Writing Configuration Schema¶
Configurations which use ZConfig
are described using
“schema”. A schema is a specification for the
allowed structure and content of the configuration. ZConfig
schema are written using a small XML-based language. The schema
language allows the schema author to specify the names of the keys
allowed at the top level and within sections, to define the types of
sections which may be used (and where), the types of each values,
whether a key or section must be specified or is optional, default
values for keys, and whether a value can be given only once or
repeatedly.
Writing Configuration Schema¶
Data types are searched in a special namespace defined by the data
type registry. The default registry has slightly magical semantics:
If the value can be matched to a standard data type when interpreted
as a basic-key, the standard data type will be used. If
that fails, the value must be a dotted-name containing at
least one dot, and a conversion function will be sought using the
search()
method of the data type registry used to load the
schema.
Schema Elements¶
For each element, the content model is shown, followed by a
description of how the element is used, and then a list of the
available attributes. For each attribute, the type of the value is
given as either the name of a ZConfig
datatype or an XML
attribute value type. Familiarity with XML’s Document Type Definition
language is helpful.
The following elements are used to describe a schema:
<schema>
description?, metadefault?, example?, import*, (sectiontype |
abstracttype)*, (section | key | multisection | multikey)*
</schema>
Document element for a ZConfig
schema.
- extends (space-separated-url-references)
- A list of URLs of base schemas from which this section type will inherit
key, section, and section type declarations. If omitted, this schema is
defined using only the keys, sections, and section types contained within
the
schema
element. - datatype (basic-key or dotted-name)
- The data type converter which will be applied to the value of this
section. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set. If any base schemas are listed in theextends
attribute, the default value for this attribute comes from the base schemas. If the base schemas all use the samedatatype
, then that data type will be the default value for the extending schema. If there are no base schemas, the default value is null , which means that theZConfig
section object will be used unconverted. If the base schemas have differentdatatype
definitions, you must explicitly define thedatatype
in the extending schema.
handler (basic-key)
- keytype (basic-key or dotted-name)
- The data type converter which will be applied to keys found in
this section. This can be used to constrain key values in
different ways; two data types which may be especially useful are
the identifier and ipaddr-or-hostname
types. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set. If any base schemas are listed in theextends
attribute, the default value for this attribute comes from the base schemas. If the base schemas all use the samekeytype
, then that key type will be the default value for the extending schema. If there are no base schemas, the default value is basic-key . If the base schemas have differentkeytype
definitions, you must explicitly define thekeytype
in the extending schema. - prefix (dotted-name)
- Prefix to be pre-pended in front of partial dotted-names that
start with a period. The value of this attribute is used in all
contexts with the
schema
element if it hasn’t been overridden by an inner element with aprefix
attribute.
<description>
PCDATA
</description>
Descriptive text explaining the purpose the container of the
description
element. Most other elements can contain
a description
element as their first child.
At most one description
element may appear in a given
context.
- format (NMTOKEN)
- Optional attribute that can be added to indicate what conventions
are used to mark up the contained text. This is intended to serve as a hint for documentation extraction tools. Suggested values are:
Value Content Format plain
text/plain
; blank lines separate paragraphsrest
reStructuredText stx
Classic Structured Text
<example>
PCDATA
</example>
An example value. This serves only as documentation.
<metadefault>
PCDATA
</metadefault>
A description of the default value, for human readers. This may include information about how a computed value is determined when the schema does not specify a default value.
<abstracttype>
description?
</abstracttype>
Define an abstract section type.
- name (basic-key)
- The name of the abstract section type; required.
<sectiontype>
description?, example?, (section | key | multisection | multikey)*
</sectiontype>
Define a concrete section type.
- datatype (basic-key or dotted-name)
- The data type converter which will be applied to the value of this
section. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set. Ifdatatype
is omitted andextends
is used, thedatatype
from the section type identified by theextends
attribute is used. - extends (basic-key)
- The name of a concrete section type from which this section type
acquires all key and section declarations. This type does
not automatically implement any abstract section type
implemented by the named section type. If omitted, this section
is defined with only the keys and sections contained within the
sectiontype
element. The new section type is called a derived section type, and the type named by this attribute is called the base type. Values for thedatatype
andkeytype
attributes are acquired from the base type if not specified. - implements (basic-key)
- The name of an abstract section type which this concrete section type implements. If omitted, this section type does not implement any abstract type, and can only be used if it is specified directly in a schema or other section type.
