Improve AttributeError message in XmlLayer when accessing missing fields

src/pyshark/packet/layers/xml_layer.py

@@ -8,28 +8,25 @@
 
 
 class XmlLayer(base.BaseLayer):
-    __slots__ = [
-        "raw_mode",
-        "_all_fields"
-    ] + base.BaseLayer.__slots__
+    __slots__ = ["raw_mode", "_all_fields"] + base.BaseLayer.__slots__
 
     def __init__(self, xml_obj=None, raw_mode=False):
-        super().__init__(xml_obj.attrib['name'])
+        super().__init__(xml_obj.attrib["name"])
         self.raw_mode = raw_mode
 
         self._all_fields = {}
 
         # We copy over all the fields from the XML object
         # Note: we don't read lazily from the XML because the lxml objects are very memory-inefficient
         # so we'd rather not save them.
-        for field in xml_obj.findall('.//field'):
+        for field in xml_obj.findall(".//field"):
             attributes = dict(field.attrib)
             field_obj = LayerField(**attributes)
-            if attributes['name'] in self._all_fields:
+            if attributes["name"] in self._all_fields:
                 # Field name already exists, add this field to the container.
-                self._all_fields[attributes['name']].add_field(field_obj)
+                self._all_fields[attributes["name"]].add_field(field_obj)
             else:
-                self._all_fields[attributes['name']] = LayerFieldsContainer(field_obj)
+                self._all_fields[attributes["name"]] = LayerFieldsContainer(field_obj)
 
     def get_field(self, name) -> typing.Union[LayerFieldsContainer, None]:
         """Gets the XML field object of the given name."""
@@ -43,7 +40,9 @@ def get_field(self, name) -> typing.Union[LayerFieldsContainer, None]:
                 return field
         return None
 
-    def get_field_value(self, name, raw=False) -> typing.Union[LayerFieldsContainer, None]:
+    def get_field_value(
+        self, name, raw=False
+    ) -> typing.Union[LayerFieldsContainer, None]:
         """Tries getting the value of the given field.
 
         Tries it in the following order: show (standard nice display), value (raw value),
@@ -65,48 +64,58 @@ def get_field_value(self, name, raw=False) -> typing.Union[LayerFieldsContainer,
     @property
     def field_names(self) -> typing.List[str]:
         """Gets all XML field names of this layer."""
-        return [self._sanitize_field_name(field_name) for field_name in self._all_fields]
+        return [
+            self._sanitize_field_name(field_name) for field_name in self._all_fields
+        ]
 
     @property
     def layer_name(self):
-        if self._layer_name == 'fake-field-wrapper':
+        if self._layer_name == "fake-field-wrapper":
             return base.DATA_LAYER_NAME
         return super().layer_name
 
     def __getattr__(self, item):
         val = self.get_field(item)
         if val is None:
-            raise AttributeError()
+            raise AttributeError(
+                f"Field '{item}' not found in layer '{self.layer_name}'. Available Fields: {', '.join(self.field_names)}"
+            )
         if self.raw_mode:
             return val.raw_value
         return val
 
     @property
     def _field_prefix(self) -> str:
         """Prefix to field names in the XML."""
-        if self.layer_name == 'geninfo':
-            return ''
-        return self.layer_name + '.'
+        if self.layer_name == "geninfo":
+            return ""
+        return self.layer_name + "."
 
     def _sanitize_field_name(self, field_name):
         """Sanitizes an XML field name
 
         An xml field might have characters which would make it inaccessible as a python attribute).
         """
-        field_name = field_name.replace(self._field_prefix, '')
-        return field_name.replace('.', '_').replace('-', '_').lower()
+        field_name = field_name.replace(self._field_prefix, "")
+        return field_name.replace(".", "_").replace("-", "_").lower()
 
     def _pretty_print_layer_fields(self, file: io.IOBase):
         for field_line in self._get_all_field_lines():
-            if ':' in field_line:
-                field_name, field_line = field_line.split(':', 1)
-                file.write(colored(field_name + ':', "green", attrs=["bold"]))
+            if ":" in field_line:
+                field_name, field_line = field_line.split(":", 1)
+                file.write(colored(field_name + ":", "green", attrs=["bold"]))
             file.write(colored(field_line, attrs=["bold"]))
 
     def _get_all_fields_with_alternates(self):
         all_fields = list(self._all_fields.values())
-        all_fields += sum([field.alternate_fields for field in all_fields
-                           if isinstance(field, LayerFieldsContainer)], [])
+        all_fields += sum(
+            [
+                field.alternate_fields
+                for field in all_fields
+                if isinstance(field, LayerFieldsContainer)
+            ],
+            [],
+        )
         return all_fields
 
     def _get_all_field_lines(self):
@@ -129,7 +138,9 @@ def _get_field_repr(self, field):
         elif field.raw_value:
             return f"{self._sanitize_field_name(field.name)}: {field.raw_value}"
 
-    def get_field_by_showname(self, showname) -> typing.Union[LayerFieldsContainer, None]:
+    def get_field_by_showname(
+        self, showname
+    ) -> typing.Union[LayerFieldsContainer, None]:
         """Gets a field by its "showname"
         This is the name that appears in Wireshark's detailed display i.e. in 'User-Agent: Mozilla...',
         'User-Agent' is the .showname