default">Spam<p>

This renders to:

              <p>Spam</p>

Most often you will want to selectively include default content, rather than always including it. For example:

              <p tal:content="python:here.getFood() or default">Spam</p>

Note: Python expressions are explained later in the chapter. If the `getFood` method returns a true value than its result will be inserted into the paragraph, otherwise it's Spam for dinner.

### Advanced Tag Repetition

You've already seen most of what you can do with the `tal:repeat` statement in Chapter 5, "Using Zope Page Templates". This section covers a few advanced features of the `tal:repeat` statement.

#### Repeat Variables

One topic that bear more explanation is repeat variables. Repeat variables provide information about the current repetition. These attributes are available on `repeat` variables:

- index - repetition number, starting from zero.
- number - repetition number, starting from one.
- even - true for even-indexed repetitions (0, 2, 4, ...).
- odd - true for odd-indexed repetitions (1, 3, 5, ...).
- start - true for the starting repetition (index 0).
- end - true for the ending, or final, repetition.
- length - length of the sequence, which will be the total number of repetitions.
- letter - count reps with lower-case letters: "a" - "z", "aa" - "az", "ba" - "bz", ..., "za" - "zz", "aaa" - "aaz", and so forth.
- Letter - upper-case version of letter.

You can access the contents of a repeat variable using path expressions or Python expressions. In path expressions, you write a three-part path consisting of the name `repeat`, the statement variable's name, and the name of the information you want, for example, `repeat/item/start`. In Python expressions, you use normal dictionary notation to get the repeat variable, then attribute access to get the information, for example, 'python:repeat['item'].start'.

#### Repetition Tips

Here are a couple practical tips that you may find useful. Some times you'd like to repeat a tag, but not have an enclosing tag. For example, you might want to repeat a number of paragraph tags, but there is no need to enclose them in another tag. You can do this with the `tal:omit-tag` statement:

              <div tal:repeat="quote here/getQuotes"
                   tal:omit-tag="">
                <p tal:content="quote">quotation</p>
              </div>

The `tal:omit-tag` statement is described later in this chapter.

While it's been mentioned before, it's worth saying again: you can nest `tal:repeat` statements inside each other. Each `tal:repeat` statement must have a different repeat variable name. Here's an example that shows a math times-table:

              <table border="1">
                <tr tal:repeat="x python:range(1, 13)">
                  <div tal:repeat="y python:range(1, 13)"
                       tal:omit-tag="">
                    <td tal:content="python:'%d x %d = %d' % (x, y, x*y)">
                      X x Y = Z
                    </td>
                  </div>
                </tr>
              </table>

This example uses Python expressions, and the `tal:omit-tag` statement both of which are covered later in this chapter.

If you've done much work with the dtml-in DTML repetition statement, you will have encountered batching. Batching is the process of chopping up a large list into smaller lists. You typically use it to display a small number of items from a large list on a web page. Think of how a search engine batches its search results. The `tal:repeat` statement does not support batching, but Zope comes with a batching utility. See the section, "Batching" later in this chapter.

Another useful feature that isn't supplied by `tal:repeat` is sorting. If you want to sort a list you can either use write your own sorting script (which is quite easy in Python) or you can use the `sequence.sort` utility function. Here's an example of how to sort a list of objects by title, and then by modification date:

              <table tal:define="objects here/objectValues;
                                 sort_on python:(('title', 'nocase', 'asc'),
                                                 ('bobobase_modification_time', 'cmp', 'desc'));
                                 sorted_objects python:sequence.sort(objects, sort_on)">
                <tr tal:repeat="item sorted_objects">
                  <td tal:content="item/title">title</td>
                  <td tal:content="item/bobobase_modification_time">
                    modification date</td>
                </tr>
              </table>

This example tries to make things clearer by defining the sort arguments outside the `sort` function. You call the `sequence.sort` function takes a sequence and a description of how to sort it. In this example the description of how to sort the sequence is defined in the `sort_on` variable. See Appendix B, "API Reference" for more information on the powerful `sequence.sort` function.

### Advanced Attribute Control

You've already met the `tal:attributes` statement. You can use it to dynamically replace tag attributes, for example, the `href` attribute on an `a` element. You can replace more than one attribute on a tag by separating attributes with semicolons:

            <a href="link"
               tal:attributes="href here/getLink;
                               class here/getClass">link</a>

You can also define attributes with XML namespaces. For example:

            <Description
                dc:Creator="creator name"
                tal:attributes="dc:Creator here/owner/getUserName">
              Description</Description>

Simply put the XML namespace prefix before the attribute name and you can create attributes with XML namespaces.

### Defining Variables

You can define your own variable using the `tal:define` attribute. There are several reasons that you might want to do this. One reason is to avoid having to write long expressions repeatedly in a template. Another is to avoid having to call expensive methods repeatedly. You can define a variable once and then use it many times in a template. For example, here's a list that defines a variable and later tests it and repeats over it:

            <ul tal:define="items container/objectIds"
                tal:condition="items">
              <li tal:repeat="item items">
                <p tal:content="item">id</p>
              </li>
            </ul>

The `tal:define` statement creates the variable `items`, which you can use it anywhere in the `ul` tag. Notice also how you can have two TAL statement on the same `ul` tag. See the section, "Interactions Between TAL Statements" later in this chapter for more information about using more than one statement on a tag. In this case the first statement assigns the variable `items` and the second uses `items` in a condition to see whether or not it is false (in this case, an empty sequence) or true. If the `items` variable is false, then the `ul` tag is not shown.

Now, suppose that instead of simply removing the list when there are no items, you want to show a message. To do this, place the following before the list:

            <h4 tal:condition="not:container/objectIds">There
            Are No Items</h4>

The expression, `not:container/objectIds` is true when `container/objectIds` is false, and vice versa. See the section, "Not Expressions" later in this chapter for more information.