- keytype (basic-key)
- The data type converter which will be applied to keys found in
this section. This can be used to constrain key values in
different ways; two data types which may be especially useful are
the identifier and ipaddr-or-hostname
types. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set. The default value is basic-key . Ifkeytype
is omitted andextends
is used, thekeytype
from the section type identified by theextends
attribute is used. - name (basic-key)
- The name of the section type; required.
- prefix (dotted-name)
- Prefix to be pre-pended in front of partial dotted-names that
start with a period. The value of this attribute is used in all
contexts in the
sectiontype
element. If omitted, the prefix specified by a containing context is used if specified.
<import>
EMPTY
</import>
Import a schema component. Exactly one of the attributes
package
and src
must be specified.
- file (file name without directory information}
- Name of the component file within a package; if not specified,
‘component.xml’ is used. This may only be given when
package
is used. (The ‘component.xml’ file is always used when importing via%import
from a configuration file.) - package (dotted-suffix)
- Name of a Python package that contains the schema component being
imported. The component will be loaded from the file identified
by the
file
attribute, or ‘component.xml’ iffile
is not specified. If the package name given starts with a dot (.
), the name used will be the current prefix and the value of this attribute concatenated. - src (url-reference)
- URL to a separate schema which can provide useful types. The
referenced resource must contain a schema, not a schema
component. Section types defined or imported by the referenced
schema are added to the schema containing the
import
; top-level keys and sections are ignored.
<key>
description?, example?, metadefault?, default*
</key>
A key
element is used to describe a key-value pair which
may occur at most once in the section type or top-level schema in
which it is listed.
- attribute (identifier)
- The name of the Python attribute which this key should be the
value of on a
SectionValue
instance. This must be unique within the immediate contents of a section type or schema. If this attribute is not specified, an attribute name will be computed by converting hyphens in the key name to underscores. - datatype (basic-key or dotted-name)
- The data type converter which will be applied to the value of this
key. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set. - default (string)
- If the key-value pair is optional and this attribute is specified,
the value of this attribute will be converted using the appropriate
data type converter and returned to the application as the
configured value. This attribute may not be specified if the
required
attribute isyes
.
handler (basic-key)
- name (basic-key)
- The name of the key, as it must be given in a configuration
instance, or
*
. If the value is*
, any name not already specified as a key may be used, and the configuration value for the key will be a dictionary mapping from the key name to the value. In this case, theattribute
attribute must be specified, and the data type for the key will be applied to each key which is found. - required (
yes|no
) - Specifies whether the configuration instance is required to
provide the key. If the value is
yes
, thedefault
attribute may not be specified and an error will be reported if the configuration instance does not specify a value for the key. If the value isno
(the default) and the configuration instance does not specify a value, the value reported to the application will be that specified by thedefault
attribute, if given, orNone
.
<multikey>
description?, example?, metadefault?, default*
</multikey>
A multikey
element is used to describe a key-value pair
which may occur any number of times in the section type or top-level
schema in which it is listed.
- attribute (identifier)
- The name of the Python attribute which this key should be the value of on a :class`SectionValue` instance. This must be unique within the immediate contents of a section type or schema. If this attribute is not specified, an attribute name will be computed by converting hyphens in the key name to underscores.
- datatype (basic-key or dotted-name)
- The data type converter which will be applied to the value of this
key. If the value is a dotted-name that begins
with a period, the value of
prefix
will be pre-pended, if set.
handler (basic-key)
- name (basic-key)
- The name of the key, as it must be given in a configuration
instance, or
+
. If the value is+
, any name not already specified as a key may be used, and the configuration value for the key will be a dictionary mapping from the key name to the value. In this case, theattribute
attribute must be specified, and the data type for the key will be applied to each key which is found. - required (
yes|no
) - Specifies whether the configuration instance is required to
provide the key. If the value is
yes
, nodefault
elements may be specified and an error will be reported if the configuration instance does not specify at least one value for the key. If the value isno
(the default) and the configuration instance does not specify a value, the value reported to the application will be a list containing one element for eachdefault
element specified as a child of themultikey
. Each value will be individually converted according to thedatatype
attribute.
<default>
PCDATA
</default>
Each default
element specifies a single default value for
a multikey
. This element can be repeated to produce a
list of individual default values. The text contained in the
element will be passed to the datatype conversion for the
multikey
.
- key (key type of the containing sectiontype}
- Key to associate with the default value. This is only used for
defaults of a
key
ormultikey
with aname
of+
; in that case this attribute is required. It is an error to use thekey
attribute with adefault
element for amultikey
with a name other than+
.
