Simple post-load validation hook

Author: tobywf

README.md

@@ -1,6 +1,8 @@
 # XML dataclasses
 
-This is a very rough prototype of how a library might look like for (de)serialising XML into  Python dataclasses. XML dataclasses build on normal dataclasses from the standard library and [`lxml`](https://pypi.org/project/lxml/) elements. Loading and saving these elements is left to the consumer for flexibility of the desired output.
+[![License: MPL 2.0](https://img.shields.io/badge/License-MPL%202.0-brightgreen.svg)](https://opensource.org/licenses/MPL-2.0)
+
+This is a prototype of how a library might look like for (de)serialising XML into  Python dataclasses. XML dataclasses build on normal dataclasses from the standard library and [`lxml`](https://pypi.org/project/lxml/) elements. Loading and saving these elements is left to the consumer for flexibility of the desired output.
 
 It isn't ready for production if you aren't willing to do your own evaluation/quality assurance. I don't recommend using this library with untrusted content. It inherits all of `lxml`'s flaws with regards to XML attacks, and recursively resolves data structures. Because deserialisation is driven from the dataclass definitions, it shouldn't be possible to execute arbitrary Python code. But denial of service attacks would very likely be feasible.
 
@@ -46,10 +48,16 @@ class Container:
     rootfiles: RootFiles
     # WARNING: this is an incomplete implementation of an OPF container
 
+    def xml_validate(self):
+        if self.version != "1.0":
+            raise ValueError(f"Unknown container version '{self.version}'")
+
 
 if __name__ == "__main__":
     nsmap = {None: CONTAINER_NS}
-    lxml_el_in = etree.parse("container.xml").getroot()
+    # see Gotchas, stripping whitespace is highly recommended
+    parser = etree.XMLParser(remove_blank_text=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))
@@ -64,6 +72,7 @@ if __name__ == "__main__":
 * 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. `lxml` does a great job at expanding namespace information when loading and simplifying it when saving
+* Post-load validation hook `xml_validate`
 
 ## Patterns
 
@@ -117,6 +126,17 @@ Children can be renamed via the `rename` function, however attempting to set a n
 
 If a class has children, it cannot have text content.
 
+### Defining post-load validation
+
+Simply implement an instance method called `xml_validate` with no parameters, and no return value (if you're using type hints):
+
+```python
+def xml_validate(self) -> None:
+    pass
+```
+
+If 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.
+
 ## Gotchas
 
 ### Whitespace

functional/container_test.py

@@ -32,7 +32,10 @@ class Container:
     version: str
     rootfiles: RootFiles
     # WARNING: this is an incomplete implementation of an OPF container
-    # (it's missing links)
+
+    def xml_validate(self):
+        if self.version != "1.0":
+            raise ValueError(f"Unknown container version '{self.version}'")
 
 
 @pytest.mark.parametrize("remove_blank_text", [True, False])

src/xml_dataclasses/serde.py

@@ -153,6 +153,14 @@ def load(cls: Type[XmlDataclass], el: Any, name: Optional[str] = None) -> XmlDat
 
     instance = cls(**attr_values, **text_values, **child_values)  # type: ignore
     instance.__nsmap__ = el.nsmap
+
+    try:
+        validate_fn = instance.xml_validate  # type: ignore
+    except AttributeError:
+        pass
+    else:
+        validate_fn()
+
     return instance
 
 

tests/load_test.py

@@ -530,3 +530,21 @@ class Foo:
     foo = load(Foo, el, "foo")
     assert isinstance(foo.bar, Child1)
     assert foo.bar.spam == "eggs"
+
+
+def test_load_with_validation():
+    class MyError(Exception):
+        pass
+
+    @xml_dataclass
+    class Foo:
+        __ns__ = None
+        bar: str
+
+        def xml_validate(self) -> None:
+            if self.bar == "baz":
+                raise MyError()
+
+    el = etree.fromstring('<foo bar="baz" />')
+    with pytest.raises(MyError):
+        load(Foo, el, "foo")