You can't use your `items` variable here, because it isn't defined yet. If you move the definition of `items` to the `h4` tag, then you can't use it in the `ul` tag any more, because it becomes a local variable of the `h4` tag. You could place the definition on some tag that enclosed both the `h4` and the `ul`, but there is a simpler solution. By placing the keyword `global` in front of the variable name, you can make the definition last from the `h4` tag to the bottom of the template:

            <h4 tal:define="global items container/objectIds"
                tal:condition="not:items">There Are No Items</h4>

You can define more than one variable using `tal:define` by separating them with semicolons. For example:

            <p tal:define="ids container/objectIds;
                           title container/title">

You can define as many variables as you wish. Each variable can have its own global or local scope. You can also refer to earlier defined variables in later definitions. For example:

            <p tal:define="title template/title;
                           tlen python:len(title);">

With judicious use of `tal:define` you can improve the efficiency and readability of your templates.

### Omitting Tags

You can remove tags with the `tal:omit-tag` statement. You will seldom need to use this TAL statement, but occasionally it's useful. The omit-tag attribute removes a tag, but does not affect the contents of the tag. For example:

            <b tal:omit-tag=""><i>this</i> stays</b>

Renders to:

            <i>this</i> stays

At this level of usage, `tal:omit-tag` operates almost like `tal:replace="default"`. However, `tal:omit-tag` is more useful when used in combination with other TAL statement such as `tal:repeat`. For example here's one way to create ten paragraph tags using `tal:repeat`:

            <span tal:repeat="n python:range(10)"
                  tal:omit-tag="">
              <p tal:content="n">1</p>
            </span>

This will produce ten paragraph tags, however, the `span` tag will not appear in the output.

The `tal:omit-tag` attribute takes an expression, though normally you'll simply use an empty expression. If the expression is true or there is no expression then the statement tag is removed. If the expression is false, then the tag is not omitted. This allows you to selectively remove tags depending on dynamic circumstances.

### Error Handling

If an error occurs in your page template, you can catch that error and show a useful error message to your user. For example, suppose your template defines a variable using form data:

            ...
            <span tal:define="global prefs request/form/prefs"
                  tal:omit-tag="" />
            ...

If Zope encounters a problem, like not being able to find the `prefs` variable in the form data, the entire page will break; you'll get an error page instead. Happily, you can avoid this kind of thing with limited error handling using the `tal:on-error` statement:

            ...
            <span tal:define="global prefs here/scriptToGetPreferences"
                  tal:omit-tag=""
                  tal:on-error="string:An error occurred">
            ...

When an error is raised while rendering a template, Zope looks for a `tal:on-error` statement to handle the error. It first looks in the current tag, then on its enclosing tag, and so on until it reaches the top-level tag. When it finds an error handler, it replaces the contents of that tag with the error handling expression. In this case, the `span` tag will contain an error message.

Typically you'll define an error handler on a tag that encloses a logical page element, for example a table. If an error crops up drawing the table, then the error handler can simply omit the table from the page, or else replace it with an error message of some sort.

For more flexible error handling you can call a script. For example:

            <div tal:on-error="structure here/handleError">
            ...
            </div>

Any error that occurs inside the `div` will call the `handleError` script. Note that the `structure` option allows the script to return HTML. Your error handling script can examine the error and take various actions depending on the error. Your script gets access to the error through the `error` variable in the namespace. For example:

            ## Script (Python) "handleError"
            ##bind namespace=_
            ##
            error=_['error']
            if error.type==ZeroDivisionError:
                return "<p>Can't divide by zero.</p>"
            else
                return """<p>An error occurred.</p>
                          <p>Error type: %s</p>
                          <p>Error value: %s</p>""" % (error.type,
                                                       error.value)

Your error handling script can take all kinds of actions, for example, it might log the error by sending email.

The `tal:on-error` statement is not meant for general purpose exception handling. For example, you shouldn't validate form input with in. You should use a script for that, since scripts allow you to do powerful exception handling. The `tal:on-error` statement is for dealing with unusual problems that can occur when rendering templates.

### Interactions Between TAL Statements

When there is only one TAL statement per element, the order in which they are executed is simple. Starting with the root element, each element's statements are executed, then each of its child elements is visited, in order, and their statements are executed, and so on.

However, it's possible to have more than one TAL statement on the same element. Any combination of statements may appear on the same element, except that the `tal:content` and `tal:replace` statements may not appear together.

When an element has multiple statements, they are executed in this order:

1. define
2. condition
3. repeat
4. content or replace
5. attributes
6. omit-tag

Since the `tal:on-error` statement is only invoked when an error occurs, it does not appear in the list.

The reasoning behind this ordering goes like this: you often want to set up variables for use in other statements, so define comes first. The very next thing to do is decide whether this element will be included at all, so condition is next; since the condition may depend on variables you just set, it comes after define. It is valuable to be able to replace various parts of an element with different values on each iteration of a repeat, so repeat comes before content, replace and attributes. Content and replace can't both be used on the same element so they occur at the same place. Omit-tag comes last since no other statement are likely to depend on it and since it should come after define and repeat.

Here's an example tag that includes several TAL statements:

            <p tal:define="x /root/a/long/path/x | nothing"
               tal:condition="x"
               tal:content="x/txt"
               tal:attributes="class x/class">Ex Text</p>

Notice how the `tal:define` statement is executed first, and the other statements rely on its results.

There are three limits you should be aware of when combining TAL statements on elements:

1. Only one of each kind of statement can be used on a single tag. Since HTML does not allow multiple attributes with the same name. For example, you can't have two `tal:define` on the same tag.
2. Both of `tal:content` and `tal:replace` cannot be used on the same tag, since their functions conflict.
3. The order in which you write TAL attributes on a tag does not affect the order in which they execute. No matter how you arrange them, the TAL statements on a tag always execute in the fixed order described earlier.

