"""
Support for Converter, the new API for serializing custom
types. Will eventually replace the `asdf.types` module.
"""
import abc
import warnings
from asdf.exceptions import AsdfWarning
from asdf.util import get_class_name, uri_match
[docs]
class Converter(abc.ABC):
"""
Abstract base class for plugins that convert nodes from the
parsed YAML tree into custom objects, and vice versa.
Implementing classes must provide the `tags` and `types`
properties and `to_yaml_tree` and `from_yaml_tree` methods.
The ``select_tag`` method is optional.
If implemented, ``select_tag`` should accept 3 parameters
obj : object
Instance of the custom type being converted. Guaranteed
to be an instance of one of the types listed in the
`types` property.
tags : list of str
List of active tags to choose from. Guaranteed to match
one of the tag patterns listed in the 'tags' property.
ctx : asdf.asdf.SerializationContext
Context of the current serialization request.
and return a str, the selected tag (should be one of tags) or
`None` which will trigger the result of ``to_yaml_tree`` to be
used to look up the next converter for this object.
"""
@classmethod
def __subclasshook__(cls, class_):
if cls is Converter:
return (
hasattr(class_, "tags")
and hasattr(class_, "types")
and hasattr(class_, "to_yaml_tree")
and hasattr(class_, "from_yaml_tree")
)
return NotImplemented # pragma: no cover
@property
@abc.abstractmethod
def tags(self):
"""
Get the YAML tags that this converter is capable of
handling. URI patterns are permitted, see
`asdf.util.uri_match` for details.
Returns
-------
iterable of str
Tag URIs or URI patterns.
"""
@property
@abc.abstractmethod
def types(self):
"""
Get the Python types that this converter is capable of
handling.
Returns
-------
iterable of str or type
If str, the fully qualified class name of the type.
"""
[docs]
@abc.abstractmethod
def to_yaml_tree(self, obj, tag, ctx):
"""
Convert an object into a node suitable for YAML serialization.
This method is not responsible for writing actual YAML; rather, it
converts an instance of a custom type to a built-in Python object type
(such as dict, list, str, or number), which can then be automatically
serialized to YAML as needed.
For container types returned by this method (dict or list),
the children of the container need not themselves be converted.
Any list elements or dict values will be converted by subsequent
calls to to_yaml_tree implementations.
The returned node must be an instance of `dict`, `list`, or `str`.
Children may be any type supported by an available Converter.
Parameters
----------
obj : object
Instance of a custom type to be serialized. Guaranteed to
be an instance of one of the types listed in the `types`
property.
tag : str
The tag identifying the YAML type that ``obj`` should be
converted into. Selected by a call to this converter's
select_tag method.
ctx : asdf.extension.SerializationContext
The context of the current serialization request.
Returns
-------
dict or list or str
The YAML node representation of the object.
"""
[docs]
@abc.abstractmethod
def from_yaml_tree(self, node, tag, ctx):
"""
Convert a YAML node into an instance of a custom type.
For container types received by this method (dict or list),
the children of the container will have already been converted
by prior calls to from_yaml_tree implementations.
Note on circular references: trees that reference themselves
among their descendants must be handled with care. Most
implementations need not concern themselves with this case, but
if the custom type supports circular references, then the
implementation of this method will need to return a generator.
Consult the documentation for more details.
Parameters
----------
node : dict or list or str
The YAML node to convert.
tag : str
The YAML tag of the object being converted.
ctx : asdf.extension.SerializationContext
The context of the current deserialization request.
Returns
-------
object
An instance of one of the types listed in the `types` property,
or a generator that yields such an instance.
"""
[docs]
class ConverterProxy(Converter):
"""
Proxy that wraps a `Converter` and provides default
implementations of optional methods.
