Appendix C: Zope Page Templates Reference

Zope Page Templates are an HTML/XML generation tool. This appendix is a reference to Zope Page Templates standards: Tag Attribute Language (TAL), TAL Expression Syntax (TALES), and Macro Expansion TAL (METAL). It also describes some ZPT-specific behaviors that are not part of the standards.

snell - Nov. 15, 2003 5:20 am:
 There is no reference to metal:block
 in:
 Zope Page Templates Reference
 Page Templates
 Advanced Page Templates
 What is it for?

Anonymous User - Nov. 23, 2003 3:01 pm:
 tal:block or metal:block can be used on tag-level
 if you have no html-tag to put your tal/metal code on.

Anonymous User - Nov. 23, 2003 3:02 pm:
 for instance, outside of the html-tag.

Anonymous User - Apr. 14, 2004 10:17 am:
 I have found a tal function that is never used or described in any of these pages or the WWW after a search
 on 5 searchpages. Except in a subproduct for Zope (Plone) (but they don't explain tal usage on their site so
 I'll ask it here)
   <tal:datetime define="today python:DateTime().earliestTime()">
     ...
   </tal:datetime>

 First thing that intrigues me: There is no standard html tag around this tal, I thought only
 tal:block could do that?
 Second thing I noticed: All the code that remained between those tags (represented by "...") appeared in the
 resulting page without problem, but there remained nothing visible of these "tal:datetime" tags.
 Third thing I noticed: straight after the tal:datetime statement comes define="...", clearly working as the
 tal:define explained later in this apendix. Why has there been no need to write "tal:" in front of it?
 Is this some faulty page template code, yet still beeing accepted by the browser?
Could this code have been replaced by a simple <tal:block tal:define="..."> ... </tal:block> or
<span
 tal:omit-tag tal:define="..."> ... </span>?
 Or is there a possibility to extend the tal-statements and why isn't it told here then?

Anonymous User - Apr. 14, 2004 10:30 am:
 You need to understand namespaces. In an HTML page, the default namespace
 is that of HTML. The browser will attempt to parse all tags in this namespace.

 To put a tag in a different namespace, which will be ignored by the browser,
 prefix it with the namespace, e.g. to put a tag in the 'tal' namespace, write:
    <tal:XXXX ... />
 The same goes for attributes. To put an attribute in the 'tal' namespace, 
 write:   <span tal:define="..." />
 If an element is in the 'tal' namespace, all its attributes are also in that
 namespace. That is why it is *not* necessary to write:  
   <tal:datetime tal:define="..." />
 The second 'tal:' is redundant. 

 The next thing to understand: TAL (the Template ATTRIBUTE language) only
 pays attention to *attributes*. It ignores tags. Therefor the XXX in 
   <tal:XXX ...> 
 can be anything -- it will be ignored by the TAL parser. It's a good idea
 to make it something descriptive.

Anonymous User - Aug. 6, 2005 6:52 pm:
 test

Anonymous User - Aug. 12, 2005 5:02 pm:
 hahaha

inflatables - Oct. 14, 2005 11:42 pm:
 test
 ----------
 http://www.inflatables-china.net

TAL Overview

The Tag Attribute Language (TAL) standard is an attribute language used to create dynamic templates. It allows elements of a document to be replaced, repeated, or omitted.

The statements of TAL are XML attributes from the TAL namespace. These attributes can be applied to an XML or HTML document in order to make it act as a template.

A TAL statement has a name (the attribute name) and a body (the attribute value). For example, a content statement might look like tal:content="string:Hello". The element on which a statement is defined is its statement element. Most TAL statements require expressions, but the syntax and semantics of these expressions are not part of TAL. TALES is recommended for this purpose.

TAL Namespace

The TAL namespace URI and recommended alias are currently defined as:

xmlns:tal="http://xml.zope.org/namespaces/tal"

snell - Nov. 2, 2003 8:34 am:
 The first two paragraphs of The Zope Book seem to have been forgotten in the recent past.  They read: 
 "Preface

 Welcome to The Zope Book. This book is designed to introduce you to Zope, the open source web application
 server.
 To make effective use of the book, you should know how to use a web browser and you should have a basic
understanding of HTML (Hyper Text Markup Language) and URLs (Uniform Resource Locators). You don't need to be
 a highly-skilled programmer in order to use Zope, but some programming background (particularly
 object-oriented programming) will be extremely helpful. "
 This statement calls for a return to that philosophy. TAL syntax basics are not explained in this document;
 therefore, the new student - who has been directed to that fact that TAL avoids many problems of dtml and,
 therefore, wants to learn it - finds it inaccessible.
 Two months into Zope I discovered an archive document which explained TAL syntax structure and would offer
 that here as a pointer. However, I cannot find it a second time!

