3.2.3.1. XMLConfig - Spring Python’s native XML format

XMLConfig is a class that scans object definitions stored in the XML format defined for Spring Python. It looks very similar to Spring Java’s 2.5 XSD spec, with some small changes.

The following is a simple definition of objects. Later sections will show other options you have for wiring things together.:

<?xml version="1.0" encoding="UTF-8"?>
<objects xmlns="http://www.springframework.org/springpython/schema/objects/1.1"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/springpython/schema/objects/1.1
               http://springpython.webfactional.com/schema/context/spring-python-context-1.1.xsd">

    <object id="MovieLister" class="springpythontest.support.testSupportClasses.MovieLister" scope="prototype">
        <property name="finder" ref="MovieFinder"/>
        <property name="description"><ref object="SingletonString"/></property>
    </object>

    <object id="MovieFinder" class="springpythontest.support.testSupportClasses.ColonMovieFinder" scope="singleton">
        <property name="filename"><value>support/movies1.txt</value></property>
    </object>

    <object id="SingletonString" class="springpythontest.support.testSupportClasses.StringHolder" lazy-init="True">
        <property name="str" value="There should only be one copy of this string"></property>
    </object>
</objects>

The definitions stored in this file are fed to an XMLConfig instance which scans it, and then sends the meta-data to the ApplicationContext. Then, when the application code requests an object named MovieLister from the container, the container utilizes an object factory to create the object and return it:

from springpython.context import ApplicationContext
from springpython.config import XMLConfig

container = ApplicationContext(XMLConfig("app-context.xml"))
service = container.get_object("MovieLister")

3.2.3.1.1. Referenced Objects

A referenced object is where an object is needed, but instead of providing the definition right there, there is, instead, a name, referring to another object definition.

Object definitions can refer to other objects in many places including: properties, constructor arguments, and objects embedded inside various collections. This is the way to break things down into smaller pieces. It also allows you more efficiently use memory and guarantee different objects are linked to the same backend object.

The following fragment, pulled from the earlier example, shows two different properties referencing other objects. It demonstrates the two ways to refer to another object:

<object id="MovieLister" class="springpythontest.support.testSupportClasses.MovieLister" scope="prototype">
    <property name="finder" ref="MovieFinder"/>
    <property name="description"><ref object="SingletonString"/></property>
</object>

This means that instead of defining the object meant to be injected into the description property right there, the container must look elsewhere amongst its collection of object definitions for an object named SingletonString.

Note

  • Referenced objects don’t have to be in same configuration

    When a referenced object is encountered, finding its definition is referred back to the container. This means ANY of the input sources provided to the container can hold this definition, REGARDLESS of format.

  • Spring Python ONLY supports global references

    While Spring Java has different levels of reference like parent, local, and global, Spring Python only supports global at this time.

In the following subsections, other types of object definitions are given. Each will also include information about embedding reference objects.

3.2.3.1.2. Inner Objects

Inner objects are objects defined inside another structure, and not at the root level of the XML document. The following shows an alternative configuration of a MovieLister where the finder uses a named inner object:

<object id="MovieLister3" class="springpythontest.support.testSupportClasses.MovieLister">
    <property name="finder">
        <object id="named" class="springpythontest.support.testSupportClasses.ColonMovieFinder">
            <property name="filename"><value>support/movies1.txt</value></property>
        </object>
    </property>
    <property name="description"><ref object="SingletonString"/></property>
</object>

The ColonMovieFinder is indeed an inner object because it was defined inside the MovieLister3 object. Objects defined at the top level have a container-level name that matches their id value. In this case, asking the container for a copy of MovieLister3 will yield the top level object. However, named objects develop a path-like name based on where they are located. In this case, the inner ColonMovieFinder object will have a container-level name of MovieLister3.finder.named.

Typically, neither your code nor other object definitions will have any need to reference MovieLister3.finder.named, but there may be cases where you need this. The id attribute of ColonMovieFinder can be left out (it is optional for inner objects) like this:

<object id="MovieLister2" class="springpythontest.support.testSupportClasses.MovieLister">
    <property name="finder">
        <object class="springpythontest.support.testSupportClasses.ColonMovieFinder">
            <property name="filename"><value>support/movies1.txt</value></property>
        </object>
    </property>
    <property name="description"><ref object="SingletonString"/></property>
</object>

That is slightly more compact, and usually alright because you usually wouldn’t access this object from anywhere. However, if you must, the name in this case is MovieLister2.finder.<anonymous> indicating an anonymous object.

