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- High level overview of the basic ASDF library

# High level overview of the basic ASDF library

This document is an attempt to make it easier to understand the design and workings of the python asdf library for those unfamiliar with it. This is expected to grow organically so at the moment it should not be considered complete or comprehensive.

Understanding the design is complicated by the fact that the library effectively inserts custom methods or classes into the objects that the pyyaml and jsonschema libraries use. Understanding what is going on thus means having some understanding of the relevant parts of the internals of both of those libraries. This overview will try to provide a small amount of context for these packages to illuminate how the code in asdf interacts with them.

There are at least two ways of outlining the design. One is to give high level overviews of the various modules and how they interact with other modules. The other is to illustrate how code is actually invoked in common operations, this often being much more informative on a practical level (at least some find that to be the case). This document will attempt to do both.

We will start with a high-level review of concepts and terms and point to where these are handled in the asdf modules.

Because of the complexity, this initial design overview will focus on issues of validation and tree construction when reading.

## Construction in progress

Before we get into further details, a word on the transition to new plugin APIs. Starting in asdf 2.8 we’ve introduced new interfaces for extending the asdf library to support additional tags and schemas. The interfaces were redesigned with the following goals in mind:

- Simplify the connection between tags and their schema content. The old “resolver” system involves sending the tag URI through a lengthy series of transformations to get the filesystem path to the schema document. This has been error-prone and difficult to troubleshoot, so the new “resource mapping” system explicitly maps schema URIs to their content, and tag URIs directly to schema URIs.

- Make it easier to separate schemas from extension code. Until now the schemas have always been provided by the same Python package that implements support for their tags, but we would like to move the schemas to language-agnostic repositories that non-Python implementations can use. To better support this, the new interface splits the old extension plugin into two new plugins, one of which is dedicated to schemas.

- Allow tag serialization support to handle arbitrary sets of URIs. Previously tag code was restricted to working with tag URIs that were identical except for version. This presented a problem for the transition of URIs from stsci.edu to asdf-format.org, so the new interface allows for supporting diverse URIs with the same code.

- Improve the terminology used in the tag serialization support classes. The old `ExtensionType` has been renamed `Converter` to indicate its purpose, and to eliminate the ambiguity betwenen YAML types and Python types. The `to_tree` and `from_tree` methods have been renamed `to_yaml_tree` and `from_yaml_tree` to better indicate which tree they’re expected to convert.

- Simplify the code and behavior of tag classes. Converters are used as instances instead of classes with a custom metaclass, Python sub-types are no longer automatically handled, URIs are treated as single values instead of broken down into various components, etc.

You can witness the gory details of this effort by clicking through the PR links on the asdf 2.8.0 roadmap.

Support for ASDF core tags has not yet been moved to the new system. Doing so would be a breaking change for users who subclass that code, so we’ll need to wait until asdf 3.0 to do that.

Some features working toward this goal are available for testing but will raise a `AsdfProvisionalAPIWarning` when used. This warning indicates that a feature may experience breaking API changes (even for minor ASDF releases) and should not be trusted in a production environment. It is likely that features that raise this warning will become stable and be added to a future ASDF version.

## Some terminology and definitions

URI vs URL (Universal Resource Identifier). This is distinguished from URL (Universal Resource Locator) primarily in that URI is a mechanism for a unique name that follows a particular syntax, but itself may not indicate where the resource is. Generally URLs are expected to be used on the web for the HTTP protocol, though for asdf, this isn’t necessarily the case as mentioned next. Recent changes to the library permit use of URIs with the asdf:// scheme, which is intended to reduce confusion over the distinction between identifiers and locations.

Resolver: Tools to map URIs and tags into actual locations of schema files, which may be local directories (the usual approach) or an actual URL for retrieval over the network. This is more complicated that it may seem for reasons explained later. The resolver system has been deprecated in favor of resource mappings; new code should use the latter instead.