Anonymous User - Nov. 3, 2003 10:57 am:
 Here is what is missing:
 http://dev.zope.org/Wikis/DevSite/Projects/ZPT/AttributeLanguage
 AttributeLanguage
 Attribute Languages
 An attribute language is a programming language designed to mesh in a particular way with documents written
 using tag-structured markup, such as HTML and XML. The statements of the language are document tags with
 special attributes, and look like this:
     <p ns:command="argument">Some Text</p>

The attribute ns:command="argument" is the statement attribute, and the entire paragraph tag is the statement
 element, the portion of the document on which this statement operates. Each statement attribute has three
 parts: the namespace prefix, the name, and the argument. The prefix identifies the language, and must be
 introduced by an XML namespace declaration in XML and XHTML documents, like this:
     xmlns:ns="http://language.creator.com/namespaces/MyAttributeLanguage"

 HTML should include these declarations, but an implementation may allow you to omit them. The statement name
 determines which language statement to apply to the statement element, and the argument determines how it is
 applied. The name is always a simple identifier, but the argument can contain just about anything, depending
 on the statement.
 How to Specify an Attribute Language
 If you create an attribute language, you need to define the following:

 The URI of the XML namespace, and (optionally) a recommended alias. 
 The names, argument syntax, and meaning of the possible statements. 
 The order in which statements on the same element are processed, and restrictions on which statements can
 share the same element.

This is not a URL, but merely a unique identifier. Do not expect a browser to resolve it successfully.

Zope does not require an XML namespace declaration when creating templates with a content-type of text/html. However, it does require an XML namespace declaration for all other content-types.

TAL Statements

These are the tal statements:

• tal:attributes - dynamically change element attributes.
• tal:define - define variables.
• tal:condition - test conditions.
• tal:content - replace the content of an element.
• tal:omit-tag - remove an element, leaving the content of the element.
• tal:on-error - handle errors.
• tal:repeat - repeat an element.
• tal:replace - replace the content of an element and remove the element leaving the content.

Expressions used in statements may return values of any type, although most statements will only accept strings, or will convert values into a string representation. The expression language must define a value named nothing that is not a string. In particular, this value is useful for deleting elements or attributes.

Anonymous User - May 10, 2004 10:18 pm:
 deleting elements or attributes? when and where? this value refered to is nothing?

remiq - May 28, 2005 6:16 pm:
 delete atributes:
 tal:attributes - dynamically change element attributes.

 delete elements:
 tal:omit-tag - remove an element, leaving the content of the element.

Order of Operations

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, to do the same.

Any combination of statements may appear on the same elements, except that the content and replace statements may not appear together.

Anonymous User - Aug. 6, 2005 6:51 pm:
 test

Due to the fact that TAL sees statements as XML attributes, even in HTML documents, it cannot use the order in which statements are written in the tag to determine the order in which they are executed. TAL must also forbid multiples of the same kind of statement on a single element, so it is sufficient to arrange the kinds of statement in a precendence list.

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

Anonymous User - Nov. 24, 2003 11:31 am:
 Add:

 It may not be apparent that there needs to be an ordering.  The reason that there must
 be one is that TAL is XML based.  The XML specification specifically states that 
 XML processors are free to rewrite the terms.  In particular, you cannot assume that
 attributes of an XML statement will be processed in the order written, particularly if
 there is another preprocessor involved.  To avoid needless proliferation of tags, and
 still permit unambigous execution of complex TAL, a precedence order was chosen according
 to the following rationalle.

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 be able to replace various parts of an element with different values on each iteration of a repeat, so repeat is next. It makes no sense to replace attributes and then throw them away, so attributes is last. The remaining statements clash, because they each replace or edit the statement element.

See Also

TALES Overview

METAL Overview

tal:attributes

tal:define

tal:condition

tal:content

tal:omit-tag

tal:on-error

tal:repeat

tal:replace

attributes: Replace element attributes

Syntax

tal:attributes syntax:

argument             ::= attribute_statement [';' attribute_statement]*
attribute_statement  ::= attribute_name expression
attribute_name       ::= [namespace-prefix ':'] Name
namespace-prefix     ::= Name

*Note: If you want to include a semi-colon (;) in an expression, it must be escaped by doubling it (;;).*

Description

The tal:attributes statement replaces the value of an attribute (or creates an attribute) with a dynamic value. You can qualify an attribute name with a namespace prefix, for example html:table, if you are generating an XML document with multiple namespaces. The value of each expression is converted to a string, if necessary.

If the expression associated with an attribute assignment evaluates to nothing, then that attribute is deleted from the statement element. If the expression evaluates to default, then that attribute is left unchanged. Each attribute assignment is independent, so attributes may be assigned in the same statement in which some attributes are deleted and others are left alone.

Anonymous User - Dec. 19, 2003 12:58 pm:
 This seems to contradict the Order of Operations, above, where it says that attribution takes place after
 replacement. That way seems better than ignoring the attribution altogether.

Anonymous User - Jan. 16, 2004 8:41 am:
 Performing attribution after replacement wouldn't make any sense; tal:replace completely replaces the
 element, where would the attribute changes be made then?

If you use tal:attributes on an element with an active tal:replace command, the tal:attributes statement is ignored.

If you use tal:attributes on an element with a tal:repeat statement, the replacement is made on each repetition of the element, and the replacement expression is evaluated fresh for each repetition.

