Remove hardcoded "json" strings

Additionally, add support for other file extenntions in the
Timestamp class.

Signed-off-by: Martin Vrachev <[email protected]>

Author: MVrachev

tuf/api/metadata.py

@@ -500,15 +500,15 @@ def __eq__(self, other: "MetadataInfo") -> bool:
 
     def to_dict(self) -> Dict:
         """Returns the dictionary representation of self. """
-        json_dict = {"version": self.version}
+        res_dict = {"version": self.version}
 
         if self.length is not None:
-            json_dict["length"] = self.length
+            res_dict["length"] = self.length
 
         if self.hashes is not None:
-            json_dict["hashes"] = self.hashes
+            res_dict["hashes"] = self.hashes
 
-        return json_dict
+        return res_dict
 
 
 class Timestamp(Signed):
@@ -534,22 +534,41 @@ def __init__(
         super().__init__(_type, version, spec_version, expires)
         self.meta = meta
 
+    @classmethod
+    def _get_snapshot_filename(cls, meta_dict):
+        """Get the snapshot file name from the metadata dict.
+        The snapshot file could be have a custom extension rather than JSON.
+        See the tuf/api/serialization folder."""
+        meta_files = list(meta_dict.keys())
+        if len(meta_files) != 1:
+            raise ValueError(
+                f"the meta_dict should contain information about the"
+                f"snapshot file only, but got {meta_dict}"
+            )
+        return meta_files[0]
+
     @classmethod
     def from_dict(cls, timestamp_dict: Mapping[str, Any]) -> "Timestamp":
         """Creates Timestamp object from its dict representation. """
         common_args = cls._common_fields_from_dict(timestamp_dict)
+        snapshot_filename = cls._get_snapshot_filename(timestamp_dict["meta"])
         meta = {
-            "snapshot.json": MetadataInfo(
-                **timestamp_dict["meta"]["snapshot.json"]
+            snapshot_filename: MetadataInfo(
+                **timestamp_dict["meta"][snapshot_filename]
             )
         }
         return cls(*common_args, meta)
 
     def to_dict(self) -> Dict[str, Any]:
         """Returns the dict representation of self. """
         timestamp_dict = self._common_fields_to_dict()
+        snapshot_filename = self._get_snapshot_filename(self.meta)
         timestamp_dict.update(
-            {"meta": {"snapshot.json": self.meta["snapshot.json"].to_dict()}}
+            {
+                "meta": {
+                    snapshot_filename: self.meta[snapshot_filename].to_dict()
+                }
+            }
         )
         return timestamp_dict
 
@@ -563,7 +582,8 @@ def update(
         """Assigns passed info about snapshot metadata to meta dict. """
         # TODO: Consider renaming this function:
         # see: https://github.com/theupdateframework/tuf/issues/1230
-        self.meta["snapshot.json"] = MetadataInfo(version, length, hashes)
+        snapshot_filename = self._get_snapshot_filename(self.meta)
+        self.meta[snapshot_filename] = MetadataInfo(version, length, hashes)
 
 
 class Snapshot(Signed):
@@ -668,13 +688,13 @@ def __eq__(self, other: "TargetInfo") -> bool:
         )
 
     def to_dict(self) -> Dict:
-        """Returns the JSON-serializable dictionary representation of self. """
-        json_dict = {"length": self.length, "hashes": self.hashes}
+        """Returns the dictionary representation of self. """
+        res_dict = {"length": self.length, "hashes": self.hashes}
 
         if self.custom is not None:
-            json_dict["custom"] = self.custom
+            res_dict["custom"] = self.custom
 
-        return json_dict
+        return res_dict
 
 
 class Targets(Signed):