This library maps XML to and from Python dataclasses. It build on normal dataclasses from the standard library and uses lxml for parsing/generating XML.
It's currently in alpha. It isn't ready for production if you aren't willing to do your own evaluation/quality assurance.
Requires Python 3.7 or higher.
- Convert XML documents to well-defined dataclasses, which work with Mypy or IDE auto-completion
- XML dataclasses are dataclasses
- Full control of parsing and generating XML via
lxml - Loading and dumping of attributes, child elements, and text content
- Required and optional attributes/child elements
- Lists of child elements are supported, as are unions and lists or unions
- Inheritance does work, but has the same limitations as dataclasses. Inheriting from base classes with required fields and declaring optional fields doesn't work due to field order. This isn't recommended
- Namespace support is decent as long as correctly declared. I've tried on several real-world examples, although they were known to be valid.
lxmldoes a great job at expanding namespace information when loading and simplifying it when saving - Post-load validation hook
xml_validate - Fields not required in the constructor are ignored by this library (via
ignored()orinit=False)
- Whitespace and comments aren't supported in the data model. They must be stripped when loading the XML
- So far, I haven't found any examples where XML can't be mapped to a dataclass, but it's likely possible given how complex XML is
- No typing/type conversions. Since XML is untyped, only string values are currently allowed. Type conversions are tricky to implement in a type-safe and extensible manner.
- Dataclasses must be written by hand, no tools are provided to generate these from, DTDs, XML schema definitions, or RELAX NG schemas
The caveats concerning untrusted content are roughly the same as with lxml, since that does the parsing. This is good, since lxml's behaviour to XML attacks are well-understood. This library recursively resolves data structures, which may have memory implications for unbounded payloads. Because loading is driven from the dataclass definitions, it shouldn't be possible to execute arbitrary Python code (not a guarantee, see license). If you must deal with untrusted content, a workaround is to use lxml to validate untrusted content with a strict schema, which you may already be doing.
Attributes can be either str or Optional[str]. Using any other type won't work. Attributes can be renamed or have their namespace modified via the rename function. It can be used either on its own, or with an existing field definition:
@xml_dataclass
class Foo:
__ns__ = None
required: str
optional: Optional[str] = None
renamed_with_default: str = rename(default=None, name="renamed-with-default")
namespaced: str = rename(ns="http://www.w3.org/XML/1998/namespace")
existing_field: str = rename(field(...), name="existing-field")For now, you can work around this limitation with properties that do the conversion, and perform post-load validation.
By default, unknown attributes raise an error. This can be disabled by passing Options to load with ignore_unknown_attributes.
Like attributes, text can be either str or Optional[str]. You must declare text content with the text function. Similar to rename, this function can use an existing field definition, or take the default argument. Text cannot be renamed or namespaced. Every class can only have one field defining text content. If a class has text content, it cannot have any children.
@xml_dataclass
class Foo:
__ns__ = None
value: str = text()
@xml_dataclass
class Foo:
__ns__ = None
content: Optional[str] = text(default=None)
@xml_dataclass
class Foo:
__ns__ = None
uuid: str = text(field(default_factory=lambda: str(uuid4())))Children must ultimately be other XML dataclasses. However, they can also be Optional, List, and Union types:
Optionalmust be at the top level. Valid:Optional[List[XmlDataclass]]. Invalid:List[Optional[XmlDataclass]]- Next,
Listshould be defined (if multiple child elements are allowed). Valid:List[Union[XmlDataclass1, XmlDataclass2]]. Invalid:Union[List[XmlDataclass1], XmlDataclass2] - Finally, if
OptionalorListwere used, a union type should be the inner-most (again, if needed)
If a class has children, it cannot have text content.
Children can be renamed via the rename function. However, attempting to set a namespace is invalid, since the namespace is provided by the child type's XML dataclass. Also, unions of XML dataclasses must have the same namespace (you can use different fields with renaming if they have different namespaces, since the XML names will be resolved as a combination of namespace and name).
By default, unknown children raise an error. This can be disabled by passing Options to load with ignore_unknown_children.
Simply implement an instance method called xml_validate with no parameters, and no return value (if you're using type hints):
def xml_validate(self) -> None:
passIf defined, the load function will call it after all values have been loaded and assigned to the XML dataclass. You can validate the fields you want inside this method. Return values are ignored; instead raise and catch exceptions.
Fields not required in the constructor are ignored by this library (new in version 0.0.6). This is useful if you want to populate a field via post-load validation.
You can simply set init=False, although you may also want to exclude the field from comparisons. The ignored function does this, and can also be used.