Examples

Replacing a link:

<a href="/sample/link.html"
 tal:attributes="href here/sub/absolute_url">

Anonymous User - June 4, 2004 9:34 am:
 path <code>here/sub/absolute_url</code> wont work for me (zope 2.7). 
 <code>here/absolute_url</code> works fine

Replacing two attributes:

<textarea rows="80" cols="20"
 tal:attributes="rows request/rows;cols request/cols">

condition: Conditionally insert or remove an element

Syntax

tal:condition syntax:

argument ::= expression

Description

The tal:condition statement includes the statement element in the template only if the condition is met, and omits it otherwise. If its expression evaluates to a true value, then normal processing of the element continues, otherwise the statement element is immediately removed from the template. For these purposes, the value nothing is false, and default has the same effect as returning a true value.

Note: Zope considers missing variables, None, zero, empty strings, and empty sequences false; all other values are true.

Examples

Test a variable before inserting it (the first example tests for existence and truth, while the second only tests for existence):

<p tal:condition="request/message | nothing"
 tal:content="request/message">message goes here</p>

<p tal:condition="exists:request/message"
 tal:content="request/message">message goes here</p>

Anonymous User - July 16, 2004 3:26 pm:
 In tal:condition="request/message | nothing", is the pipe used to explicitly define the default value, as
with traditional OR syntax, or is it just considered part of this particular conditional statement? Or is the
 default value defined as the content already stated between the tags? In this case, "message goes here"?

Test for alternate conditions:

<div tal:repeat="item python:range(10)">
<p tal:condition="repeat/item/even">Even</p>
<p tal:condition="repeat/item/odd">Odd</p>
</div>

content: Replace the content of an element

Syntax

tal:content syntax:

argument ::= (['text'] | 'structure') expression

Description

Rather than replacing an entire element, you can insert text or structure in place of its children with the tal:content statement. The statement argument is exactly like that of tal:replace, and is interpreted in the same fashion. If the expression evaluates to nothing, the statement element is left childless. If the expression evaluates to default, then the element's contents are unchanged.

The default replacement behavior is text, which replaces angle-brackets and ampersands with their HTML entity equivalents. The structure keyword passes the replacement text through unchanged, allowing HTML/XML markup to be inserted. This can break your page if the text contains unanticipated markup (eg. text submitted via a web form), which is the reason that it is not the default.

Examples

Inserting the user name:

<p tal:content="user/getUserName">Fred Farkas</p>

Anonymous User - Apr. 30, 2004 11:32 am:
Following this example, how could I write in TAL a condition that checks if the current user is, for example,
 "smith" ?

Anonymous User - Oct. 19, 2004 4:17 pm:
 try this:
 <p tal:condition="python:user.getUserName=='smith'"></p>

Inserting HTML/XML:

<p tal:content="structure here/getStory">marked <b>up</b>
content goes here.</p>

See Also

tal:replace

define: Define variables

Syntax

tal:define syntax:

argument       ::= define_scope [';' define_scope]*
define_scope   ::= (['local'] | 'global') define_var
define_var     ::= variable_name expression
variable_name  ::= Name

*Note: If you want to include a semi-colon (;) in an expression, it must be escaped by doubling it (;;).*

Description

The tal:define statement defines variables. You can define two different kinds of TAL variables: local and global. When you define a local variable in a statement element, you can only use that variable in that element and the elements it contains. If you redefine a local variable in a contained element, the new definition hides the outer element's definition within the inner element. When you define a global variables, you can use it in any element processed after the defining element. If you redefine a global variable, you replace its definition for the rest of the template.

Note: local variables are the default

If the expression associated with a variable evaluates to nothing, then that variable has the value nothing, and may be used as such in further expressions. Likewise, if the expression evaluates to default, then the variable has the value default, and may be used as such in further expressions.

Examples

Defining a global variable:

tal:define="global company_name string:Zope Corp, Inc."

Defining two variables, where the second depends on the first:

tal:define="mytitle template/title; tlen python:len(mytitle)"

omit-tag: Remove an element, leaving its contents

Syntax

tal:omit-tag syntax:

argument ::= [ expression ]

Description

The tal:omit-tag statement leaves the contents of an element in place while omitting the surrounding start and end tags.

If the expression evaluates to a false value, then normal processing of the element continues and the tags are not omitted. If the expression evaluates to a true value, or no expression is provided, the statement element is replaced with its contents.

Zope treats empty strings, empty sequences, zero, None, and nothing as false. All other values are considered true, including default.

Examples

Unconditionally omitting a tag:

<div tal:omit-tag="" comment="This tag will be removed">
  <i>...but this text will remain.</i>
</div>

Conditionally omitting a tag:

<b tal:omit-tag="not:bold">I may be bold.</b>

The above example will omit the b tag if the variable bold is false.

Creating ten paragraph tags, with no enclosing tag:

<tal:block repeat="n python:range(10)">
  <p tal:content="n">1</p>
</tal:block>

on-error: Handle errors

Syntax

tal:on-error syntax:

argument ::= (['text'] | 'structure') expression

Description

The tal:on-error statement provides error handling for your template. When a TAL statement produces an error, the TAL interpreter searches for a tal:on-error statement on the same element, then on the enclosing element, and so forth. The first tal:on-error found is invoked. It is treated as a tal:content statement.