If you want to override the ordering of TAL statements, you must do so by enclosing the element in another element, possibly `div` or `span`, and placing some of the statements on this new element. For example suppose you want to loop over a series of items but skip some. Here's an attempt to write a template that loops over the numbers zero to nine and skips three:

            <!-- broken template -->
            <ul>
              <li tal:repeat="n python:range(10)"
                  tal:condition="python:n != 3"
                  tal:content="n">
                1
              </li>
            </ul>

This template doesn't work because the the condition is tested before the repeat is executed. The upshot is that the `n` variable is not defined until after it is tested. Here's a way around this problem:

            <ul>
              <div tal:repeat="n python:range(10)"
                   tal:omit-tag="">
                <li tal:condition="python:n != 3"
                    tal:content="n">
                  1
                </li>
              </div>
            </ul>

This template solves the problem by defining the `n` variable on an enclosing `div` tag. Notice that the `div` tag will not appear in the output on account of it's `tal:omit-tag` statement. This may be ugly, but it works. Perhaps future versions of Page Templates will solve this problem in a nicer fashion.

### Form Processing

You can process forms in DTML using a common pattern called the "form/action pair". A form/action pair consists of two DTML methods or document: one that contains a form that collects input from the user, and one that contains an action that is taken on that input and returns the user a response. The form calls the action. See Chapter 4, "Dynamic Content with DTML" for more information on the form/action pattern.

Zope Page Templates don't work particularly well with the form/action pattern since it assumes that input processing and response presentation are handled by the same object (the action). Instead of the form/action pattern you should use form/action/response pattern with Page Templates. The form and response should be Page Templates and the action should be a script. The form template gathers the input and call the action script. The action script should process the input and return a response template. This pattern is more flexible than the form/action pattern since it allows the script to return any of a number of different response objects.

For example here's a part of a form template:

            ...
            <form action="action">
              <input type="text" name="name">
              <input type="text" name="age:int">
              <input type="submit">
            </form>
            ...

This form could be processed by this script:

            ## Script (Python) "action"
            ##parameters=name, age
            ##
            container.addPerson(name, age)
            return container.responseTemplate()

This script calls a method to process the inputs and then returns another template, the response. You can render a Page Template from Python by calling it. The response template typically contains an acknowledgment that the form has been correctly processed.

The action script can do all kinds of things. It can validate input, handle errors, send email, and more. Here's a sketch of how to validate input with an script:

            ## Script (Python) "action"
            ##
            if not context.validateData(request):
                # if there's a problem return the form page template
                # along with an error message
                return context.formTemplate(error_message='Invalid data')

            # otherwise return the thanks page
            return context.responseTemplate()

This script validates the form input and returns the form template with an error message if there's a problem. You can pass Page Templates extra information with keyword arguments. The keyword arguments are available to the template as via the `options` built-in variable. So the form template in this example might include a section like this:

            <b tal:condition="options/error_message | nothing"
               tal:content="options/error_message">
              Error message goes here.
            </b>

This example shows how you can display an error message that is passed to the template via keyword arguments.

Depending on your application you may choose to redirect the user to a response Page Template instead of returning it directly. This results in twice as much network activity, but might be useful because it changes the URL displayed in the user's browser to the URL of the Page Template, rather than that of the action script.

If you insist on doing a crummy job of things, you can always create a lame version of the form-action pair using Page Templates. You should only do this when you don't care about error handling and when the response will always be the same, not matter what the user submits. Since Page Templates don't have an equivalent of dtml-call, you can use one of any number of hacks to call an input processing method without inserting its results. For example:

            <span tal:define="unused here/processInputs"
                  tal:omit-tag=""/>

This sample calls the `processInputs` method and assigns the result to the `unused` variable.

## Expressions

You've already encountered Page Template expressions. Expressions provide values to template statements. For example, path expressions describe objects by giving them paths such as `request/form/age`, or `user/getUserName`. In this section you'll learn about all the different types of expressions, and variables.

### Built-in Variables

Variables are names that you can use in expressions. You have already seen some examples of the built-in variables such as `template`, `user`, `repeat`, and `request`. Here is the complete list of the other built-in variables and their uses:

`nothing`
  A false value, similar to a blank string, that you can use in `tal:replace` or `tal:content` to erase a tag or its contents. If you set an attribute to `nothing`, the attribute is removed from the tag (or not inserted), unlike a blank string.
`default`
  A special value that doesn't change anything when used in `tal:replace`, `tal:content`, or `tal:attributes`. It leaves the template text in place.
`options`
  The keyword arguments, if any, that were passed to the template. Note: options are only available when a template is called from Python. When a template is rendered from the web, no options are present.
`attrs`
  A dictionary of attributes of the current tag in the template. The keys are the attributes names, and the values are the original values of the attributes in the template. This variable is rarely needed.
`root`
  The root Zope object. Use this to get Zope objects from fixed locations, no matter where your template is placed or called.
`here`
  The object on which the template is being called. This is often the same as the container, but can be different if you are using acquisition. Use this to get Zope objects that you expect to find in different places depending on how the template is called. The `here` variable is analogous to the `context` variable in Python-based scripts.
`container`
  The container (usually a Folder) in which the template is kept. Use this to get Zope objects from locations relative to the template's permanent home. The `container` and `here` variables refer to the same object when a template is called from its normal location. However, when a template is applied to another object (for example, a ZSQL Method) the `container` and `here` will not refer to the same object.
`modules`
  The collection of Python modules available to templates. See the section on writing Python expressions.

You'll find examples of how to use these variables through out this chapter.

### String Expressions

String expressions allow you to easily mix path expressions with text. All of the text after the leading `string:` is taken and searched for path expressions. Each path expression must be preceded by a dollar sign ('$'). Here are some examples:

            "string:Just text. There's no path here."
            "string:copyright $year, by me."

If the path expression has more than one part, or needs to be separated from the text that follows it, it must be surrounded by braces ('{}'). For example:

            "string:Three ${vegetable}s, please."
            "string:Your name is ${user/getUserName}!"