It is important to realize that inner objects have all the same privileges as top-level objects, meaning that they can also utilize reference objects, collections, and inner objects themselves.

3.2.3.1.3. Collections

Spring Java supports many types of collections, including lists, sets, frozen sets, maps, tuples, and Java-style properties. Spring Python supports these as well. The following configuration shows usage of dict, list, props, set, frozenset, and tuple:

<object id="ValueHolder" class="springpythontest.support.testSupportClasses.ValueHolder">
    <constructor-arg><ref object="SingletonString"/></constructor-arg>
    <property name="some_dict">
        <dict>
            <entry><key><value>Hello</value></key><value>World</value></entry>
            <entry><key><value>Spring</value></key><value>Python</value></entry>
            <entry><key><value>holder</value></key><ref object="SingletonString"/></entry>
            <entry><key><value>another copy</value></key><ref object="SingletonString"/></entry>
        </dict>
    </property>
    <property name="some_list">
        <list>
            <value>Hello, world!</value>
            <ref object="SingletonString"/>
            <value>Spring Python</value>
        </list>
    </property>
    <property name="some_props">
        <props>
            <prop key="administrator">administrator@example.org</prop>
            <prop key="support">support@example.org</prop>
            <prop key="development">development@example.org</prop>
        </props>
    </property>
    <property name="some_set">
        <set>
            <value>Hello, world!</value>
            <ref object="SingletonString"/>
            <value>Spring Python</value>
        </set>
    </property>
    <property name="some_frozen_set">
        <frozenset>
            <value>Hello, world!</value>
            <ref object="SingletonString"/>
            <value>Spring Python</value>
        </frozenset>
    </property>
    <property name="some_tuple">
        <tuple>
            <value>Hello, world!</value>
            <ref object="SingletonString"/>
            <value>Spring Python</value>
        </tuple>
    </property>
</object>
  • some_dict is a Python dictionary with four entries.
  • some_list is a Python list with three entries.
  • some_props is also a Python dictionary, containing three values.
  • some_set is an instance of Python’s mutable set.
  • some_frozen_set is an instance of Python’s frozen set.
  • some_tuple is a Python tuple with three values.

Note

Java uses maps, Python uses dictionaries

While java calls key-based structures maps, Python calls them dictionaries. For this reason, the code fragment shows a “dict” entry, which is one-to-one with Spring Java’s “map” definition.

Java also has a Property class. Spring Python translates this into a Python dictionary, making it more like an alternative to the configuring mechanism of dict.

3.2.3.1.4. Constructors

Python functions can have both positional and named arguments. Positional arguments get assembled into a tuple, and named arguments are assembled into a dictionary, before being passed to a function call. Spring Python takes advantage of that option when it comes to constructor calls. The following block of configuration data shows defining positional constructors:

<object id="AnotherSingletonString" class="springpythontest.support.testSupportClasses.StringHolder">
    <constructor-arg value="attributed value"/>
</object>

<object id="AThirdSingletonString" class="springpythontest.support.testSupportClasses.StringHolder">
    <constructor-arg><value>elemental value</value></constructor-arg>
</object>

Spring Python will read these and then feed them to the class constructor in the same order as shown here.

The following code configuration shows named constructor arguments. Spring Python converts these into keyword arguments, meaning it doesn’t matter what order they are defined:

<object id="MultiValueHolder" class="springpythontest.support.testSupportClasses.MultiValueHolder">
    <constructor-arg name="a"><value>alt a</value></constructor-arg>
    <constructor-arg name="b"><value>alt b</value></constructor-arg>
</object>

<object id="MultiValueHolder2" class="springpythontest.support.testSupportClasses.MultiValueHolder">
    <constructor-arg name="c"><value>alt c</value></constructor-arg>
    <constructor-arg name="b"><value>alt b</value></constructor-arg>
</object>

This was copied from the code’s test suite, where a test case is used to prove that order doesn’t matter. It is important to note that positional constructor arguments are fed before named constructors, and that overriding a the same constructor parameter both by position and by name is not allowed by Python, and will in turn, generate a run-time error.

It is also valuable to know that you can mix this up and use both.