Warning
The datatype of this attribute is that of the section type containing the actual keys, not necessarily that of the section type which defines the key. If a derived section overrides the key type of the base section type, the actual key type used is that of the derived section.
This can lead to confusing errors in schemas, though the
ZConfig
package checks for this when the schema is
loaded. This situation is particularly likely when a derived
section type uses a key type which collapses multiple default
keys which were not collapsed by the base section type.
Consider this example schema:
<schema>
<sectiontype name="base" keytype="identifier">
<key name="+" attribute="mapping">
<default key="foo">some value</default>
<default key="FOO">some value</default>
</key>
</sectiontype>
<sectiontype name="derived" keytype="basic-key"
extends="base"/>
<section type="derived" name="*" attribute="section"/>
</schema>
When this schema is loaded, a set of defaults for the
derived section type is computed. Since
basic-key is case-insensitive (everything is
converted to lower case), foo
and Foo
are both
converted to foo
, which clashes since key
only
allows one value for each key.
<section>
description?, example?
</section>
A section
element is used to describe a section which may
occur at most once in the section type or top-level schema in which
it is listed.
- attribute (identifier)
- The name of the Python attribute which this section should be the
value of on a
SectionValue
instance. This must be unique within the immediate contents of a section type or schema. If this attribute is not specified, an attribute name will be computed by converting hyphens in the section name to underscores, in which case thename
attribute may not be*
or+
.
handler (basic-key)
- name (basic-key)
- The name of the section, as it must be given in a configuration
instance,
*
, or+
. If the value is*
or this attribute is omitted, any name not already specified as a key may be used. If the value is*
or+
, theattribute
attribute must be specified. If the value is*
, any name is allowed, or the name may be omitted. If the value is+
, any name is allowed, but some name must be provided. - required (
yes|no
) - Specifies whether the configuration instance is required to
provide the section. If the value is
yes
, an error will be reported if the configuration instance does not include the section. If the value isno
(the default) and the configuration instance does not include the section, the value reported to the application will beNone
. - type (basic-key)
- The section type which matching sections must implement. If the value names an abstract section type, matching sections in the configuration file must be of a type which specifies that it implements the named abstract type. If the name identifies a concrete type, the section type must match exactly.
<multisection>
description?, example?
</multisection>
A multisection
element is used to describe a section which
may occur any number of times in the section type or top-level
schema in which it is listed.
- attribute (identifier)
- The name of the Python attribute which matching sections should be
the value of on a
SectionValue
instance. This is required and must be unique within the immediate contents of a section type or schema. TheSectionValue
instance will contain a list of matching sections.
handler (basic-key)
- name (basic-key)
- For a
multisection
, any name not already specified as a key may be used. If the value is*
or+
, theattribute
attribute must be specified. If the value is*
or this attribute is omitted, any name is allowed, or the name may be omitted. If the value is+
, any name is allowed, but some name must be provided. No other value for thename
attribute is allowed for amultisection
. - required (
yes|no
) - Specifies whether the configuration instance is required to
provide at least one matching section. If the value is
yes
, an error will be reported if the configuration instance does not include the section. If the value isno
(the default) and the configuration instance does not include the section, the value reported to the application will beNone
. - type (basic-key)
- The section type which matching sections must implement. If the value names an abstract section type, matching sections in the configuration file must be of types which specify that they implement the named abstract type. If the name identifies a concrete type, the section type must match exactly.
Schema Components¶
ZConfig
supports schema components that can be
provided by disparate components, and allows them to be knit together
into concrete schema for applications. Components cannot add
additional keys or sections in the application schema.
A schema component is allowed to define new abstract and section
types. Components are identified using a dotted-name, similar to a
Python module name. For example, one component may be
zodb.storage
.
Schema components are stored alongside application code since they
directly reference datatype code. Schema components are provided by
Python packages. The component definition is normally stored in the
file ‘component.xml’; an alternate filename may be specified
using the file
attribute of the import
element.
Components imported using the %import
keyword from a
configuration file must be named ‘component.xml’.
The component defines the types provided by that component; it must
have a component
element as the document element.
The following element is used as the document element for schema components. Note that schema components do not allow keys and sections to be added to the top-level of a schema; they serve only to provide type definitions.
<component>
description?, (abstracttype | sectiontype)*
</component>
The top-level element for schema components.