The name doesn't matter, but it might be useful to use the _ prefix as a convention.
(This is a simplified real world example - the container can also include optional links child elements.)
<?xml version="1.0"?>
<container version="1.0" xmlns="urn:oasis:names:tc:opendocument:xmlns:container">
<rootfiles>
<rootfile full-path="OEBPS/content.opf" media-type="application/oebps-package+xml" />
</rootfiles>
</container>from dataclasses import dataclass
from typing import List
from lxml import etree # type: ignore
from xml_dataclasses import xml_dataclass, rename, load, dump, NsMap, XmlDataclass
CONTAINER_NS = "urn:oasis:names:tc:opendocument:xmlns:container"
@xml_dataclass
@dataclass
class RootFile:
__ns__ = CONTAINER_NS
full_path: str = rename(name="full-path")
media_type: str = rename(name="media-type")
@xml_dataclass
@dataclass
class RootFiles:
__ns__ = CONTAINER_NS
rootfile: List[RootFile]
# see Gotchas, this workaround is required for type hinting
@xml_dataclass
@dataclass
class Container(XmlDataclass):
__ns__ = CONTAINER_NS
version: str
rootfiles: RootFiles
# WARNING: this is an incomplete implementation of an OPF container
def xml_validate(self) -> None:
if self.version != "1.0":
raise ValueError(f"Unknown container version '{self.version}'")
if __name__ == "__main__":
nsmap: NsMap = {None: CONTAINER_NS}
# see Gotchas, stripping whitespace and comments is highly recommended
parser = etree.XMLParser(remove_blank_text=True, remove_comments=True)
lxml_el_in = etree.parse("container.xml", parser).getroot()
container = load(Container, lxml_el_in, "container")
lxml_el_out = dump(container, "container", nsmap)
print(etree.tostring(lxml_el_out, encoding="unicode", pretty_print=True))This can be a real pain to get right. Unfortunately, if you need this, you may have to resort to:
@xml_dataclass
@dataclass
class Child:
__ns__ = None
pass
@xml_dataclass
@dataclass
class Parent(XmlDataclass):
__ns__ = None
children: ChildIt's important that @dataclass be the last decorator, i.e. the closest to the class definition (and so the first to be applied). Luckily, only the root class you intend to pass to load/dump has to inherit from XmlDataclass, but all classes should have the @dataclass decorator applied.
If you are able to, it is strongly recommended you strip whitespace and comments from the input via lxml:
parser = etree.XMLParser(remove_blank_text=True, remove_comments=True)By default, lxml preserves whitespace. This can cause a problem when checking if elements have no text. The library does attempt to strip these; literally via Python's strip(). But lxml is likely faster and more robust.
Similarly, comments are included by default, and because loading is strict, they will be considered as nodes that the dataclass has not declared. It is recommended to omit them during parsing.
On dataclasses, optional fields also usually have a default value to be useful. But this isn't required; Optional is just a type hint to say None is allowed. This would occur e.g. if an element has no children.
For loading XML dataclasses, whether or not a field is required is determined by if it has a default/default_factory defined. If so, and it's missing, that default is used. Otherwise, an error is raised.
For dumping, the default isn't considered. Instead, if a value is marked as Optional and the value is None, it isn't written.
This makes sense in many cases, but possibly not every case.
- Stringified type hints and postponed annotations should now resolve correctly.
- Allow
NsMapto beNone/optional - thanks bphunter1972!
- Fix issue passing options when loading children - thanks tim-lansen!
- Warn if comments are found/don't treat comments as child elements in error messages
- Allow lenient loading of undeclared attributes or children
- Allow ignored fields via
init=falseor theignoredfunction
- Fixed type hinting for consumers. While the library passed mypy validation, it was hard to get XML dataclasses in a codebase to pass mypy validation
- Improved type resolving. This lead to easier field definitions, as
attrandchildare no longer needed because the type of the field is inferred
- Added support for union types on children
This project uses pre-commit to run some linting hooks when committing. When you first clone the repo, please run:
pre-commit install
You may also run the hooks at any time:
pre-commit run --all-files
Dependencies are managed via poetry. To install all dependencies, use:
poetry install
This will also install development dependencies such as black, isort, pylint, mypy, and pytest. Pre-defined tasks make it easy to run these, for example
poetry run task lint- this runsblack,isort,mypy, andpylintpoetry run task test- this runspytestwith coverage
For a full list of tasks, see poetry run task --list.
This library is licensed under the Mozilla Public License Version 2.0. For more information, see LICENSE.