Global config: A global library configuration feature that was added in asdf 2.8. Allows plugins to be added or removed at runtime and `AsdfFile` defaults to be modified by the user. Accessed via the `get_config` method on the top-level `asdf` module. For example, the default ASDF Standard version for new files can be set like this:

    asdf.get_config().default_version = "1.3.0"

Or a resource mapping plugin added at runtime like this:

    asdf.get_config().add_resource_mapping({"http://somewhere.org/resources/foo", b"foo resource content"})

Entry point: A Python packaging feature that allows asdf to use plugins provided by other packages. Entry points are registered when a package is installed and become available to asdf without any additional effort on the part of the user. See pypa-packaging:entry-points for more information.

Resource mapping: An asdf plugin that provides access to “resources” which are binary blobs associated with a URI. These resources are mostly schemas, but any resource may provided by a mapping. Resource mappings are provided via entry points or added at runtime using a method on the global config object. This feature is intended to replace the deprecated “resolver” mechanism.

Extension: An extension to the ASDF Standard that defines additional YAML tags. In the future an extension may include other additional features such as binary block compressors or filters, but currently only tags are supported.

Extension implementation: An asdf plugin that implements an extension to the ASDF Standard. This is the asdf library’s support for an extended set of YAML tags. The library currently provides two interfaces for implementing extensions: the `AsdfExtension` class and the new, still-experimental `Extension` class. Extension implementations are provided via entry points or added at runtime using a method on the global config object. The `AsdfFile` also permits adding additional extensions on a per-instance basis, but use of that feature is discouraged and may be removed in asdf 3.0.

Tag code/tag class: A class responsible for converting a family of tags into Python objects and vice versa. Each extension implementation includes a list of such classes. For the original `AsdfExtension` API, the tag classes all implement the `ExtensionType` interface. For the new API, tag classes implement `Converter`.

Validator: Tool to confirm that the YAML conforms to the schemas that apply. A lot goes on in this area and it is pretty complex in the implementation.

Tree building: The YAML content is built into a tree in two stages. The YAML parser converts the raw YAML into a custom Python structure. It is that structure that is validated. Then if no errors are found, the tree is converted into a tree where tagged nodes get converted into corresponding Python objects (usually, an option exists to prevent this from happening, which is useful for some applications), e.g., WCS object or numpy arrays (well, not quite that simply for numpy arrays).

The above is a simplified view of what happens when an ASDF file is read.

Most of resolver tools and code is in `resolver.py` (but not all).

Most of the validation code is in `schema.py`.

The code that builds the trees is spread in many places: `tagged.py`, `treeutil.py`, `types.py` as well as all the extension code that supplies code to handle the tags within (and often the the associated schemas).

A note on the location of schemas and tag code; there is a bit of schizophrenic aspect to this since schema should be language agnostic and in that view, not bundled with specific language library code. But currently nearly all of the implementation is in Python so while the long-term goal is to keep them separate, it is more convenient to keep them together for now. You will see cases where they are separate and some where they are bundled. The introduction of a separate plugin for providing access to schemas (the “resource mapping”) is intended to allow extension authors to keep the schema documents in a separate language-neutral repository.

## Actions that happen when an AsdfFile is instantiated

The asdf plugins (new and old-style extensions as well as resource mappings) registered as entry points can be obtained by calling methods in `entry_points.py`. These methods are invoked by `config.AsdfConfig` the first time library needs to use the plugins, and thereafter are cached within that config object. Both extensions and resource mappings are stored wrapped in proxy objects (`ExtensionProxy` and `ResourceMappingProxy`, respectively) that carry additional metadata like the package name and version of the entry point, and add some convenience methods on top of what the extension developer provides. Additionally, `ExtensionProxy` allows the library to treat both new-style `Extension` instances and old-style `AsdfExtension` instances similarly.

To see the list of extensions loaded by the library, call `asdf.get_config().extensions`. To see the list of resource mappings, call `asdf.get_config().resource_mappings`. Both of these properties are lazy-loaded and then cached, so the first call will take a moment to complete but subsequent calls will return immediately.

When an `AsdfFile` class is instantiated, one thing that happens on the `__init__` is that `self._process_plugin_extensions()` is called. This method retrieves the extensions from the global config and selects those that are compatible with the `AsdfFile`’s ASDF Standard version. It returns the resulting list, which is assigned to the `_plugin_extensions` variable. The term “plugin extensions” contrasts with “user extensions” which are additional extensions provided by the user as an argument to `AsdfFile.__init__`.