A local variable

error

is set. This variable has these attributes:

type

the exception type

value

the exception instance

traceback

the traceback object

Anonymous User - Apr. 26, 2004 4:12 pm:
 Despite what the documentation claims, there does not seem to be a traceback object. A browse of the source
 code in TALDefs.py shows type, value, lineno and offset, but no traceback.

The simplest sort of tal:on-error statement has a literal error string or nothing for an expression. A more complex handler may call a script that examines the error and either emits error text or raises an exception to propagate the error outwards.

Examples

Simple error message:

<b tal:on-error="string: Username is not defined!" 
 tal:content="here/getUsername">Ishmael</b>

Removing elements with errors:

<b tal:on-error="nothing"
   tal:content="here/getUsername">Ishmael</b>

Calling an error-handling script:

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

Here's what the error-handling script might look like:

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

See Also

Python Tutorial: Errors and Exceptions

Python Built-in Exceptions

repeat: Repeat an element

Syntax

tal:repeat syntax:

argument      ::= variable_name expression
variable_name ::= Name

Description

The tal:repeat statement replicates a sub-tree of your document once for each item in a sequence. The expression should evaluate to a sequence. If the sequence is empty, then the statement element is deleted, otherwise it is repeated for each value in the sequence. If the expression is default, then the element is left unchanged, and no new variables are defined.

The variable_name is used to define a local variable and a repeat variable. For each repetition, the local variable is set to the current sequence element, and the repeat variable is set to an iteration object.

Repeat Variables

You use repeat variables to access information about the current repetition (such as the repeat index). The repeat variable has the same name as the local variable, but is only accessible through the built-in variable named repeat.

nvram - May 19, 2005 6:22 pm:
 This section desperately needs an example of how to use 'repeat' within a 'pyton:' clause (in my case I want
 a string with the repeat/item/index ordinal for a form input name).
All of my guesses have been wrong, and ~50 documentation searches in vain. It's gotta be simple, but it's not
 obvious.
 Something like:

 <table>
   <tr tal:repeat="item here/cart">
       <td tal:content="python:('%03d' % repeat.item.number) ">001</td>
       <td tal:content="item/description">Widget</td>
       <td tal:content="item/price">$1.50</td>
   </tr>
 </table>

Anonymous User - Oct. 28, 2005 10:15 am:
 This is the correct way of doing it:

 <table>
    <tr tal:repeat="item here/cart">
        <td tal:content="python:('%03d' % repeat['item'].number) ">001</td>
        <td tal:content="item/description">Widget</td>
        <td tal:content="item/price">$1.50</td>
    </tr>
  </table>

The following information is available from the repeat variable:

• 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.
• first - true for the first item in a group - see note below
• last - true for the last item in a group - see note below
• length - length of the sequence, which will be the total number of repetitions.
• letter - repetition number as a lower-case letter: "a" - "z", "aa" - "az", "ba" - "bz", ..., "za" - "zz", "aaa" - "aaz", and so forth.
• Letter - upper-case version of letter.
• roman - repetition number as a lower-case roman numeral: "i", "ii", "iii", "iv", "v", etc.
• Roman - upper-case version of roman.

You can access the contents of the 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".

With the exception of start, end, and index, all of the attributes of a repeat variable are methods. Thus, when you use a Python expression to access them, you must call them, as in "python:repeat['item'].length()".

Note that first and last are intended for use with sorted sequences. They try to divide the sequence into group of items with the same value. If you provide a path, then the value obtained by following that path from a sequence item is used for grouping, otherwise the value of the item is used. You can provide the path by passing it as a parameter, as in "python:repeat['item'].first(color)", or by appending it to the path from the repeat variable, as in "repeat/item/first/color".

Examples

Iterating over a sequence of strings::

<p tal:repeat="txt python:'one', 'two', 'three'">
   <span tal:replace="txt" />
</p>

Inserting a sequence of table rows, and using the repeat variable to number the rows:

<table>
  <tr tal:repeat="item here/cart">
      <td tal:content="repeat/item/number">1</td>
      <td tal:content="item/description">Widget</td>
      <td tal:content="item/price">$1.50</td>
  </tr>
</table>

Nested repeats:

<table border="1">
  <tr tal:repeat="row python:range(10)">
    <td tal:repeat="column python:range(10)">
      <span tal:define="x repeat/row/number; 
                        y repeat/column/number; 
                        z python:x*y"
            tal:replace="string:$x * $y = $z">1 * 1 = 1</span>
    </td>
  </tr>
</table>

Insert objects. Seperates groups of objects by meta-type by drawing a rule between them:

<div tal:repeat="object objects">
  <h2 tal:condition="repeat/object/first/meta_type"
    tal:content="object/meta_type">Meta Type</h2>
  <p tal:content="object/getId">Object ID</p>
  <hr tal:condition="repeat/object/last/meta_type" />
</div>

Note, the objects in the above example should already be sorted by meta-type.

replace: Replace an element

Syntax

tal:replace syntax:

argument ::= (['text'] | 'structure') expression

Description

The tal:replace statement replaces an element with dynamic content. It replaces the statement element with either text or a structure (unescaped markup). The body of the statement is an expression with an optional type prefix. The value of the expression is converted into an escaped string if you prefix the expression with text or omit the prefix, and is inserted unchanged if you prefix it with structure. Escaping consists of converting "&" to "&amp;", "<" to "&lt;", and ">" to "&gt;".

