Association Rules

Association definitions, or rules, are Python classes, all based on Association. The base class provides only a framework, much like an abstract base class; all functionality must be implemented in sub-classes.

Any subclass that is intended to produce an association is referred to as a rule. Any rule subclass must have a name that begins with the string Asn_. This is to ensure that any other classes involved in defining the definition of the rule classes do not get used as rules themselves, such as the Association itself.

Association Dynamic Definition

Associations are created by matching members to rules. However, an important concept to remember is that an association is defined by both the rule matched, and by the initial member that matched it. The following example will illustrate this concept.

For JWST Level 3, many associations created must have members that all share the same filter. To avoid writing rules for each filter, the rules have a condition that states that it doesn’t matter what filter is specified, as long as the association contains all the same filter.

To accomplish this, the association defines a constraint where filter must have a valid value, but can be any valid value. When the association is first attempted to be instantiated with a member, and that member has a valid filter, the association is created. However, the constraint on filter value in the newly created association is modified to match exactly the filter value that the first member had. Now, when other members are attempted to be added to the association, the filter of the new members must match exactly with what the association is expecting.

This dynamic definition allows rules to be written where each value of a specific attribute of a member does not have to be explicitly stated. This provides for very robust, yet concise, set of rule definitions.

User-level API

Core Keys

To be repetitive, the basic association is simply a dict (default) or list. The structure of the dict is completely determined by the rules. However, the base class defines the following keys:


The type of the association.


The name of the rule.


A version number for any associations created by this rule.


The version of the generator library in use.

These keys are accessed in the same way any dict key is accessed:

asn = Asn_MyAssociation()

#--> MyAssociation

Core Methods

These are the methods of an association rule deal with creation or returning the created association. A rule may define other methods, but the following are required to be implemented.


Create an association.


Add a member to the current association.


Return the string serialization of the association.


Return the association from its serialization.


To create an association based on a member, the create method of the rule is called:

(association, reprocess_list) = Asn_SomeRule.create(member)

create returns a 2-tuple: The first element is the association and the second element is a list of reprocess instances.

If the member matches the conditions for the rule, an association is returned. If the member does not belong, None is returned for the association.

Whether an association is created or not, it is possible a list of reprocess instances may be returned. This list represents the expansion of the pool in Member Attributes that are Lists


To add members to an existing association, one uses the Association.add method:

(matches, reprocess_list) = association.add(new_member)

If the association accepts the member, the matches element of the 2-tuple will be True.

Typically, one does not deal with a single rule, but a collection of rules. For association creation, one typically uses an AssociationRegistry to collect all the rules a pool will be compared against. Association registries provide extra functionality to deal with a large and varied set of association rules.

Saving and Loading

Once created, an association can be serialized using its Association.dump method. Serialization creates a string representation of the association which can then be saved as one wishes. Some code that does a basic save looks like:

file_name, serialized = association.dump()
with open(file_name, 'w') as file_handle:

Note that dump returns a 2-tuple. The first element is the suggested file name to use to save the association. The second element is the serialization.

To retrieve an association, one uses the Association.load method:

with open(file_name, 'r') as file_handle:
    association = Association.load(file_handle)

Association.load will only validate the incoming data against whatever schema or other validation checks the particular subclass calls for. The generally preferred method for loading an association is through the jwst.associations.load_asn() function.

Defining New Associations

All association rules are based on the Association base class. This class will not create associations on its own; subclasses must be defined. What an association is and how it is later used is completely left to the subclasses. The base class itself only defines the framework required to create associations. The rest of this section will discuss the minimum functionality that a subclass needs to implement in order to create an association.

Class Naming

The AssociationRegistry is used to store the association rules. Since rules are defined by Python classes, a way of indicating what the final rule classes are is needed. By definition, rule classes are classes that begin with the string Asn_. Only these classes are used to produce associations.

Core Attributes

Since rule classes will potentially have a large number of attributes and methods, the base Association class defines two attributes: data, which contains the actual association, and meta, the structure that holds auxiliary information needed for association creation. Subclasses may redefine these attributes as they see fit. However, it is suggested that they be used as conceptually defined here.

data Attribute

data contains the association itself. Currently, the base class predefines data as a dict. The base class itself is a subclass of MutableMapping. Any instance behaves as a dict. The contents of that dict is the contents of the data attribute. For example:

asn = Asn_MyAssociation()['value'] = 'a value'

assert asn['value'] == 'a value'
# True

asn['value'] = 'another value'
assert['value'] == 'another value'
# True


Instantiating a rule, in and of itself, does nothing more than setup the constraints that define the rule, and basic structure initialization.

Implementing create()

The base class function performs the following steps:

  • Instantiates an instance of the rule

  • Calls add() to attempt to add the member to the instance

If add() returns matches==False, then create returns None as the new association.

Any override of this method is expected to first call super. On success, any further initialization may be performed.

Implementing add()

The add() method adds members to an association.

If a member does belong to the association, the following events occur:

Constraint Modification

Any wildcard constraints are modified so that any further matching must match exactly the value provided by the current member.

self._init_hook() is executed

If a new association is being created, the rule’s _init_hook method is executed, if defined. This allows a rule to do further initialization before the member is officially added to the association.

self._add() is executed

The rule class must define _add(). This method officially adds the member to the association.

Implementing dump() and load()

The base Association class defines the dump() and load() methods to serialize the data structure pointing to by the data attribute. If the new rule uses the data attribute for storing the association information, no further overriding of these methods is necessary.

However, if the new rule does not define data, then these methods will need be overridden.

Rule Registration

In order for a rule to be used by generate, the rule must be loaded into an AssociationRegistry. Since a rule is just a class that is defined as part of a, most likely, larger module, the registry needs to know what classes are rules. Classes to be used as rules are marked with the RegistryMarker.rule decorator as follows:

from jwst.associations import (Association, RegistryMarker)

class MyRule(Association):

Then, when the rule file is used to create an AssociationRegistry, the class MyRule will be included as one of the available rules:

.. doctest-skip::
>>> from jwst.associations import AssociationRegistry
>>> registry = AssociationRegistry('', include_default=False)
>>> print(registry)
    {'MyRule': <class 'abc.MyRule'>}