The extension lists are used by `AsdfFile` to create the file’s `ExtensionList` and `ExtensionManager` instances, which manage extensions for the old and new extension APIs, respectively. These instances are created lazily when the `extension_list` and `extension_manager` properties are first accessed, to help speed up the initial construction of the `AsdfFile`.

The `extension_manager` is responsible for mapping tag URIs to schema URIs for validation and retrieving type converters (instances of the `Converter` interface) by Python type or by YAML tag URI. `extension_list` handles the same duties, but for old-style extensions. `extension_manager` takes precedence over `extension_list` throughout the asdf library, so `extension_list` will only be consulted if `extension_manager` can’t handle a particular tag or Python type.

## On the subject of resolvers and tag/url mapping

The `AsdfFile` class has `tag_mapping` and `url_mapping` properties that each return the `extension_list` properties of the same name. These objects implement the original support for mapping tag URIs to schema content that in the new API is provided by resource mappings.

`tag_mapping` and `url_mapping` are each `resolver.Resolver` instances that are generated from the mapping lists in the old-style extensions. These lists consist of 2-tuples. In the first case it is a mechanism to map the tag string to a url string, typically with an expected prefix or suffix to the tag (suffix is typical) so that given a full tag, it generates a url that includes the suffix. This permits one mapping to cover many tag variants (The details of mapping machinery with examples are given in a later section since understanding this is essential to defining new tags and corresponding schemas).

The URL mapping works in a similar way, except that it consists of 2-tuples where the first element is the common elements of the url, and the second part maps it to an actual location (url or file path). Again the second part may include a place holder for the suffix or prefix, and code to generate the path to the schema file.

The use of the resolver object turns these lists into functions so that supplied the appropriate input that matches something in the list, it gives the corresponding output.

## Outline of how an ASDF file is opened and read into the corresponding Python object.

The starting point can be found in `asdf.py` essentially through the following chain (many calls and steps left out to keep it simpler to follow)

When `asdf.open("myasdffile.asdf")` is called, it is aliased to `asdf.open_asdf` which first creates an instance of `asdf.AsdfFile` (let’s call the instance `af`), then calls `af._open_impl()` and then `af._open_asdf`. That invokes a call to `generic_io.get_file()`.

`generic.py` basically contains code to handle all the variants of I/O possible (files, streaming, http access, etc). In this case it returns a `RealFile` instance that wraps a local file system file.

Next the file is examined to see if it is an ASDF file (first by examining the first few lines in the header). If it passes those checks, the header (yaml) section of the file is extracted through a proxy mechanism that signals an end of file when the end of the yaml is reached, but otherwise looks like a file object.

The yaml parsing phase described below normally returns a “tagged_tree”. That is (somewhat simplified), it returns the data structure that yaml would normally return without any object conversion (i.e., all nodes are either dicts, lists, or scalar values), except that they are objects that now support a tag attribute that indicates if a tag was associated with that node and what the tag was.

This reader object is passed to the yaml parser by calling `yamlutil.load_tree`. A simple explanation for what goes on here is necessary to understand how this all works. Yaml supports various kinds of loaders. For security reasons, the “safe” loader is used (note that both C and python versions are supported through an indirection of the `_yaml_base_loader` defined at the beginning of that module that determines whether the C version is available). The loaders are recursive mechanisms that build the tree structure. Note that `yamlutil.load_tree` creates a temporary subclass of `AsdfLoader` and attaches a reference to the AsdfFile instance as the `.ctx` attribute of that temporary subclass.

One of the hooks that pyyaml supplies is the ability to overload the method `construct_object`. That’s what the class `yamlutil.AsdfLoader` does. pyyaml calls this method at each node in the tree to see if anything special should be done. One could perform conversion to predefined objects here, but instead it does the following: it sees if the node.tag attribute is handled by yaml itself (examples?) it calls that constructor which returns the type yaml converts it to. Otherwise:

> - it converts the node to the type indicated (dict, list, or scalar type) by yaml for that node.