Notice how in the example above, you need to surround the `vegetable` path with braces so that Zope doesn't mistake it for `vegetables`.

Since the text is inside of an attribute value, you can only include a double quote by using the entity syntax `&quot;`. Since dollar signs are used to signal path expressions, a literal dollar sign must be written as two dollar signs ('$$'). For example:

            "string:Please pay $$$dollars_owed"
            "string:She said, &quot;Hello world.&quot;"

Some complex string formatting operations (such as search and replace or changing capitalization) can't easily be done with string expressions. For these cases, you should use Python expressions or Scripts.

### Path Expressions

Path expressions refer to objects with a path that resembles a URL path. A path describes a traversal from object to object. All paths begin with a known object (such as a built-in variable, a repeat variable, or a user defined variable) and depart from there to the desired object. Here are some example paths expressions:

            template/title
            container/files/objectValues
            user/getUserName
            container/master.html/macros/header
            request/form/address
            root/standard_look_and_feel.html

With path expressions you can traverse from an object to its sub-objects including properties and methods. You can also use acquisition in path expressions. See the section entitled "Calling Scripts from the Web" in Chapter 8, "Advanced Zope Scripting" for more information on acquisition and path traversal.

Zope restricts object traversal in path expressions in the same way that it restricts object access via URLs. You must have adequate permissions to access an object in order to refer to it with a path expression. See Chapter 6, "Users and Security" for more information about object access controls.

#### Alternate Paths

The path `template/title` is guaranteed to exist every time the template is used, although it may be a blank string. Some paths, such as `request/form/x`, may not exist during some renderings of the template. This normally causes an error when Zope evaluates the path expression.

When a path doesn't exist, you may have a fall-back path or value that you would like to use instead. For instance, if `request/form/x` doesn't exist, you might want to use `here/x` instead. You can do this by listing the paths in order of preference, separated by vertical bar characters ('|'):

              <h4 tal:content="request/form/x | here/x">Header</h4>

Two variables that are very useful as the last path in a list of alternates are `nothing` and `default`. For example, `default` tells `tal:content` to leave the dummy content. Different TAL statements interpret `default` and `nothing` differently. See Appendix C, "Zope Page Templates Reference" for more information.

You can also use a non-path expression as the final part in an alternate-path expression. For example:

              <p tal:content="request/form/age|python:18">age</p>

In this example, if the `request/form/age` path doesn't exist, then the value is the number 18. This form allows you to specify default values to use which can't be expressed as paths. Note, you can only use a non-path expression as the last alternative.

You can also test the existence of a path directly with the exists expression type prefix. See the section "Exists Expressions" below for more information on exists expressions.

### Not Expressions

Not expressions let you negate the value of other expressions. For example:

            <p tal:condition="not:here/objectIds">
              There are no contained objects.
            </p>

Not expressions return true when the expression they are applied to is false, and vice versa. In Zope, non-existent variables, zero, empty strings, empty sequences, nothing, and None are considered false, while everything else is true.

There isn't much reason to use not expressions with Python expressions since you can use the Python `not` keyword instead.

### Nocall Expressions

An ordinary path expression tries to render the object that it fetches. This means that if the object is a function, Script, Method, or some other kind of executable thing, then expression will evaluate to the result of calling the object. This is usually what you want, but not always. For example, if you want to put a DTML Document into a variable so that you can refer to its properties, you can't use a normal path expression because it will render the Document into a string.

If you put the `nocall:` expression type prefix in front of a path, it prevents the rendering and simply gives you the object. For example:

            <span tal:define="doc nocall:here/aDoc"
                  tal:content="string:${doc/getId}: ${doc/title}">
            Id: Title</span>

This expression type is also valuable when you want to define a variable to hold a function or class from a module, for use in a Python expression.

Nocall expressions can also be used on functions, rather than objects:

            <p tal:define="join nocall:modules/string/join">

This expression defines the `join` variable as a function ('string.join'), rather than the result of calling a function.

### Exists Expressions

An exists expression is true if its path exists, and otherwise is false. For example here's one way to display an error message only if it is passed in the request:

            <h4 tal:define="err request/form/errmsg | nothing"
                tal:condition="err"
                tal:content="err">Error!</h4>

You can do the same thing more easily with an exists expression:

            <h4 tal:condition="exists:request/form/errmsg"
                tal:content="request/form/errmsg">Error!</h4>

You can combine exists expressions with not expressions, for example:

            <p tal:condition="not:exists:request/form/number">Please enter
            a number between 0 and 5</p>

Note that in this example you can't use the expression, "not:request/form/number", since that expression will be true if the `number` variable exists and is zero.

### Python Expressions

The Python programming language is a simple and expressive one. If you have never encountered it before, you should read one of the excellent tutorials or introductions available at the Python website.

A Page Template Python expression can contain anything that the Python language considers an expression. You can't use statements such as `if` and `while`. In addition, Zope imposes some security restrictions to keep you from accessing protected information, changing secured data, and creating problems such as infinite loops. See Chapter Chapter 8, "Advanced Zope Scripting" for more information on Python security restrictions.

#### Comparisons

One place where Python expressions are practically necessary is in `tal:condition` statements. You usually want to compare two strings or numbers, and there isn't any other way to do that without Python expressions. You can use the comparison operators `<` (less than), `>` (greater than), `==` (equal to), and `!=` (not equal to). You can also use the boolean operators `and`, `not`, and `or`. For example:

              <p tal:repeat="widget widgets">
                <span tal:condition="python:widget.type == 'gear'>
                Gear #<span tal:replace="repeat/widget/number>1</span>:
                <span tal:replace="widget/name">Name</span>
                </span>
              </p>

This example loops over a collection of objects, testing each object's `type` attribute.