If the value is nothing, then the element is simply removed. If the value is default, then the element is left unchanged.

Examples

The two ways to insert the title of a template:

<span tal:replace="template/title">Title</span>
<span tal:replace="text template/title">Title</span>

Inserting HTML/XML:

<div tal:replace="structure table" />

Inserting nothing:

<div tal:replace="nothing">This element is a comment.</div>

See Also

tal:content

TALES Overview

The Template Attribute Language Expression Syntax (TALES) standard describes expressions that supply TAL and METAL with data. TALES is one possible expression syntax for these languages, but they are not bound to this definition. Similarly, TALES could be used in a context having nothing to do with TAL or METAL.

TALES expressions are described below with any delimiter or quote markup from higher language layers removed. Here is the basic definition of TALES syntax:

Expression  ::= [type_prefix ':'] String
type_prefix ::= Name

Here are some simple examples:

a/b/c
path:a/b/c
nothing
path:nothing
python: 1 + 2
string:Hello, ${user/getUserName}

The optional type prefix determines the semantics and syntax of the expression string that follows it. A given implementation of TALES can define any number of expression types, with whatever syntax you like. It also determines which expression type is indicated by omitting the prefix.

If you do not specify a prefix, Zope assumes that the expression is a path expression.

TALES Expression Types

These are the TALES expression types supported by Zope:

• path expressions - locate a value by its path.
• exists expressions - test whether a path is valid.
• nocall expressions - locate an object by its path.
• not expressions - negate an expression
• string expressions - format a string
• python expressions - execute a Python expression

Built-in Names

These are the names that always available to TALES expressions in Zope:

• nothing - special value used by to represent a non-value (e.g. void, None, Nil, NULL).
• default - special value used to specify that existing text should not be replaced. See the documentation for individual TAL statements for details on how they interpret default.
• options - the keyword arguments passed to the template. These are generally available when a template is called from Methods and Scripts, rather than from the web.
• repeat - the repeat variables; see the tal:repeat documentation.
• attrs - a dictionary containing the initial values of the attributes of the current statement tag.
• CONTEXTS - the list of standard names (this list). This can be used to access a built-in variable that has been hidden by a local or global variable with the same name.
• root - the system's top-most object: the Zope root folder.
• here - the object to which the template is being applied.
• container - The folder in which the template is located.
• template - the template itself.
• request - the publishing request object.
• user - the authenticated user object.
• modules - a collection through which Python modules and packages can be accessed. Only modules which are approved by the Zope security policy can be accessed.

Note the names root, here, container, template, request, user, and modules are optional names supported by Zope, but are not required by the TALES standard.

See Also

TAL Overview

METAL Overview

exists expressions

nocall expressions

not expressions

string expressions

path expressions

python expressions

TALES Exists expressions

Syntax

Exists expression syntax:

exists_expressions ::= 'exists:' path_expression

Description

Exists expressions test for the existence of paths. An exists expression returns true when the path expressions following it expression returns a value. It is false when the path expression cannot locate an object.

Examples

Testing for the existence of a form variable:

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

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

TALES Nocall expressions

Syntax

Nocall expression syntax:

nocall_expression ::= 'nocall:' path_expression

Description

Nocall expressions avoid rendering the results of a path expression.

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.

Examples

Using nocall to get the properties of a document:

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

Using nocall expressions on a functions:

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

This example defines a variable join which is bound to the string.join function.

TALES Not expressions

Syntax

Not expression syntax:

not_expression ::= 'not:' expression

Description

Not expression evaluate the expression string (recursively) as a full expression, and returns the boolean negation of its value. If the expression supplied does not evaluate to a boolean value, not will issue a warning and coerce the expression's value into a boolean type based on the following rules:

1. the number 0 is false
2. positive and negative numbers are true
3. an empty string or other sequence is false
4. a non-empty string or other sequence is true
5. a non-value (e.g. void, None, Nil, NULL, etc) is false
6. all other values are implementation-dependent.

If no expression string is supplied, an error should be generated.

Zope considers all objects not specifically listed above as false to be true.

Examples

Testing a sequence:

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

TALES Path expressions

Syntax

Path expression syntax:

PathExpr    ::= Path [ '|' Expression ]
Path        ::= variable [ '/' PathSegment ]*
variable    ::= Name
PathSegment ::= ( '?' variable ) | PathChar+
PathChar    ::= AlphaNumeric | ' ' | '_' | '-' | '.' | ',' | '~'

Description

A path expression consists of a path optionally followed by a vertical bar (|) and alternate expression. A path consists of one or more non-empty strings separated by slashes. The first string must be a variable name (a built-in variable or a user defined variable), and the remaining strings, the path segments, may contain letters, digits, spaces, and the punctuation characters underscore, dash, period, comma, and tilde.

A limited amount of indirection is possible by using a variable name prefixed with ? as a path segment. The variable must contain a string, which replaces that segment before the path is traversed.