> - it obtains the appropriate tag class (an AsdfType subclass) from the AsdfFile instance (using `ctx.type_index.fix_yaml_tag` to deal with version issues to match the most appropriate tag class). The new extension API does not support this “fix YAML tag” feature so file’s ExtensionManager is not used here.

> - it wraps all the node alternatives in a special asdf `Tagged` class instance variant where that object contains a ._tag attribute that is a reference to the corresponding Tag class.

The loading process returns a tree of these Tagged object instances. This tagged_tree is then returned to the `af` instance (still running the `_open_asdf()` method) this tree is passed to to the `_validate()` method (This is the major reason that the tree isn’t directly converted to an object tree since jsonschema would not be able to use the final object tree for validation, besides issues relate to the fact that things that don’t validate may not be convertible to the designated object.)

The validate machinery is a bit confusing since there are essentially two basic approaches to how validation is done. One type of validation is for validation of schema files themselves, and the other for schemas for tags.

The schema.py file is fairly involved and the details are covered elsewhere. When the validator machinery is constructed, it uses the fundamental validation files (schemas). But this doesn’t handle the fact that the file being validated is yaml, not json and that there are items in yaml not part of json so special handling is needed. And the way it is handled is through a internal mechanism of the jsonschema library. There is a method that jsonschema calls recursively for a validator and it is called iter_errors. The subclass of the jsonschema validator class is defined as schema.ASDFValidator and this method is overloaded in this class. Despite its name, it’s primary purpose is to validate the special features that yaml has, namely applying schemas associated with tags (this is not part of the normal jsonschema scheme [ahem]). It is in this method that it looks for a tag for a node and if it exists and in the tag_index, loads the appropriate schema and applies it to the node. (jsonschemas are normally only associated with a whole json entity rather than specific nodes). While the purpose of this method is to iteratively handle errors that jsonschema detects, it has essentially been repurposed as the means of interjecting handling tag schemas.

In order to prevent repeated loading of the same schema, the lru caching scheme is used (from functools in the standard library) where the last n cached schemas are saved (details of how this works were recently changed to prevent a serious memory leak)

In any event, a lot is going on behind the scenes in validation and it deserves its own description elsewhere.

After validation, the tagged tree is then passed to yamlutil.tagged_tree_to_custom_tree() where the nodes in the tree that have special tag code convert the nodes into the appropriate Python objects that the base asdf and extensions are aware of. This is accomplished by that function defining a walker “callback” function (defined within that function as to pick up the af object intrinsically). The function then passes the callback walker to treeutil.walk_and_modify() where the tree will be traversed recursively applying the tag code associated with the tag to the more primitive tree representation replacing such nodes with Python objects. The tree traversal starts from the top, but the objects are created from the bottom up due to recursion (well, not quite that simple).

Understanding how this works is described more fully later on.

The result is what af.tree is set to, after doing another tree traversal looking for special type hooks for each node. It isn’t clear if there is yet any use of that feature.

## Not quite that simple

## Outline of schema.py

This module is somewhat confusing due to the many functions and methods with some variant of validate in their name. This will try to make clear what they do (a renaming of these may be in order).

Here is a list of the functions/classes in `schema.py` and their purpose and where they sit in the order of things

default_ext_resolver

_type_to_tag: Handles mapping python types to yaml_tags, with the addition of support for OrderedDicts.

The next 5 functions are put in the `YAML_VALIDATORS` dictionary to ultimately be used by `_create_validator` to create the json validator object

validate_tag: Obtain the relevant tag for the supplied instance (either built ins or custom objects) and check that it matches the tag supplied to the function.

validate_propertyOrder: Not really a validator but rather as a trick to indicate that properties should retain their order.

validate_flowStyle: Not really a validator but rather as a trick to store what style to use to write the elements (for yaml objects and arrays)

validate_style: Not really a validator but rather as a trick to store info on what style to use to write the string.

validate_type: Used to deal with date strings

(It may make sense to rename the above to be more descriptive of the action than where they are stuck in the validation machinery; e.g., `set_propertyOrder`)

validate_fill_default: Set the default values for all properties that have a subschema that defines a default. Called indirectly in `fill_defaults`

validate_remove_default: does the opposite; remove all properties where value equals subschema default. Called indirectly in `remove_defaults` (For this and the above, validate in the name mostly confuses although it is used by the json validator.)