Sometimes you want to choose different values inside a single statement based on one or more conditions. You can do this with the `test` function, like this:

              You <span tal:define="name user/getUserName"
                   tal:replace="python:test(name=='Anonymous User',
                                           'need to log in', default)">
                    are logged in as
                    <span tal:replace="name">Name</span>
                  </span>

The `test` function works like an if/then/else statement. See Appendix A, "DTML Reference" for more information on the `test` function. Here's another example of how you can use the `test` function:

              <tr tal:define="oddrow repeat/item/odd"
                  tal:attributes="class python:test(oddrow, 'oddclass',
                                                    'evenclass')">

Without the `test` function you'd have to write two `tr` elements each with a different condition, one for even rows, and the other for odd rows.

#### Using other Expression Types

You can use other expression types inside of a Python expression. Each expression type has a corresponding function with the same name, including: `path()`, `string()`, `exists()`, and `nocall()`. This allows you to write expressions such as:

              "python:path('here/%s/thing' % foldername)"
              "python:path(string('here/$foldername/thing'))"
              "python:path('request/form/x') or default"

The final example has a slightly different meaning than the path expression, "request/form/x | default", since it will use the default text if "request/form/x" doesn't exists or if it is false.

#### Getting at Zope Objects

Much of the power of Zope involves tying together specialized objects. Your Page Templates can use Scripts, SQL Methods, Catalogs, and custom content objects. In order to use these objects you have to know how to get access to them within Page Templates.

Object properties are usually attributes, so you can get a template's title with the expression "template.title". Most Zope objects support acquisition, which allows you to get attributes from "parent" objects. This means that the Python expression "here.Control_Panel" will acquire the Control Panel object from the root Folder. Object methods are attributes, as in "here.objectIds" and "request.set". Objects contained in a Folder can be accessed as attributes of the Folder, but since they often have Ids that are not valid Python identifiers, you can't use the normal notation. For example, you cannot use this Python expression:

              "python:here.penguin.gif"'.

You must write:

              "python:getattr(here, 'penguin.gif')"

since Python doesn't support attribute names with periods.

Some objects, such as `request`, `modules`, and Zope Folders support Python item access, for example:

              request['URL']
              modules['math']
              here['thing']

When you use item access on a Folder, it doesn't try to acquire the name, so it will only succeed if there is actually an object with that Id contained in the Folder.

As shown in previous chapters, path expressions allow you to ignore details of how you get from one object to the next. Zope tries attribute access, then item access. You can write:

              "here/images/penguin.gif"

instead of:

              "python:getattr(here.images, 'penguin.gif')"

and:

              "request/form/x"

instead of:

              "python:request.form['x']"

The trade-off is that path expressions don't allow you to specify those details. For instance, if you have a form variable named "get", you must write:

              "python:request.form['get']"

since this path expression:

              "request/form/get"

will evaluate to the "get" method of the form dictionary.

If you prefer you can use path expressions inside Python expressions using the `path()` function, as described above.

#### Using Scripts

Script objects are often used to encapsulate business logic and complex data manipulation. Any time that you find yourself writing lots of TAL statements with complicated expressions in them, you should consider whether you could do the work better in a Script. If you have trouble understanding your template statements and expressions, then it's better to simplify your Page Template and use Scripts for the complex stuff.

Each Script has a list of parameters that it expects to be given when it is called. If this list is empty, then you can use the Script by writing a path expression. Otherwise, you will need to use a Python expression in order to supply the argument, like this:

              "python:here.myscript(1, 2)"
              "python:here.myscript('arg', foo=request.form['x'])"

If you want to return more than one item of data from a Script to a Page Template, it is a good idea to return it in a dictionary. That way, you can define a variable to hold all the data, and use path expressions to refer to each item. For example, suppose the `getPerson` script returns a dictionary with `name` and `age` keys:

              <span tal:define="person here/getPerson"
                    tal:replace="string:${person/name} is ${person/age}">
              Name is 30</span> years old.

Of course, it's fine to return Zope objects and Python lists as well.

#### Calling DTML

Unlike Scripts, DTML Methods and Documents don't have an explicit parameter list. Instead, they expect to be passed a client, a mapping, and keyword arguments. They use these parameters to construct a namespace. See Chapter 7 for more information on explicitly calling DTML.

When the Zope publishes a DTML object through the web, it passes the context of the object as the client, and the REQUEST as the mapping. When one DTML object calls another, it passes its own namespace as the mapping, and no client.

If you use a path expression to render a DTML object, it will pass a namespace with `request`, `here`, and the template's variables already on it. This means that the DTML object will be able to use the same names as if it were being published in the same context as the template, plus the variable names defined in the template.

#### Python Modules

The Python language comes with a large number of modules, which provide a wide variety of capabilities to Python programs. Each module is a collection of Python functions, data, and classes related to a single purpose, such as mathematical calculations or regular expressions.

Several modules, including "math" and "string", are available in Python expressions by default. For example, you can get the value of pi from the math module by writing "python:math.pi". To access it from a path expression, however, you need to use the `modules` variable, "modules/math/pi".

The "string" module is hidden in Python expressions by the "string" expression type function, so you need to access it through the `modules` variable. You can do this directly in an expression in which you use it, or define a global variable for it, like this:

              tal:define="global mstring modules/string"
              tal:replace="python:mstring.join(slist, ':')"

In practice you'll rarely need to do this since you can use string methods most of the time rather than having to rely on functions in the string module.

Modules can be grouped into packages, which are simply a way of organizing and naming related modules. For instance, Zope's Python-based Scripts are provided by a collection of modules in the "PythonScripts" subpackage of the Zope "Products" package. In particular, the "standard" module in this package provides a number of useful formatting functions that are standard in the DTML "var" tag. The full name of this module is "Products.PythonScripts.standard", so you could get access to it using either of the following statements:

              tal:define="global pps modules/Products/PythonScripts/standard"
              tal:define="global pps python:modules['Products.PythonScripts.standard']"

Most Python modules cannot be accessed from Page Templates, DTML, or Scripts unless you add Zope security assertions to them. This procedure is outside the scope of this book. See the Zope Developer's Guide for more information.