For example:

request/cookies/oatmeal
nothing
here/some-file 2001_02.html.tar.gz/foo
root/to/branch | default

request/name | string:Anonymous Coward
here/?tname/macros/?mname

When a path expression is evaluated, Zope attempts to traverse the path, from left to right, until it succeeds or runs out of paths segments. To traverse a path, it first fetches the object stored in the variable. For each path segment, it traverses from the current object to the subobject named by the path segment. Subobjects are located according to standard Zope traversal rules (via getattr, getitem, or traversal hooks).

Once a path has been successfully traversed, the resulting object is the value of the expression. If it is a callable object, such as a method or template, it is called.

If a traversal step fails, and no alternate expression has been specified, an error results. Otherwise, the alternate expression is evaluated.

The alternate expression can be any TALES expression. For example, request/name | string:Anonymous Coward is a valid path expression. This is useful chiefly for providing default values, such as strings and numbers, which are not expressable as path expressions. Since the alternate expression can be a path expression, it is possible to "chain" path expressions, as in first | second | third | nothing.

If no path is given the result is nothing.

Since every path must start with a variable name, you need a set of starting variables that you can use to find other objects and values. See the TALES overview for a list of built-in variables. Variable names are looked up first in locals, then in globals, then in the built-in list, so the built-in variables act just like built-ins in Python; They are always available, but they can be shadowed by a global or local variable declaration. You can always access the built-in names explicitly by prefixing them with CONTEXTS. (e.g. CONTEXTS/root, CONTEXTS/nothing, etc).

Examples

Inserting a cookie variable or a property:

<span tal:replace="request/cookies/pref | here/pref">
  preference
</span>

Inserting the user name:

<p tal:content="user/getUserName">
  User name
</p>

TALES Python expressions

Syntax

Python expression syntax:

Any valid Python language expression

Description

Python expressions evaluate Python code in a security-restricted environment. Python expressions offer the same facilities as those available in Python-based Scripts and DTML variable expressions.

Security Restrictions

Python expressions are subject to the same security restrictions as Python-based scripts. These restrictions include:

access limits

Python expressions are subject to Zope permission and role security restrictions. In addition, expressions cannot access objects whose names begin with underscore.

write limits

Python expressions cannot change attributes of Zope objects.

Despite these limits malicious Python expressions can cause problems. See The Zope Book for more information.

Built-in Functions

Python expressions have the same built-ins as Python-based Scripts with a few additions.

These standard Python built-ins are available: None, abs, apply, callable, chr, cmp, complex, delattr, divmod, filter, float, getattr, hash, hex, int, isinstance, issubclass, list, len, long, map, max, min, oct, ord, repr, round, setattr, str, tuple.

The range and pow functions are available and work the same way they do in standard Python; however, they are limited to keep them from generating very large numbers and sequences. This limitation helps protect against denial of service attacks.

In addition, these utility functions are available: DateTime, test, and same_type. See DTML functions for more information on these functions.

Finally, these functions are available in Python expressions, but not in Python-based scripts:

holdenweb - Nov. 13, 2004 11:08 am:
 Is there some way to stop the HTML escaping of path: values so this can be used as a substitution mechanism,
 or is there some other better mechanism to use?

path(string)

Evaluate a TALES path expression.

string(string)

Evaluate a TALES string expression.

exists(string)

Evaluates a TALES exists expression.

nocall(string)

Evaluates a TALES nocall expression.

Python Modules

A number of Python modules are available by default. You can make more modules available. You can access modules either via path expressions (for example modules/string/join) or in Python with the modules mapping object (for example modules["string"].join). Here are the default modules:

woller - Jan. 24, 2004 5:38 pm:
 It is not clear at all where to go to "see ZTUtils" for instance. 
 Or any of the other references.... 
 There should really be some links here!

string

The standard Python string module. Note: most of the functions in the module are also available as methods on string objects.

random

The standard Python random module.

math

The standard Python math module.

sequence

A module with a powerful sorting function. See sequence for more information.

Products.PythonScripts.standard

Various HTML formatting functions available in DTML. See Products.PythonScripts.standard for more information.

ZTUtils

Batch processing facilities similar to those offered by dtml-in. See ZTUtils for more information.

AccessControl

Security and access checking facilities. See AccessControl for more information.

Examples

Using a module usage (pick a random choice from a list):

<span tal:replace="python:modules['random'].choice(['one', 
                     'two', 'three', 'four', 'five'])">
  a random number between one and five
</span>

Anonymous User - May 28, 2004 4:26 pm:
 When do you use that syntax instead of an earlier example that used a path ("python:modules/string/join") to
 get at a function? Is it just author preference which to use?

