27. 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: Template 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.

27.1. TAL Overview

The Template 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, an 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.

27.1.1. TAL Namespace

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

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

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.

27.1.2. 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.

27.1.3. 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.

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 precedence 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.

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 unambiguous execution of complex TAL, a precedence order was chosen according to the following rationale.

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.

27.2. attributes: Replace element attributes

27.2.1. 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 (;;).

27.2.2. 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.

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.

27.2.3. Examples

Replacing a link:

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

Replacing two attributes:

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

27.3. condition: Conditionally insert or remove an element

27.3.1. Syntax

tal:condition syntax:

argument ::= expression

27.3.2. 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.

27.3.3. 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>

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>

27.4. content: Replace the content of an element

27.4.1. Syntax

tal:content syntax:

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

27.4.2. 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 (e.g.. text submitted via a web form), which is the reason that it is not the default.

27.4.3. Examples

Inserting the user name:

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

Inserting HTML/XML:

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

27.5. define: Define variables

27.5.1. 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 (;;).

27.5.2. 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.

27.5.3. 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)"

27.6. omit-tag: Remove an element, leaving its contents

27.6.1. Syntax

tal:omit-tag syntax:

argument ::= [ expression ]

27.6.2. 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.

27.6.3. 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:

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

27.7. on-error: Handle errors

27.7.1. Syntax

tal:on-error syntax:

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

27.7.2. 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

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.

27.7.3. Examples

Simple error message:

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

Removing elements with errors:

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

Calling an error-handling script:

<div tal:on-error="structure context/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)

27.8. repeat: Repeat an element

27.8.1. Syntax

tal:repeat syntax:

argument      ::= variable_name expression
variable_name ::= Name

27.8.2. 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.

27.8.3. 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.

27.8.3.1. 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”.

27.8.4. 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 context/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. Separate 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.

27.9. replace: Replace an element

27.9.1. Syntax

tal:replace syntax:

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

27.9.2. 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 “&”, “<” to “<”, and “>” to “>”.

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

27.9.3. 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>

27.10. 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.

27.10.1. 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

27.10.2. Built-in Names

These are the names 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.
• context- - 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, context, container, template, request, user, and modules are optional names supported by Zope, but are not required by the TALES standard.

27.11. TALES Exists expressions

27.11.1. Syntax

Exists expression syntax:

exists_expressions ::= 'exists:' path_expression

27.11.2. 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.

27.11.3. 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.

27.12. TALES Nocall expressions

27.12.1. Syntax

Nocall expression syntax:

nocall_expression ::= 'nocall:' path_expression

27.12.2. 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.

27.12.3. Examples

Using nocall to get the properties of a document:

<span tal:define="doc nocall:context/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.

27.13. TALES Not expressions

27.13.1. Syntax

Not expression syntax:

not_expression ::= 'not:' expression

27.13.2. Description

Not expression evaluates 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.

27.13.3. Examples

Testing a sequence:

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

27.14. TALES Path expressions

27.14.1. Syntax

Path expression syntax:

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

27.14.2. 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
context/some-file 2009_02.html.tar.gz/foo
root/to/branch | default
request/name | string:Anonymous Coward
context/?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 sub-object named by the path segment. Sub-objects 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 expressible 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).

27.14.3. Examples

Inserting a cookie variable or a property:

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

Inserting the user name:

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

27.15. TALES Python expressions

27.15.1. Syntax

Python expression syntax:

Any valid Python language expression

27.15.2. 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.

27.15.2.1. 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.

27.15.2.2. 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 to avoid accidental long execution times.

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

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.

27.15.2.3. 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:

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.

27.15.3. 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>

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>

27.16. TALES String expressions

27.16.1. Syntax

String expression syntax:

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

27.16.2. 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 $$.

27.16.3. 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>

27.17. 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

27.17.1. 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.

27.17.2. 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.

27.18. define-macro: Define a macro

27.18.1. Syntax

metal:define-macro syntax:

argument ::= Name

27.18.2. 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.

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

27.18.3. Examples

Simple macro definition:

<p metal:define-macro="copyright">
  Copyright 2009, <em>Foobar</em> Inc.
</p>

27.19. define-slot: Define a macro customization point

27.19.1. Syntax

metal:define-slot syntax:

argument ::= Name

27.19.2. 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.

Slot names must be unique within a macro.

27.19.3. 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.

27.20. fill-slot: Customize a macro

27.20.1. Syntax

metal:fill-slot syntax:

argument ::= Name

27.20.2. 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.

27.20.3. Examples

Given this macro:

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

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>

27.21. use-macro: Use a macro

27.21.1. Syntax

metal:use-macro syntax:

argument ::= expression

27.21.2. 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.

27.21.3. 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.

27.22. 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.

27.22.1. 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 implicitly 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">

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