- prefix (dotted-name)
- Prefix to be pre-pended in front of partial dotted-names that
start with a period. The value of this attribute is used in all
contexts within the
component
element if it hasn’t been overridden by an inner element with aprefix
attribute.
Referring to Files in Packages¶
The extends
attribute of the schema
element is
used to refer to files containing base schema; sometimes it makes
sense to refer to a base schema relative to the Python package that
provides it. For this purpose, ZConfig supports the special
package:
URL scheme.
The package:
URL scheme is straightforward, and contains three
parts: the scheme name, the package name, and a relative path. The
relative path is searched for using the named package’s
__path__
if it’s a conventional filesystem package, or using
the package’s loader if that supports resource access (such as the
loader for eggs and other ZIP-file based packages).
The basic form of the package:
URL is:
package:package.name:relative-path
The package name must be fully specified; the current prefix, if any, is not used. If the named package is contained in an egg or ZIP file, the resource identified by the relative path must reside in the same egg or ZIP file.
The package:
URL scheme is generally available everywhere
ZConfig supports loading text from URLs directly, but applications
using ZConfig do not automatically acquire general support for this.
Schema Document Type Definition¶
The following is the XML Document Type Definition for ZConfig
schema:
<!--
*************************************************************************
Copyright (c) 2002, 2003 Zope Foundation and Contributors.
All Rights Reserved.
This software is subject to the provisions of the Zope Public License,
Version 2.1 (ZPL). A copy of the ZPL should accompany this distribution.
THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
FOR A PARTICULAR PURPOSE.
*************************************************************************
Please note that not all documents that conform to this DTD are
legal ZConfig schema. The ZConfig reference manual describes many
constraints that are important to understanding ZConfig schema.
-->
<!-- DTD for ZConfig schema documents. -->
<!ELEMENT schema (description?, metadefault?, example?,
import*,
(sectiontype | abstracttype)*,
(section | key | multisection | multikey)*)>
<!ATTLIST schema
extends NMTOKEN #IMPLIED
prefix NMTOKEN #IMPLIED
handler NMTOKEN #IMPLIED
keytype NMTOKEN #IMPLIED
datatype NMTOKEN #IMPLIED>
<!ELEMENT component (description?, (sectiontype | abstracttype)*)>
<!ATTLIST component
prefix NMTOKEN #IMPLIED>
<!ELEMENT import EMPTY>
<!ATTLIST import
file CDATA #IMPLIED
package NMTOKEN #IMPLIED
src CDATA #IMPLIED>
<!ELEMENT description (#PCDATA)*>
<!ATTLIST description
format NMTOKEN #IMPLIED>
<!ELEMENT metadefault (#PCDATA)*>
<!ELEMENT example (#PCDATA)*>
<!ELEMENT sectiontype (description?, example?
(section | key | multisection | multikey)*)>
<!ATTLIST sectiontype
name NMTOKEN #REQUIRED
prefix NMTOKEN #IMPLIED
keytype NMTOKEN #IMPLIED
datatype NMTOKEN #IMPLIED
implements NMTOKEN #IMPLIED
extends NMTOKEN #IMPLIED>
<!ELEMENT abstracttype (description?)>
<!ATTLIST abstracttype
name NMTOKEN #REQUIRED
prefix NMTOKEN #IMPLIED>
<!ELEMENT default (#PCDATA)*>
<!ATTLIST default
key CDATA #IMPLIED>
<!ELEMENT key (description?, metadefault?, example?, default*)>
<!ATTLIST key
name CDATA #REQUIRED
attribute NMTOKEN #IMPLIED
datatype NMTOKEN #IMPLIED
handler NMTOKEN #IMPLIED
required (yes|no) "no"
default CDATA #IMPLIED>
<!ELEMENT multikey (description?, metadefault?, example?, default*)>
<!ATTLIST multikey
name CDATA #REQUIRED
attribute NMTOKEN #IMPLIED
datatype NMTOKEN #IMPLIED
handler NMTOKEN #IMPLIED
required (yes|no) "no">
<!ELEMENT section (description?, example?)>
<!ATTLIST section
name CDATA "*"
attribute NMTOKEN #IMPLIED
type NMTOKEN #REQUIRED
handler NMTOKEN #IMPLIED
required (yes|no) "no">
<!ELEMENT multisection (description?, example?)>
<!ATTLIST multisection
name CDATA "*"
attribute NMTOKEN #IMPLIED
type NMTOKEN #REQUIRED
handler NMTOKEN #IMPLIED
required (yes|no) "no">