3.2.3.1.5. Values

For those of you that used Spring Python before XMLConfig, you may have noticed that expressing values isn’t as succinct as the old format. A good example of the old PyContainer format would be:

<component id="user_details_service" class="springpython.security.userdetails.InMemoryUserDetailsService">
    <property name="user_dict">
        {
            "basichiblueuser"  : ("password1", ["ROLE_BASIC", "ASSIGNED_BLUE",   "LEVEL_HI"], True),
            "basichiorangeuser": ("password2", ["ROLE_BASIC", "ASSIGNED_ORANGE", "LEVEL_HI"], True),
            "otherhiblueuser"  : ("password3", ["ROLE_OTHER", "ASSIGNED_BLUE",   "LEVEL_HI"], True),
            "otherhiorangeuser": ("password4", ["ROLE_OTHER", "ASSIGNED_ORANGE", "LEVEL_HI"], True),
            "basicloblueuser"  : ("password5", ["ROLE_BASIC", "ASSIGNED_BLUE",   "LEVEL_LO"], True),
            "basicloorangeuser": ("password6", ["ROLE_BASIC", "ASSIGNED_ORANGE", "LEVEL_LO"], True),
            "otherloblueuser"  : ("password7", ["ROLE_OTHER", "ASSIGNED_BLUE",   "LEVEL_LO"], True),
            "otherloorangeuser": ("password8", ["ROLE_OTHER", "ASSIGNED_ORANGE", "LEVEL_LO"], True)
        }
    </property>
</component>

Note

Why do I see components and not objects?

In the beginning, PyContainer was used and it tagged the managed instances as components. After replacing PyContainer with a more sophisticated IoC container, the instances are now referred to as objects, however, to maintain this legacy format, you will see component tags inside PyContainerConfig-based definitions.

While this is very succinct for expressing definitions using as much Python as possible, that format makes it very hard to embed referenced objects and inner objects, since PyContainerConfig uses Python’s eval method to convert the material.

The following configuration block shows how to configure the same thing for XMLConfig:

<object id="user_details_service" class="springpython.security.userdetails.InMemoryUserDetailsService">
    <property name="user_dict">
        <dict>
            <entry>
                <key><value>basichiblueuser</value></key>
                <value><tuple>
                    <value>password1</value>
                    <list><value>ROLE_BASIC</value><value>ASSIGNED_BLUE</value><value>LEVEL_HI</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>basichiorangeuser</value></key>
                <value><tuple>
                    <value>password2</value>
                    <list><value>ROLE_BASIC</value><value>ASSIGNED_ORANGE</value><value>LEVEL_HI</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>otherhiblueuser</value></key>
                <value><tuple>
                    <value>password3</value>
                    <list><value>ROLE_OTHER</value><value>ASSIGNED_BLUE</value><value>LEVEL_HI</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>otherhiorangeuser</value></key>
                <value><tuple>
                    <value>password4</value>
                    <list><value>ROLE_OTHER</value><value>ASSIGNED_ORANGE</value><value>LEVEL_HI</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>basicloblueuser</value></key>
                <value><tuple>
                    <value>password5</value>
                    <list><value>ROLE_BASIC</value><value>ASSIGNED_BLUE</value><value>LEVEL_LO</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>basicloorangeuser</value></key>
                <value><tuple>
                    <value>password6</value>
                    <list><value>ROLE_BASIC</value><value>ASSIGNED_ORANGE</value><value>LEVEL_LO</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>otherloblueuser</value></key>
                <value><tuple>
                    <value>password7</value>
                    <list><value>ROLE_OTHER</value><value>ASSIGNED_BLUE</value><value>LEVEL_LO</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
            <entry>
                <key><value>otherloorangeuser</value></key>
                <value><tuple>
                    <value>password8</value>
                    <list><value>ROLE_OTHER</value><value>ASSIGNED_ORANGE</value><value>LEVEL_LO</value></list>
                    <value>True</value>
                </tuple></value>
            </entry>
        </dict>
    </property>
</object>

Of course this is more verbose than the previous block. However, it opens the door to having a much higher level of detail:

<object id="user_details_service2" class="springpython.security.userdetails.InMemoryUserDetailsService">
    <property name="user_dict">
        <list>
            <value>Hello, world!</value>
            <dict>
                <entry>
                    <key><value>yes</value></key>
                    <value>This is working</value>
                </entry>
                <entry>
                    <key><value>no</value></key>
                    <value>Maybe it's not?</value>
                </entry>
            </dict>
            <tuple>
                <value>Hello, from Spring Python!</value>
                <value>Spring Python</value>
                <dict>
                    <entry>
                        <key><value>yes</value></key>
                        <value>This is working</value>
                    </entry>
                    <entry>
                        <key><value>no</value></key>
                        <value>Maybe it's not?</value>
                    </entry>
                </dict>
                <list>
                    <value>This is a list element inside a tuple.</value>
                    <value>And so is this :)</value>
                </list>
            </tuple>
            <set>
                <value>1</value>
                <value>2</value>
                <value>1</value>
            </set>
            <frozenset>
                <value>a</value>
                <value>b</value>
                <value>a</value>
            </frozenset>
        </list>
    </property>
</object>