[these could be renamed as well since they do more than validate]

_create_validator: Creates an `ASDFValidator` class on the fly that uses the `jsonchema.validators` class created. This `ASDFValidator` class overrides the `iter_errors` method that is used to handle yaml tag cases (using the `._tag` attribute of the node to obtain the corresponding schema for that tag; e.g., it calls `load_schema` to obtain the right schema when called for each node in the jsonschema machinery). What isn’t clear to me is why this is done on the fly and at least cached since it really only handles two variants of calls (basically which JSONSCHEMA version is to be used). Otherwise it doesn’t appear to vary except for that. Admittedly, this is only created at the top level. This is called by `get_validator`.

class OrderedLoader: Inherits from the `_yaml_base_loader`, but otherwise does nothing new in the definition. But the following code defines `construct_mapping`, and then adds it as a method.

construct_mapping: Defined outside the `OrderedLoader` class but to be added to the `OrderedLoader` class by use of the base class add_constructor method. This function flattens the mapping and returns an `OrderedDict` of the property attributes (This needs some deep understanding of how the yaml parser actually works, which is not covered here. Apparently mappings can be represented as nested trees as the yaml is originally parsed. Or something like that.)

_load_schema: Loads json or yaml schemas (using the `OrderedLoader`).

_make_schema_loader: Defines the function load_schema using the provided resolver and _load_schema.

_make_jsonschema_refresolver: Sets the schema loader for http, https, file, tag using a dictionary where these access methods are the keys and the schema loader returning only the schema (and not the uri). These all appear to use the same schema loader.

_load_draft4_metaschema:

load_schema: Loads a schema from the specified location (this is cached). Called for every tag encountered (uses resolver machinery). Most of the complexity is in resolving json references. Calls `_make_schema_loader, resolver, reference.resolve_fragment, load_schema`

get_validator: Calls `_create_validator`. Is called by validate to return the created validator.

validate_large_literals: Ensures tree has no large literals (raises error if it does)

validate: Uses `get_validator` to get a validator object and then calls its validate method, and validates any large literals using `validate_large_literals`.

fill_defaults: Inserts attributes missing with the default value

remove_defaults: Where the tree has attributes with value equal to the default, strip the attribute.

check_schema: Checks schema against the metaschema.

Illustration of the where these are called:

`af._open_asdf` calls `af.validate` which calls `af._validate` which then calls `schema.validate` with the tagged tree as the first argument (it can be called again if there is a custom schema).

in schema.py

`validate -> get_validator -> _create_validator` (returns `ASDFValidator`). There are two levels of validation, those passed to the json_validation machinery for the schemas themselves, and those that the tag machinery triggers when the jsonschema validator calls through `iter_errors`. The first level handles all the tricks at the top. the `ASDFValidator` uses `load_schema` which in turn calls `_make_schema_loader`, then `_load_schema`. `_load_schema` uses the `OrderedLoader` to load the schemas.

Got that?

## How the ASDF library works with pyyaml

### A Tree Identifier

There are three flavors of trees in the process of reading ASDF files, one will see many references to each in the code and description below.

pyyaml native tree. This consists of standard Python containers like dict and list, and primitive values like string, integer, float, etc.

Tagged tree. These are similar to pyyaml native trees, but with the basic types wrapped in a class that has has an attribute that identifies the tag associated with that node so that later processing can apply the appropriate conversion code to convert to the final Python object.

Custom tree. This is a tree where all nodes are converted to the destination Python objects. For example, a numpy array or GWCS object.

### Brief overview of how pyyaml constructs a Python tree

Understanding the process of creating Python objects from yaml requires some understanding of how pyyaml works. We will not go into all the details of pyyaml, but instead concentrate on one phase of its loading process. First an outline of the phases of processing that pyyaml goes through in loading a yaml file:

1. scanning: Converting the text into lexical tokens. Done in scanner.py

2. parsing: Converting the lexical tokens into parsing events. Done in parser.py.