## Macros

So far, you've seen how page templates can be used to add dynamic behavior to individual web pages. Another feature of page templates is the ability to reuse look and feel elements across many pages.

For example, with Page Templates, you can have a site that has a standard look and feel. No matter what the "content" of a page, it will have a standard header, side-bar, footer, and/or other page elements. This is a very common requirement for web sites.

You can reuse presentation elements across pages with macros. Macros define a section of a page that can be reused in other pages. A macro can be an entire page, or just a chunk of a page such as a header or footer. After you define one or more macros in one Page Template, you can use them in other Page Templates.

### Using Macros

You can define macros with tag attributes similar to TAL statements. Macro tag attributes are called Macro Expansion Tag Attribute Language (METAL) statements. Here's an example macro definition:

            <p metal:define-macro="copyright">
              Copyright 2001, <em>Foo, Bar, and Associates</em> Inc.
            </p>

This `metal:define-macro` statement defines a macro named "copyright". The macro consists of the `p` tag and its contents (including all contained tags).

Macros defined in a Page Template are stored in the template's `macro` attribute. You can use macros from other page template by referring to them through the `macros` attribute of the Page Template in which they are defined. For example, suppose the `copyright` macro is in a Page Template called "master_page". Here's how to use `copyright` macro from another Page Template:

            <hr>
            <b metal:use-macro="container/master_page/macros/copyright">
              Macro goes here
            </b>

In this Page template, the `b` tag will be completely replaced by the macro when Zope renders the page:

            <hr>
            <p>
              Copyright 2001, <em>Foo, Bar, and Associates</em> Inc.
            </p>

If you change the macro (for example, if the copyright holder changes their name) then all Page Templates that use the macro will automatically reflect the change.

Notice how the macro is identified by a path expression using the `metal:use-macro` statement. The `metal:use-macro` statement replaces the statement element with the named macro.

### Macro Details

The `metal:define-macro` and `metal:use-macro` statements are pretty simple. However there are a few subtleties worth mentioning.

A macro's name must be unique within the Page Template in which it's defined. You can define more than one macro in a template, but they all need to have different names.

Normally you'll refer to a macro in a `metal:use-macro` statement with a path expression. However, you can use any expression type you wish so long as it returns a macro. For example:

            <p metal:use-macro="python:here.getMacro()">
              Replaced with a dynamically determined macro,
              which is located by the getMacro script.
            </p>

Using Python expressions to locate macros lets you dynamically vary which macro your template uses.

You can use the `default` variable with the `metal:use-macro` statement:

            <p metal:use-macro="default">
              This content remains - no macro is used
            </p>

The result is the same as using default with `tal:content` and `tal:replace`, the statement element doesn't change.

If you try to use the `nothing` variable with `metal:use-macro` you will get an error, since `nothing` is not a macro. If you want to use `nothing` to conditionally include a macro, you should instead enclose the `metal:use-macro` statement with a `tal:condition` statement.

Zope handles macros first when rendering your templates. Then Zope evaluates TAL expressions. For example, consider this macro:

            <p metal:define-macro="title"
               tal:content="template/title">
              template's title
            </p>

When you use this macro it will insert the title of the template in which the macro is used, not the title of the template in which the macro is defined. In other words, when you use a macro, it's like copying the text of a macro into your template and then rendering your template.

If you check the Expand macros when editing option on the Page Template Edit view, then any macros that you use will be expanded in your template's source. This is Zope's default behavior, and in general this is what you want, since it allows you to edit a complete and valid page. Sometimes, however, especially when you're editing in the ZMI, rather than using a WYSIWYG editing tool, it's more convenient not to expand macros when editing. In these cases, simply uncheck the option.

### Using Slots

Macros are much more useful if you can override parts of them when you use them. You can do this by defining slots in the macro that you can fill in when you use the template. For example, consider a side bar macro:

            <p metal:define-macro="sidebar">
              Links
              <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/products">Products</a></li>
                <li><a href="/support">Support</a></li>
                <li><a href="/contact">Contact Us</a></li>
              </ul>
            </p>

This macro is fine, but suppose you'd like to include some additional information in the sidebar on some pages. One way to accomplish this is with slots:

            <p metal:define-macro="sidebar">
              Links
              <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/products">Products</a></li>
                <li><a href="/support">Support</a></li>
                <li><a href="/contact">Contact Us</a></li>
              </ul>
              <span metal:define-slot="additional_info"></span>
            </p>

When you use this macro you can choose to fill the slot like so:

            <p metal:fill-slot="container/master.html/macros/sidebar">
              <b metal:fill-slot="additional_info">
                Make sure to check out our <a href="/specials">specials</a>.
              </b>
            </p>

When you render this template the side bar will include the extra information that you provided in the slot:

            <p>
              Links
              <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/products">Products</a></li>
                <li><a href="/support">Support</a></li>
                <li><a href="/contact">Contact Us</a></li>
              </ul>
              <b>
                Make sure to check out our <a href="/specials">specials</a>.
              </b>
            </p>

Notice how the `span` element that defines the slot is replaced with the `b` element that fills the slot.

### Customizing Default Presentation

A common use of slot is to provide default presentation which you can customize. In the slot example in the last section, the slot definition was just an empty `span` element. However, you can provide default presentation in a slot definition. For example, consider this revised sidebar macro:

            <div metal:define-macro="sidebar">
              <p metal:define-slot="links">
              Links
              <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/products">Products</a></li>
                <li><a href="/support">Support</a></li>
                <li><a href="/contact">Contact Us</a></li>
              </ul>
              </p>
              <span metal:define-slot="additional_info"></span>
            </div>

Now the sidebar is fully customizable. You can fill the links slot to redefine the sidebar links. However, if you choose not to fill the `links` slot then you'll get the default links, which appear inside the slot.