3.2.3.1.6. XMLConfig and basic Python types

Objects of most commonly used Python types - str, unicode, int, long, float, decimal.Decimal, bool and complex - may be expressed in XMLConfig using a shorthand syntax which allows for a following XMLConfig file:

<?xml version="1.0" encoding="UTF-8"?>
<objects xmlns="http://www.springframework.org/springpython/schema/objects/1.1"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/springpython/schema/objects/1.1
            http://springpython.webfactional.com/schema/context/spring-python-context-1.1.xsd">

    <str id="MyString">My string</str>
    <unicode id="MyUnicode">Zażółć gęślą jaźń</unicode>
    <int id="MyInt">10</int>
    <long id="MyLong">100000000000000000000000</long>
    <float id="MyFloat">3.14</float>
    <decimal id="MyDecimal">12.34</decimal>
    <bool id="MyBool">False</bool>
    <complex id="MyComplex">10+0j</complex>

</objects>

_objects-xmlconfig-inheritance

3.2.3.1.7. Object definition inheritance

XMLConfig’s definitions may be stacked up into hierarchies of abstract parents and their children objects. A child object not only inherits all the properties and constructor arguments from its parent but it can also easily override any of the inherited values. This can save a lot of typing when configuring non-trivial application contexts which would otherwise need to repeat the same configuration properties over many objects definitions.

An abstract object is identified by having an abstract=”True” attribute and the child ones are those which have a parent attribute set to ID of an object from which the properties or constructor arguments should be inherited. Child objects must not specify the class attribute, its value is taken from their parents.

An object may be both a child and an abstract one.

Here’s a hypothetical configuration of a set of services exposed by a server. Note how you can easily change the CRM environment you’re invoking by merely changing the concrete service’s (get_customer_id or get_customer_profile) parent ID:

<?xml version="1.0" encoding="UTF-8"?>
<objects xmlns="http://www.springframework.org/springpython/schema/objects/1.1"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/springpython/schema/objects/1.1
            http://springpython.webfactional.com/schema/context/spring-python-context-1.1.xsd">

    <object id="service" class="springpythontest.support.testSupportClasses.Service" scope="singleton" abstract="True" lazy-init="True">
        <property name="ip"><value>192.168.1.153</value></property>
    </object>

    <object id="crm_service_dev" parent="service" abstract="True">
        <property name="port"><value>3392</value></property>
    </object>

    <object id="crm_service_test" parent="service" abstract="True">
        <property name="port"><value>3393</value></property>
    </object>

    <object id="get_customer_id" parent="crm_service_dev">
        <property name="path"><value>/soap/invoke/get_customer_id</value></property>
    </object>

    <object id="get_customer_profile" parent="crm_service_test">
        <property name="path"><value>/soap/invoke/get_customer_profile</value></property>
    </object>

</objects>

Here’s how you can override inherited properties; both get_customer_id and get_customer_profile object definitions will inherit the path property however the actual objects returned by the container will use local, overridden, values of the property:

<?xml version="1.0" encoding="UTF-8"?>
<objects xmlns="http://www.springframework.org/springpython/schema/objects/1.1"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/springpython/schema/objects/1.1
            http://springpython.webfactional.com/schema/context/spring-python-context-1.1.xsd">

    <object id="service" class="springpythontest.support.testSupportClasses.Service" scope="singleton" abstract="True" lazy-init="True">
        <property name="ip"><value>192.168.1.153</value></property>
        <property name="port"><value>3392</value></property>
        <property name="path"><value>/DOES-NOT-EXIST</value></property>
    </object>

    <object id="get_customer_id" parent="service">
        <property name="path"><value>/soap/invoke/get_customer_id</value></property>
    </object>

    <object id="get_customer_profile" parent="service">
        <property name="path"><value>/soap/invoke/get_customer_profile</value></property>
    </object>

</objects>

If you need to get an abstract object from a container, use the .get_object’s ignore_abstract parameter, otherwise springpython.container.AbstractObjectException will be raised. Observe the difference:

# .. skip creating the context

# No exception will be raised, even though 'service' is an abstract object
service = ctx.get_object("service", ignore_abstract=True)

# Will show the object
print service

# Will raise AbstractObjectException
service = ctx.get_object("service")