String processing (capitalize the user name):

<p tal:content="python:user.getUserName().capitalize()">
  User Name
</p>

Basic math (convert an image size to megabytes):

<p tal:content="python:image.getSize() / 1048576.0">
  12.2323
</p>

String formatting (format a float to two decimal places):

<p tal:content="python:'%0.2f' % size">
  13.56
</p>

Anonymous User - Mar. 24, 2004 12:17 am:
 How create 12345 to 12,345.00

Anonymous User - May 28, 2004 3:08 pm:
 One way:

 <p tal:define="ps python:modules['Products.PythonScripts.standard'];
                varA python:12345;
                varB python:'%0.2f' % ( varA );
                varC python:ps.thousands_commas( varB )"
    tal:content="string:$$$varC">
     Will print $12,345.00
 </p>

TALES String expressions

Syntax

String expression syntax:

string_expression ::= ( plain_string | [ varsub ] )*
varsub            ::= ( '$' Path ) | ( '${' Path '}' )
plain_string      ::= ( '$$' | non_dollar )*
non_dollar        ::= any character except '$'

Description

String expressions interpret the expression string as text. If no expression string is supplied the resulting string is empty. The string can contain variable substitutions of the form $name or ${path}, where name is a variable name, and path is a path expression. The escaped string value of the path expression is inserted into the string. To prevent a $ from being interpreted this way, it must be escaped as $$.

Examples

Basic string formatting:

<span tal:replace="string:$this and $that">
  Spam and Eggs
</span>

Using paths:

<p tal:content="string:total: ${request/form/total}">
  total: 12
</p>

Including a dollar sign:

<p tal:content="string:cost: $$$cost">
  cost: $42.00
</p>

METAL Overview

The Macro Expansion Template Attribute Language (METAL) standard is a facility for HTML/XML macro preprocessing. It can be used in conjunction with or independently of TAL and TALES.

Macros provide a way to define a chunk of presentation in one template, and share it in others, so that changes to the macro are immediately reflected in all of the places that share it. Additionally, macros are always fully expanded, even in a template's source text, so that the template appears very similar to its final rendering.

Anonymous User - Nov. 17, 2003 4:25 am:
 NOT MENTIONED:
 A single Page Template can accomodate multiple macros.
The method employed to mark the beginning and, above all, the end of one macro is not discussed here.
Therefore,
 a second macro can accidently be called inadvertently when calling the first one.

METAL Namespace

The METAL namespace URI and recommended alias are currently defined as:

xmlns:metal="http://xml.zope.org/namespaces/metal"

Just like the TAL namespace URI, this URI is not attached to a web page; it's just a unique identifier.

Zope does not require an XML namespace declaration when creating templates with a content-type of text/html. However, it does require an XML namespace declaration for all other content-types.

METAL Statements

METAL defines a number of statements:

• metal:define-macro - Define a macro.
• metal:use-macro - Use a macro.
• metal:define-slot - Define a macro customization point.
• metal:fill-slot - Customize a macro.

Although METAL does not define the syntax of expression non-terminals, leaving that up to the implementation, a canonical expression syntax for use in METAL arguments is described in TALES Specification.

See Also

TAL Overview

TALES Overview

metal:define-macro

metal:use-macro

metal:define-slot

metal:fill-slot

define-macro: Define a macro

Syntax

metal:define-macro syntax:

argument ::= Name

Description

The metal:define-macro statement defines a macro. The macro is named by the statement expression, and is defined as the element and its sub-tree.

Anonymous User - Nov. 17, 2003 4:27 am:
 NOT MENTIONED:
 Sub-tree, where does it end?

In Zope, a macro definition is available as a sub-object of a template's macros object. For example, to access a macro named header in a template named master.html, you could use the path expression master.html/macros/header.

Examples

Simple macro definition:

<p metal:define-macro="copyright">
  Copyright 2001, <em>Foobar</em> Inc.
</p>

See Also

metal:use-macro

metal:define-slot

define-slot: Define a macro customization point

Syntax

metal:define-slot syntax:

argument ::= Name

Description

The metal:define-slot statement defines a macro customization point or slot. When a macro is used, its slots can be replaced, in order to customize the macro. Slot definitions provide default content for the slot. You will get the default slot contents if you decide not to customize the macro when using it.

The metal:define-slot statement must be used inside a metal:define-macro statement.

Anonymous User - Nov. 17, 2003 4:30 am:
 NOT MENTIONED:
 "inside" a metal:define-macro implies that the macro has a beginning and an end. Inside is then before the
 end. How does one mark the end of a macro?

Anonymous User - Dec. 8, 2003 4:01 pm:
 by the closing tag

Slot names must be unique within a macro.

Examples

Simple macro with slot:

<p metal:define-macro="hello">
  Hello <b metal:define-slot="name">World</b>
</p>

This example defines a macro with one slot named name. When you use this macro you can customize the b element by filling the name slot.

See Also

metal:fill-slot

fill-slot: Customize a macro

Syntax

metal:fill-slot syntax:

argument ::= Name

Description

The metal:fill-slot statement customizes a macro by replacing a slot in the macro with the statement element (and its content).

The metal:fill-slot statement must be used inside a metal:use-macro statement.

Slot names must be unique within a macro.

If the named slot does not exist within the macro, the slot contents will be silently dropped.

Examples

Given this macro:

<p metal:define-macro="hello">
  Hello <b metal:define-slot="name">World</b>
</p>

Anonymous User - Oct. 17, 2004 1:45 am:
 One looks like an example of usage in an HTML template and the other is an example of usage in an xml
 template.

