feat: add file default value

beet/core/file.py

@@ -43,6 +43,10 @@ class File(Generic[ValueType, SerializeType]):
     content: Union[ValueType, SerializeType, None] = None
     source_path: Optional[FileSystemPath] = None
 
+    def __post_init__(self):
+        if self.content is self.source_path is None:
+            self.content = self.default()
+
     def merge(self: FileType, other: FileType) -> bool:
         """Merge the given file or return False to indicate no special handling."""
         return False
@@ -65,8 +69,8 @@ def ensure_source_path(self) -> FileSystemPath:
         if self.source_path:
             return self.source_path
         raise ValueError(
-            f"{self.__class__.__name__} object must be initialized with "
-            "either a value, raw bytes or a source path."
+            f"Expected {self.__class__.__name__} object to be initialized with "
+            "a source path."
         )
 
     def ensure_serialized(self) -> SerializeType:
@@ -91,6 +95,14 @@ def __eq__(self, other: Any) -> bool:
             or self.ensure_deserialized() == other.ensure_deserialized()
         )
 
+    @classmethod
+    def default(cls) -> ValueType:
+        """Return the file's default value."""
+        raise ValueError(
+            f"{cls.__name__} object must be initialized with "
+            "either a value, serialized data, or a source path."
+        )
+
     @classmethod
     def serialize(cls, content: Union[ValueType, SerializeType]) -> SerializeType:
         """Serialize file content."""
@@ -264,6 +276,10 @@ class JsonFileBase(TextFileBase[ValueType]):
 
     data: FileDeserialize[ValueType] = FileDeserialize()
 
+    @classmethod
+    def default(cls) -> ValueType:
+        return {}  # type: ignore
+
     @classmethod
     def to_str(cls, content: ValueType) -> str:
         return dump_json(content)

beet/library/data_pack.py

@@ -43,7 +43,7 @@
     TextFileBase,
     TextFileContent,
 )
-from beet.core.utils import extra_field
+from beet.core.utils import JsonDict, extra_field
 
 from .base import Namespace, NamespaceFile, NamespacePin, NamespaceProxyDescriptor, Pack
 
@@ -69,6 +69,10 @@ class Function(TextFileBase[List[str]], NamespaceFile):
 
     lines = FileDeserialize()  # type: FileDeserialize[List[str]]
 
+    @classmethod
+    def default(cls) -> List[str]:
+        return []
+
     @classmethod
     def to_str(cls, content: List[str]) -> str:
         return "\n".join(content) + "\n"
@@ -147,6 +151,10 @@ def merge(self: TagFileType, other: TagFileType) -> bool:  # type: ignore
                 values.append(deepcopy(value))
         return True
 
+    @classmethod
+    def default(cls) -> JsonDict:
+        return {"values": []}
+
 
 class BlockTag(TagFile):
     """Class representing a block tag."""