You can even take this technique further by defining slots inside of slots. This allows you to override default presentation with a fine degree of precision. Here's a sidebar macro that defines slots within slots:

            <div metal:define-macro="sidebar">
              <p metal:define-slot="links">
              Links
              <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/products">Products</a></li>
                <li><a href="/support">Support</a></li>
                <li><a href="/contact">Contact Us</a></li>
                <span metal:define-slot="additional_links"></span>
              </ul>
              </p>
              <span metal:define-slot="additional_info"></span>
            </div>

If you wish to customize the sidebar links you can either fill the `links` slot to completely override the links, or you can fill the `additional_links` slot to insert some extra links after the default links. You can nest slots as deeply as you wish.

### Combining METAL and TAL

You can use both METAL and TAL statements on the same elements. For example:

            <ul metal:define-macro="links"
                tal:repeat="link here/getLinks">
              <li>
                <a href="link url"
                   tal:attributes="url link/url"
                   tal:content="link/name">link name</a>
              </li>
            </ul>

Since METAL statements are evaluated before TAL statements, there are no conflicts. This example is also interesting since it customizes a macro without using slots. The macro calls the `getLinks` Script to determine the links. You can thus customize your site's links be redefining the `getLinks` Script at different locations within your site.

It's not always easy to figure out the best way customize look and feel in different parts of your site. In general you should use slots to override presentation elements, and you should use Scripts to provide content dynamically. In the case of the links example, it's arguable whether links are content or presentation. Scripts probably provide a more flexible solution, especially if your site includes link content objects.

### Whole Page Macros

Rather than using macros for chunks of presentation shared between pages, you can use macros to define entire pages. Slots make this possible. Here's an example macro that defines an entire page:

            <html metal:define-macro="page">
              <head>
                <title tal:content="here/title">The title</title>
              </head>

              <body>
                <h1 metal:define-slot="headline"
                    tal:content="here/title">title</h1>

                <p metal:define-slot="body">
                  This is the body.
                </p>

                <span metal:define-slot="footer">
                  <p>Copyright 2001 Fluffy Enterprises</p>
                </span>

              </body>
            </html>

This macro defines a page with three slots, `headline`, `body`, and `footer`. Notice how the `headline` slot includes a TAL statement to dynamically determine the headline content.

You can then use this macro in templates for different types of content, or different parts of your site. For example here's how a template for news items might use this macro:

           <html metal:use-macro="container/master.html/macros/page">

             <h1 metal:fill-slot="headline">
               Press Release:
               <span tal:replace="here/getHeadline">Headline</span>
             </h1>

             <p metal:fill-slot="body"
                tal:content="here/getBody">
               News item body goes here
             </p>

           </html>

This template redefines the `headline` slot to include the words, "Press Release" and call the `getHeadline` method on the current object. It also redefines the `body` slot to call the `getBody` method on the current object.

The powerful thing about this approach is that you can now change the `page` macro and the press release template will be automatically updated. For example you could put the body of the page in a table and add a sidebar on the left and the press release template would automatically use these new presentation elements.

This is a much more flexible solution to control page look and feel then the DTML `standard_html_header` and `standard_html_footer` solution. In fact, Zope comes with a stock page template in the root folder named `standard_template.pt` that includes a whole page macro with a `head` and `body` slot. Here's how you might use this macro in a template:

            <html metal:use-macro="here/standard_template.pt/macros/page">
              <div metal:fill-slot="body">
                <h1 tal:content="here/title">Title</h1>
                <p tal:content="here/getBody">Body text goes here</p>
              </div>
            </html>

Using the `standard_template.pt` macro is very similar to using other whole page macros. The only subtlety worth pointing out is the path used to locate the macro. In this example the path begins with `here`. This means that Zope will search for the `standard_template.pt` object using acquisition starting at the object that the template is applied to. This allows you to customize the look and feel of templates by creating custom `standard_template.pt` objects in various locations. This is exactly the same trick that you can use to customize look and feel by overriding `standard_html_header` and `standard_html_footer` in site locations. However, with `standard_template.pt` you have more choices. You can choose to start the path to the macro with `root` or with `container`, as well as with `here`. If the path begins with `root` then you will always get the standard template which is located in the root folder. If the path begins with `container` then Zope will search for a standard template using acquisition starting in the folder where the template is defined. This allows you to customize look and feel of templates, but does not allow you to customize the look and feel of different objects based on their location in the site.

## Caching Templates

While rendering Page Templates normally is quite fast, sometimes it's not fast enough. For frequently accessed pages, or page that take a long time to render, you may want to trade some dynamic behavior for speed. Caching lets you do this. For more information on caching see the "Cache Manager" section of Chapter 3, "Basic Objects".

You can cache Page Templates using a cache manager in the same way that you cache other objects. To cache a Page Template, you must associate it with a cache manager. You can either do this by going to the Cache view of you Page Template and selecting the cache manager, or by going to the Associate view of your cache manager and locating your Page Template.

Here's an example of how to cache a Page Template. First create a Python-based script name `long.py` with these contents:

          ## Script (Python) "long.py"
          ##
          for i in range(500):
            for j in range(500):
              for k in range(5):
                pass
          return 'Done'

The purpose of this script is to take up a noticeable amount of execution time. Now create a Page Template that uses this script, for example:

          <html>
            <body>
              <p tal:content="here/long.py">results</p>
            </body>
          </html>

Now view this page. Notice how it takes a while to render. Now let's radically improve its rendering time with caching. Create a Ram Cache Manager if you don't already have one. Make sure to create it within the same folder as your Page Template, or in a higher level. Now visit the Cache view of your Page Template. Choose the Ram Cache Manager you just created and click Save Changes. Click the Cache Settings link to see how your Ram Cache Manager is configured. By default, your cache stores objects for one hour (3600 seconds). You may want to adjust this number depending on your application. Now return to your Page Template and view it again. It should take a while for it to render. Now reload the page, and watch it render immediately. You can reload the page again and again, and it will always render immediately since the page is now cached.

