The built-in extension

The ability to serialize the following types is provided by ASDF’s built-in extension:

The built-in extension is packaged with ASDF and is automatically used when reading and writing files. Users can not control the use of the built-in extension and in general they need not concern themselves with the details of its implementation. However, it is useful to be aware that the built-in extension is always in effect when reading and writing ASDF files.

Custom types

For the purposes of this documentation, a “custom type” is any data type that can not be serialized by the built-in extension.

In order for a particular custom type to be serialized, a special class called a “tag type” (or “tag” for short) must be implemented. Each tag type defines how the corresponding custom type will be serialized and deserialized. More details on how tag types are implemented can be found in Writing ASDF extensions. Users should never have to refer to tag implementations directly; they simply enable ASDF to recognize and process custom types.

In addition to tag types, each custom type must have a corresponding schema, which is used for validation. The definition of the schema is closely tied to the definition of the tag type. More details on schema validation can be found in Schema validation.

All schemas and their associated tag types have versions that move in sync. The version will change whenever a schemas (and therefore the tag type implementation) changes.


In order for the tag types and schemas to be used by ASDF, they must be packaged into an extension class. In general, the details of extensions are transparent to users of ASDF. However, users need to be aware of extensions in the following two scenarios:

  • when storing custom data types to files to be written
  • when reading files that contain custom data types

These scenarios require the use of custom extensions (the built-in extension is always used). There are two ways to use custom extensions, which are detailed below in Extensions from other packages and Explicit use of extensions.

Writing custom types to files

ASDF is not capable of serializing any custom type unless an extension is provided that defines how to serialize that type. Attempting to do so will cause an error when trying to write the file. For details on writing custom tag types and extensions, see Writing ASDF extensions.

Reading files with custom types

The ASDF software is capable of reading files that contain custom data types even if the extension that was used to create the file is not present. However, the extension is required in order to properly deserialize the original type.

If the necessary extension is not present, the custom data types will simply appear in the tree as a nested combination of basic data types. The structure of this data will mirror the structure of the schema used serialize the custom type.

In this case, a warning will occur by default to indicate to the user that the custom type in the file was not recognized and can not be deserialized. To suppress these warnings, users should pass ignore_unrecognized_tag=True to

Even if an extension for the custom type is present, it does not guarantee that the type can be deserialized successfully. Instantiating the custom type may involve additional software dependencies, which, if not present, will cause an error when the type is deserialized. Users should be aware of the dependencies that are required for instantiating custom types when reading ASDF files.

Custom types, extensions, and versioning

All tag types and schemas are versioned. This allows changes to tags and schemas to be recorded, and it allows ASDF to define behavior with respect to version compatibility.

Tag and schema versions may change for several reasons. One common reason is to reflect a change to the API of the custom type that a tag represents. This typically corresponds to an update to the version of the software that defines that custom type.

Since ASDF is designed to be an archival file format, it attempts to maintain backwards compatibility with all older tag and schema versions, at least when reading files. However, there are some caveats, which are described below.

Reading files

When ASDF encounters a tagged object in a file, it will compare the version of the tag in the file with the version of the corresponding tag type (if one is provided by an available extension).

In general, when reading files ASDF abides by the following principles:

  • If a tag type is available and its version matches that of the tag in the file, ASDF will return an instance of the original custom type.
  • If no corresponding tag type is found in any available extension, ASDF will return a basic data structure representing the type. A warning will occur unless the option ignore_unrecognized_tag=True was given. (see Reading files with custom types).
  • If a tag type is available but its version is older than that in the file (meaning that the file was written using a newer version of the tag type), ASDF will attempt to deserialize the tag using the existing tag type. If this fails, ASDF will return a basic data structure representing the type, and a warning will occur.
  • If a tag type is available but its version is newer than that in the file, ASDF will attempt to deserialize the tag using the existing tag type. If this fails, ASDF will return a basic data structure representing the type, and a warning will occur.

In cases where the available tag type version does not match the version of the tag in the file, warnings can be enabled by passing ignore_version_mismatch=False to These warnings are ignored by default.

Writing files

In general, ASDF makes no guarantee of being able to write older versions of tag types.

Explicit version support

Some tag types explicitly support reading only particular versions of the tag and schema (see asdf.CustomType.supported_versions). In these cases, deserialization is only possible if the version in the file matches one of the explicitly supported versions. Otherwise, ASDF will return a basic data structure representing the type, and a warning will occur.


While ASDF makes every attempt to deserialize stored objects even in the case of a tag version mismatch, deserialization will not always be possible. In most cases, if the versions do not match, ASDF will be able to return a basic data structure representing the original type.

However, tag version mismatches often indicate a mismatch between the versions of the software packages that define the type being serialized. In some cases, these version incompatibilities may lead to errors when attempting to read a file (especially when multiple tags/packages are involved). In these cases, the best course of action is to try to install the necessary versions of the packages (and extensions) involved.

Extensions from other packages

Some external packages may define extensions that allow ASDF to recognize some or all of the types that are defined by that package. Such packages may install the extension class as part of the package itself (details for developers can be found in Overriding built-in extensions).

If the package installs its extension, then ASDF will automatically detect the extension and use it when processing any files. No specific action is required by the user in order to successfully read and write custom types defined by the extension for that particular package.

Users can use the extensions command of the asdftool command line tool in order to determine which packages in the current Python environment have installed ASDF extensions:

$ asdftool extensions -s
Extension Name: 'bizbaz' (from bizbaz 1.2.3) Class:
Extension Name: 'builtin' (from asdf 2.0.0) Class: asdf.extension.BuiltinExtension

The output will always include the built-in extension, but may also display other extensions from other packages, depending on what is installed.

Explicit use of extensions

Sometimes no packaged extensions are provided for the types you wish to serialize. In this case, it is necessary to explicitly provide any necessary extension classes when reading and writing files that contain custom types.

Both and the AsdfFile constructor take an optional extensions keyword argument to control which extensions are used when reading or creating ASDF files.

Consider the following example where there exists a custom type MyCustomType that needs to be written to a file. An extension is defined MyCustomExtension that contains a tag type that can serialize and deserialize MyCustomType. Since MyCustomExtension is not installed by any package, we will need to pass it directly to the AsdfFile constructor:

import asdf


af = asdf.AsdfFile(extensions=MyCustomExtension())
af.tree = {'thing': MyCustomType('foo') }
# This call would cause an error if the proper extension was not
# provided to the constructor

Note that the extension class must actually be instantiated when it is passed as the extensions argument.

To read the file, we pass the same extension to

import asdf

af ='custom.asdf', extensions=MyCustomExtension())

If necessary, it is also possible to pass a list of extension instances to and the AsdfFile constructor:

extensions = [MyCustomExtension(), AnotherCustomExtension()]
af = asdf.AsdfFile(extensions=extensions)

Passing either a single extension instance or a list of extension instances to either or the AsdfFile constructor will not override any extensions that are installed in the environment. Instead, the custom types provided by the explicitly provided extensions will be added to the list of any types that are provided by installed extensions.

Extension checking

When writing ASDF files using this software, metadata about the extensions that were used to create the file will be added to the file itself. For extensions that were provided with another software package, the metadata includes the version of that package.

When reading files with extension metadata, ASDF can check whether the required extensions are present before processing the file. If a required extension is not present, or if the wrong version of a package that provides an extension is installed, ASDF will issue a warning.

It is possible to turn these warnings into errors by using the strict_extension_check parameter of If this parameter is set to True, then opening the file will fail if the required extensions are missing.