3. composing: Converting the parsing events into a tree structure of pyyaml objects. Done in composer.py

4. loading: Converting the pyyaml tree into a Python object tree. Done in constructor.py

We will focus on the last step since that is where asdf integrates with how pyyaml works.

The key object in that module is `BaseConstructor` and its subclasses (asdf uses `SafeConstructor` for security purposes). Note that the pyyaml code is severely deficient in docstrings and comments. The key method that kicks off the conversion is `construct_document()`. Its responsibilities are to call the `construct_object()` method on the top node, “drain” any generators produced by construction (more on this later), and finally reset internal data structures once construction is complete.

The actual process seems somewhat mysterious because what is going on is that it is using generators in place of vanilla code to construct the children for mutable items. The general scheme is that each constructor for mutable elements (see as an example the `SafeConstructor.construct_yaml_seq()` method) is written as a generator that is expected to be asked a value twice. The first value returned is an empty object of the expected type (e.g., empty dict or list) and when asked a second time, it populates the previous object returned (and returns None, which is not used). (In rare exceptions, when called with `deep=True`, it does immediately populate the child nodes.)

Normally the generator is appended to the loader’s state_generators attribute (a list) for later use. Any generators not handled in the recursive chain are handled when contruct_object returns to `construct_document`, where it iteratively asks each generator to complete populating its referenced object. Since that step of populating the object may in turn create new generators on the `state_generator` list, it only stops when no more generators appear on the list.

Why is this done? One reason is to handle references (anchors and aliases) that may be circular.

Suppose one had the following yaml source:

    A: &a
        x: 1
        B:
            item1: 42
            item2: life, the universe, and everything
        circular: *a

Without generators, it would not be possible to handle this case since the node identified by anchor `a` has not been fully constructed when pyyaml encounters a reference to that anchor among the same node’s descendants. The use of the generator allows creation of the container object to reference to before it is populated so that the above construction will work when constructing the tree. To follow the above example in more detail, the construction creates a dictionary for `a` and then returns to the `construct_document()` method, which then starts handling the generators put on the list (there is only one in this case). The generator then populates the contents of `a`. For the attribute `B` it encounters a new mutable container, and puts its generator on the list to handle, and then makes a reference to `a` which now is defined. One last time it handles the generator for `B` and since each item in that is not a container, the construction completes.

Pyyaml tracks pending objects in a recursive objects dict and throws an exception if generators fail to handle reference cycles. (The conversion of the tagged tree to the custom tree, performed later does not use the same technique; explained later)

### How ASDF hooks into pyyaml construction

ASDF makes use of this by adding generators to this process by defining a new construct method `construct_undefined()` that handles all ASDF tag cases. This is added to the pyyaml dict of construct methods under the key of `None`. When pyyaml doesn’t find a tag, that is what it uses as a key to handle unknown tags. Thus the construction is redirected to ASDF code. That code returns a generator in the case of mutable ASDF objects in line with how yaml works with mutable objects.

Historical note: Versions older than 2.6.0 did not work this way. Instead, those versions completely replaced the pyyaml method `construct_object()` with their own version that did not use generators as pyyaml did.

### How conversion to ASDF objects is done

The current means of conversion is simpler to use by tag code, but also more subtle to understand how it actually works (for many, that means harder ;-)

The YAML loading process produces a tagged tree of basic Python types. The conversion of these into ASDF types is kicked off when the `AsdfFile` method `_open_asdf()` calls `yamlutil.tagged_tree_to_custom_tree()`. This function defines a walker function that is to be used with `treeutil.walk_and_modify()`. Most of what the walker function does is handle tag issues (e.g., can the tag be appropriately mapped to the tag creation code) and then returns the appropriate ASDF type by calling `tag_type.from_tree_tagged()`.

A note on tree traversal. One can traverse a tree in three ways: inorder, preorder, and postorder (`asdf.info()` uses a breadth-first traversal, yet another exciting option, which we won’t describe here). These respectively mean whether nodes are visited in the horizontal ordering of the nodes displayed on a graphs (inorder), descending the tree from the root, doing the left node first, before the right node (preorder), or from the bottom up, doing both leaf nodes before the parent node (postorder). In generating the pyyaml tree, preorder works since it builds the tree from the root as one would expect in constructing the tree. But in converting the tagged tree into the custom tree, postorder is the natural course, where the children are generated first so that the parent node can refer to the final objects.