If you change your Page Template, then it will be removed from the cache. So the next time you view it, it will take a while to render. But after that it will render quickly since it will be cached again.

Caching is a simple but very powerful technique for improving performance. You don't have to be a wizard to use caching, and it can provide great speed-ups. It's well worth your time to use caching for performance-critical applications.

## Page Template Utilities

Zope Page Templates are powerful but simple. Unlike DTML, Page Templates don't give you a lot of convenience features for things like batching, drawing trees, sorting, etc. The creators of Page Templates wanted to keep them simple. However, you may miss some of the built-in features that DTML provides. To address these needs, Zope comes with utilities designed to enhance Page Templates.

### Batching Large Sets of Information

When a user queries a database and gets a hundred results, it's often better to show them several pages with only twenty results per page, rather than putting all the results on one page. Breaking up large lists into smaller lists is called batching.

Unlike DTML, which provides batching built into the language, Page Templates support batching by using a special `Batch` object that comes from the `ZTUtils` utility module. See Appendix B, "API Reference", for more information on the `ZTUtils` Python module.

Here's a simple example, showing how to create a `Batch` object:

            <ul tal:define="lots python:range(100);
                            batch python:modules['ZTUtils'].Batch(lots,
                                                                  size=10,
                                                                  start=0)">
              <li tal:repeat="num batch"
                  tal:content="num">0
              </li>
            </ul>

This example renders a list with 10 items (in this case, the numbers 0 through 9). The `Batch` object chops a long list up into groups or batches. In this case it broke a one hundred item list up into batches of ten items.

You can display a different batch of ten items by passing a different start number:

            <ul tal:define="lots python:range(100);
                            batch python:modules['ZTUtils'].Batch(lots,
                                                                  size=10,
                                                                  start=13)">

This batch starts with the fourteenth item and ends with the twenty third item. In other words, it displays the numbers 13 through 22. It's important to notice that the batch `start` argument is the index of the first item. Indexes count from zero, rather than from one. So index 13 points to the fourteenth item in the sequence. Python uses indexes to refer to list items.

Normally when you use batches you'll want to include navigation elements on the page to allow users to go from batch to batch. Here's a full-blow batching example that shows how to navigate between batches:

            <html>
              <head>
                <title tal:content="template/title">The title</title>
              </head>
              <body tal:define="employees  here/getEmployees;
                                start python:path('request/start') or 0;
                                batch python:modules['ZTUtils'].Batch(employees,
                                                                      size=10,
                                                                      start=start);
                                previous python:batch.previous;
                                next python:batch.next">

              <p>
                <a tal:condition="previous"
                   tal:attributes="href string:${request/URL0}?start:int=${previous/first}"
                   href="previous_url">previous</a>
                <a tal:condition="next"
                   tal:attributes="href string:${request/URL0}?start:int=${next/first}"
                   href="next_url">next</a>
              </p>

              <ul tal:repeat="employee batch" >
                <li>
                  <span tal:replace="employee/name">Bob Jones</span>
                  makes $<span tal:replace="employee/salary">100,000</span>
                  a year.
                </li>
              </ul>

              </body>
            </html>

This example iterates over batches of results from the `getEmployees` ZSQL Method. It draws a previous and a next link as necessary to allow you to page through all the results a batch at a time.

Take a look at the `tal:define` statement on the `body` element. It defines a bunch of batching variables. The `employees` variable is a potentially big list of employee objects returned by the `getEmployees` ZSQL Method. The second variable, `start`, is either set to the value of `request/start` or to zero if there is no `start` variable in the request. The `start` variable keeps track of where you are in the list of employees. The `batch` variable is a batch of ten items from the lists of employees. The batch starts at the location specified by the `start` variable. The `previous` and `next` variables refer to the previous and next batches (if any). Since all these variables are defined on the `body` element, they are available to all elements inside the body.

Next let's look at the navigation links. They create hyper links to browse previous and next batches. The `tal:condition` statement first tests to see if there is a previous and next batch. If there is a previous or next batch, then the link is rendered, otherwise there is not link. The `tal:attributes` statement creates a link to the previous and next batches. The link is simply the URL or the current page ('request/URL0') along with a query string indicating the start index of the batch. For example, if the current batch starts with index 10, then the previous batch will start with an index of 0. The `first` variable of a batch gives its staring index, so in this case, `previous.start` would be 0.

It's not important to fully understand the workings of this example. Simply copy it, or use a batching example created by the Z Search Interface. Later when you want to do more complex batching you can experiment by changing the example code. Don't forget to consult Appendix B, "API Reference" for more information on the `ZTUtils` module and `Batch` objects.

### Miscellaneous Utilities

Zope provides a couple Python modules which may come in handy when using Page Templates. The `string`, `math`, and `random` modules can be used in Python expressions for string formatting, math function, and pseudo-random number generation. These same modules are available from DTML and Python-based scripts. See Appendix B, "API Reference" for more information on these modules.

The `Products.PythonScripts.standard` module is designed to provide utilities to Python-based scripts, but it's also useful for Page Templates. It includes various string and number formatting functions. See Appendix B, "API Reference" for more information.

As mentioned earlier in the chapter, the `sequence` module provides a handy `sort` function. See Appendix B, "API Reference" for the details.

Finally the `AccessControl` module includes a function and a class which you'll need if you want to test access and to get the authenticated user. See Appendix B, "API Reference" for more information.

## Conclusion

This chapter covers all the nooks and crannies of Page Templates, and after reading it you may feel a little overwhelmed. Don't worry, you don't need to know everything in this chapter to effectively use Page Templates. You should understand the different path types and macros, but you can come back to the rest of the material when you need it. The advanced features that you've learned about in this chapter are there for you when you need them. It's encouraging to know that when you're ready you can do some pretty impressive tricks with Page Templates.