# AsdfExtension

class `asdf.extension.``AsdfExtension`

  Bases: `object`

  Abstract base class defining an extension to ASDF.

  Attributes Summary

  `tag_mapping` | A list of 2-tuples or callables mapping YAML tag prefixes to JSON Schema URL prefixes.
  `types` | A list of `asdf.CustomType` subclasses that describe how to store custom objects to and from ASDF.
  `url_mapping` | A list of 2-tuples or callables mapping JSON Schema URLs to other URLs.

  Attributes Documentation

  `tag_mapping`

    A list of 2-tuples or callables mapping YAML tag prefixes to JSON Schema URL prefixes.

    For each entry:

    - If a 2-tuple, the first part of the tuple is a YAML tag prefix to match. The second part is a string, where case the following are available as Python formatting tokens:
      - `{tag}`: the complete YAML tag.
      - `{tag_suffix}`: the part of the YAML tag after the matched prefix.
      - `{tag_prefix}`: the matched YAML tag prefix.
    - If a callable, it is passed the entire YAML tag must return the entire JSON schema URL if it matches, otherwise, return `None`.

    Note that while JSON Schema URLs uniquely define a JSON Schema, they do not have to actually exist on an HTTP server and be fetchable (much like XML namespaces).

    For example, to match all YAML tags with the `tag:nowhere.org:custom` prefix to the ``http://nowhere.org/schemas/custom/` URL prefix:

        return [('tag:nowhere.org:custom/',
                 'http://nowhere.org/schemas/custom/{tag_suffix}')]

  `types`

    A list of `asdf.CustomType` subclasses that describe how to store custom objects to and from ASDF.

  `url_mapping`

    A list of 2-tuples or callables mapping JSON Schema URLs to other URLs. This is useful if the JSON Schemas are not actually fetchable at their corresponding URLs but are on the local filesystem, or, to save bandwidth, we have a copy of fetchable schemas on the local filesystem. If neither is desirable, it may simply be the empty list.

    For each entry:

    - If a 2-tuple, the first part is a URL prefix to match. The second part is a string, where the following are available as Python formatting tokens:
      - `{url}`: The entire JSON schema URL
      - `{url_prefix}`: The matched URL prefix
      - `{url_suffix}`: The part of the URL after the prefix.
    - If a callable, it is passed the entire JSON Schema URL and must return a resolvable URL pointing to the schema content. If it doesn’t match, should return `None`.

    For example, to map a remote HTTP URL prefix to files installed alongside as data alongside Python module:

        return [('http://nowhere.org/schemas/custom/1.0.0/',
                asdf.util.filepath_to_url(
                    os.path.join(SCHEMA_PATH, 'stsci.edu')) +
                '/{url_suffix}.yaml'
               )]