An important part of this conversion process is handled by an instance of the class `treeutil._TreeModificationContext`. This class does much the same trick that pyyaml does with generators. Although pyyaml creates references between basic python objects, these references must be converted to references between ASDF objects, and doing so requires a similar mechanism for building the ASDF objects. The `_TreeModificationContext` object (hereafter context object) holds the incomplete generators in a way similar to the pyyaml `construct_document` function.

There are differences though. The class `TreeModificationContext` provides methods to indicate if nodes are pending (i.e., incomplete), and there is a special value `PendingValue` that is a signal that the node hasn’t been handled yet (e.g., it may be referencing something yet to be done). If `PendingValue` persists to the end, it indicates a failure to handle circular references in the tag code. This approach was taken because one of the earlier prototype implementations did something like this, passing dict and list subclasses that would throw an exception if a `PendingValue` element was accessed. That would have been more friendly to extension developers, but it was discarded because it wasn’t thought it was worth turning all those high performance containers into slower asdf subclasses. We may want to revisit this if we decide to implement a tree that tracks “dirty” nodes and only writes to disk those that have changed, since in that case we’ll need custom container subclasses anyway. We could also consider writing our own dict/list subclass in C so we could have our cake and eat it too.

The `walk_and_modify` code handles the case where the tag code returns a generator instead of a value. This generator is expected to be a similar kind of generator to what pyyaml uses, but differing in that instead of returning an empty container object it will populate whatever elements it can complete (e.g, all non-mutable ones), and complete the population of all the mutable members on the second iteration (which may, in turn, generate new generators for mutable elements contained within). When it detects a generator, the `walk_and_modify` code retrieves the first yielded value, then saves the generator in the context. When the top level of the context is reached (it handles nesting by indicating how many times it has been entered as a context), it starts “draining” the saved generators by doing the second iteration on them. Like pyyaml, this second iteration may produce yet more generators that get saved, and thus keeps iterating on the saved generators until none are left.

It is not possible to construct reference cycles in immutable objects within pure Python code, and thus the generators are only needed for mutable constructs (e.g., dicts and lists).

Historical note: versions of the ASDF library prior to 2.6.0 required tag code when converting from a tagged object to a custom object to call `tagged_tree_to_custom_tree` on any values of attributes that may be arbitrarily nested objects. That no longer is needed with the latest code since any attribute that contains a mapping or sequence object automatically uses a generator, so population of that attribute is automatically deferred until the context is exited. Thus there is no need to explicitly call a function to populate it.

More explicitly, the `_recurse` function defined within `walk_and_modify` (in this postorder case) calls `_handle_children()` on the node in question first. If the node contains children, they are each fed back into `_recurse` and transformed into their final objects. A new node is populated with these transformed children, and that is the node that gets handed to `tag.from_tree_tagged()`. The effect is that the tag class receives a structure containing only transformed children, so it has no need to call `tagged_tree_to_custom_tree` on its own.

## Future plans for SerializationContext

Currently, the `AsdfFile` itself is used as a container for serialization parameters and is passed to various methods in block.py, reference.py, schema.py, yamlutil.py, in `ExtensionType` subclasses, and others. This doesn’t work very well for a couple of reasons. For one, the intention of `AsdfFile.write_to` is to “export” a copy of the file to disk without changing the in-memory `AsdfFile`, but since serialization parameters are read from the `AsdfFile`, the code currently modifies the open file as part of the write (and doesn’t change it back). The second issue is that requiring an `AsdfFile` instance in so many method signatures forces the code (or users themselves) to create an empty dummy `AsdfFile` just to use the method.

The new `Converter` interface also accepts a `ctx` variable, but instead of an `AsdfFile` it’s an instance of `asdf.extension.SerializationContext`. This new object will serve the purpose of configuring serialization parameters and keeping necessary state, which means that the `AsdfFile` can go unmodified. The `asdf.extension.SerializationContext` will be relatively lightweight and creating it will not incur as much of a performance penalty as creating an `AsdfFile`.