Anonymous User - June 14, 2004 3:41 pm:
the tag <metal:headslot ...> ... </metal:headslot> is an XML / XHTML compliant tag entity, but
will not be
 rendered by most browsers ('metal:headslot' is not a standard HTML tag).
 Regardless, the TAL parser will parse the tag for any logic it contains...
 By putting the metal: at the beginning, the tag's attribute scope falls under the METAL namespace; this
 essentially means that any metal attributes within the opening tag don't need to have 'metal:' attached to
 them
 (example: <metal:some-block define-slot="..."> ... </metal:some-block> 
 rather than 
 <metal:some-other-block metal:define-slot="..."> ... </metal:some-block>)

Anonymous User - Apr. 8, 2004 4:52 pm:
 Why do some define-slots look like:

 <p metal:define-slot="head_slot" />

 while others look like:

 <metal:headslot define-slot="head_slot" />

  what is that "headslot" thingie? It's not the same as "head_slot"...

You can fill the name slot like so:

<p metal:use-macro="container/master.html/macros/hello">
  Hello <b metal:fill-slot="name">Kevin Bacon</b>
</p>

whijo - Mar. 24, 2005 12:15 am:
 I find that I stil can't get to grips with constructs like this:
 <metal:cssslot fill-slot="css_slot">
   <metal:cssslot define-slot="css_slot" />
 </metal:cssslot>

 I understand it to somehow cater for a default fill which might be overwritten by another template...but I'm
 not confident with that one! Unfortunately the only reference I've found to it, in the so-called "Definitive
 Guide to Plone" fails to get the point through (to me at least). Is this actually a way to allow target
 templates to add their own css to those that may already be provided via the containing template?

See Also

metal:define-slot

use-macro: Use a macro

Syntax

metal:use-macro syntax:

argument ::= expression

Description

The metal:use-macro statement replaces the statement element with a macro. The statement expression describes a macro definition.

In Zope the expression will generally be a path expression referring to a macro defined in another template. See "metal:define-macro" for more information.

The effect of expanding a macro is to graft a subtree from another document (or from elsewhere in the current document) in place of the statement element, replacing the existing sub-tree. Parts of the original subtree may remain, grafted onto the new subtree, if the macro has slots. See metal:define-slot for more information. If the macro body uses any macros, they are expanded first.

When a macro is expanded, its metal:define-macro attribute is replaced with the metal:use-macro attribute from the statement element. This makes the root of the expanded macro a valid use-macro statement element.

Examples

Basic macro usage:

<p metal:use-macro="container/other.html/macros/header">
  header macro from defined in other.html template
</p>

This example refers to the header macro defined in the other.html template which is in the same folder as the current template. When the macro is expanded, the p element and its contents will be replaced by the macro. Note: there will still be a metal:use-macro attribute on the replacement element.

Anonymous User - July 14, 2004 1:31 pm:
 Is there any way to pass arguments to a macro explicitly instead of relying on the implicit attributes from
 the context from which the macro is called?

See Also

metal:define-macro

metal:fill-slot

ZPT-specific Behaviors

The behavior of Zope Page Templates is almost completely described by the TAL, TALES, and METAL specifications. ZPTs do, however, have a few additional features that are not described in the standards.

HTML Support Features

When the content-type of a Page Template is set to text/html, Zope processes the template somewhat differently than with any other content-type. As mentioned under TAL Namespace, HTML documents are not required to declare namespaces, and are provided with tal and metal namespaces by default.

HTML documents are parsed using a non-XML parser that is somewhat more forgiving of malformed markup. In particular, elements that are often written without closing tags, such as paragraphs and list items, are not treated as errors when written that way, unless they are statement elements. This laxity can cause a confusing error in at least one case; A <div> element is block-level, and therefore technically not allowed to be nested in a <p> element, so it will cause the paragraph to be implicity closed. The closing </p> tag will then cause a NestingError, since it is not matched up with the opening tag. The solution is to use <span> instead.

Unclosed statement elements are always treated as errors, so as not to cause subtle errors by trying to infer where the element ends. Elements which normally do not have closing tags in HTML, such as image and input elements, are not required to have a closing tag, or to use the XHTML <tag /> form.

Certain boolean attributes, such as checked and selected, are treated differently by tal:attributes. The value is treated as true or false (as defined by tal:condition). The attribute is set to attr="attr" in the true case and omitted otherwise. If the value is default, then it is treated as true if the attribute already exists, and false if it does not. For example, each of the following lines:

<input type="checkbox" checked tal:attributes="checked default">
<input type="checkbox" tal:attributes="checked string:yes">
<input type="checkbox" tal:attributes="checked python:42">

...will render as:

<input type="checkbox" checked="checked">

...while each of these:

<input type="checkbox" tal:attributes="checked default">
<input type="checkbox" tal:attributes="checked string:">
<input type="checkbox" tal:attributes="checked nothing">

...will render as:

<input type="checkbox">

nederhoed - May 12, 2005 4:26 am:
To create a <select> tag in a zpt page that reselects the selected value on submit, you can use the
following
 code:
 <select size="1" name="person">
 <option 
    value="tammo"
    tal:attributes="selected python:request.get('person')=='tammo'">Tammo</option>
 <option 
    value="anita" 
    tal:attributes="selected python:request.get('person')=='anita'">Anita</option>
 </select>

This works correctly in all browsers in which it has been tested.