"""
def __init__(self, delegate, extension):
if not isinstance(delegate, Converter):
msg = "Converter must implement the asdf.extension.Converter interface"
raise TypeError(msg)
self._delegate = delegate
self._extension = extension
self._class_name = get_class_name(delegate)
# Sort these out up-front so that errors are raised when the extension is loaded
# and not in the middle of the user's session. The extension will fail to load
# and a warning will be emitted, but it won't crash the program.
relevant_tags = set()
for tag in delegate.tags:
if isinstance(tag, str):
relevant_tags.update(t.tag_uri for t in extension.tags if uri_match(tag, t.tag_uri))
else:
msg = "Converter property 'tags' must contain str values"
raise TypeError(msg)
if len(relevant_tags) > 1 and not hasattr(delegate, "select_tag"):
# we cannot use isinstance here because Converter supports
# virtual subclasses
if Converter in delegate.__class__.__mro__:
# prior to asdf 3.0 Converter provided a default select_tag
# to provide backwards compatibility allow Converter subclasses
# to be registered with >1 tag but produce a warning
msg = (
"Converter handles multiple tags for this extension, "
"but does not implement a select_tag method. "
"This previously worked because Converter subclasses inherited "
"the now removed select_tag. This will be an error in a future "
"version of asdf"
)
warnings.warn(msg, AsdfWarning)
else:
msg = "Converter handles multiple tags for this extension, but does not implement a select_tag method."
raise RuntimeError(msg)
self._tags = sorted(relevant_tags)
self._types = []
if not len(self._tags) and not hasattr(delegate, "select_tag"):
# this converter supports no tags so don't inspect the types
return
for typ in delegate.types:
if isinstance(typ, (str, type)):
self._types.append(typ)
else:
msg = "Converter property 'types' must contain str or type values"
raise TypeError(msg)
@property
def tags(self):
"""
Get the list of tag URIs that this converter is capable of
handling.
Returns
-------
list of str
"""
return self._tags
@property
def types(self):
"""
Get the Python types that this converter is capable of
handling.
Returns
-------
list of type or str
"""
return self._types
[docs]
def select_tag(self, obj, ctx):
"""
Select the tag to use when converting an object to YAML.
Parameters
----------
obj : object
Instance of the custom type being converted.
ctx : asdf.extension.SerializationContext
Serialization parameters.
Returns
-------
str or None
Selected tag or `None` to defer conversion.
"""
method = getattr(self._delegate, "select_tag", None)
if method is None:
return self._tags[0]
return method(obj, self._tags, ctx)
[docs]
def to_yaml_tree(self, obj, tag, ctx):
"""
Convert an object into a node suitable for YAML serialization.
Parameters
----------
obj : object
Instance of a custom type to be serialized.
tag : str
The tag identifying the YAML type that ``obj`` should be
converted into.
ctx : asdf.extension.SerializationContext
Serialization parameters.
Returns
-------
object
The YAML node representation of the object.
"""
return self._delegate.to_yaml_tree(obj, tag, ctx)
[docs]
def from_yaml_tree(self, node, tag, ctx):
"""
Convert a YAML node into an instance of a custom type.
Parameters
----------
tree : dict or list or str
The YAML node to convert.
tag : str
The YAML tag of the object being converted.
ctx : asdf.extension.SerializationContext
Serialization parameters.
Returns
-------
object
"""
return self._delegate.from_yaml_tree(node, tag, ctx)
@property
def delegate(self):
"""
Get the wrapped converter instance.
Returns
-------
asdf.extension.Converter
"""
return self._delegate
@property
def extension(self):
"""
Get the extension that provided this converter.
Returns
-------
asdf.extension.ExtensionProxy
"""
return self._extension
@property
def package_name(self):
"""
Get the name of the Python package of this converter's
extension. This may not be the same package that implements
the converter's class.
Returns
-------
str or None
Package name, or `None` if the extension was added at runtime.
"""
return self.extension.package_name
@property
def package_version(self):
"""
Get the version of the Python package of this converter's
extension. This may not be the same package that implements
the converter's class.
Returns
-------
str or None
Package version, or `None` if the extension was added at runtime.
"""
return self.extension.package_version
@property
def class_name(self):
"""
Get the fully qualified class name of this converter.
Returns
-------
str
"""
return self._class_name
def __eq__(self, other):
if isinstance(other, ConverterProxy):
return other.delegate is self.delegate and other.extension is self.extension
return False
def __hash__(self):
return hash((id(self.delegate), id(self.extension)))
def __repr__(self):
package_description = "(none)" if self.package_name is None else f"{self.package_name}=={self.package_version}"
return f"<ConverterProxy class: {self.class_name} package: {package_description}>"