Rewrite serialization to allow access to byte form

This lets you bypass a file completely if using an
object store.

python/lsst/base/packages.py

@@ -257,6 +257,10 @@ class Packages:
     This is essentially a wrapper around a dict with some conveniences.
     """
 
+    formats = {".pkl": "pickle",
+               ".pickle": "pickle",
+               ".yaml": "yaml"}
+
     def __init__(self, packages):
         assert isinstance(packages, Mapping)
         self._packages = packages
@@ -279,6 +283,24 @@ def fromSystem(cls):
         packages.update(getEnvironmentPackages())  # Should be last, to override products with LOCAL versions
         return cls(packages)
 
+    @classmethod
+    def fromBytes(cls, data, format):
+        """Construct the object from a byte representation.
+
+        Parameters
+        ----------
+        data : `bytes`
+            The serialized form of this object in bytes.
+        format : `str`
+            The format of those bytes. Can be ``yaml`` or ``pickle``.
+        """
+        if format == "pickle":
+            return pickle.loads(data)
+        elif format == "yaml":
+            return yaml.load(data, Loader=yaml.SafeLoader)
+        else:
+            raise ValueError(f"Unexpected serialization format given: {format}")
+
     @classmethod
     def read(cls, filename):
         """Read packages from filename.
@@ -295,14 +317,32 @@ def read(cls, filename):
         packages : `Packages`
         """
         _, ext = os.path.splitext(filename)
-        if ext in (".pickle", ".pkl"):
-            with open(filename, "rb") as ff:
-                return pickle.load(ff)
-        elif ext == ".yaml":
-            with open(filename, "r") as ff:
-                return yaml.load(ff, Loader=yaml.SafeLoader)
+        if ext not in cls.formats:
+            raise ValueError(f"Format from {ext} extension in file {filename} not recognized")
+        with open(filename, "rb") as ff:
+            data = ff.read()
+        return cls.fromBytes(data, cls.formats[ext])
+
+    def toBytes(self, format):
+        """Convert the object to a serialized bytes form using the
+        specified format.
+
+        Parameters
+        ----------
+        format : `str`
+            Format to use when serializing. Can be ``yaml`` or ``pickle``.
+
+        Returns
+        -------
+        data : `bytes`
+            Byte string representing the serialized object.
+        """
+        if format == "pickle":
+            return pickle.dumps(self)
+        elif format == "yaml":
+            return yaml.dump(self).encode("utf-8")
         else:
-            raise ValueError(f"Unable to determine how to read file {filename} from extension {ext}")
+            raise ValueError(f"Unexpected serialization format requested: {format}")
 
     def write(self, filename):
         """Write to file.
@@ -315,14 +355,10 @@ def write(self, filename):
             ``.pickle`` and ``.yaml``
         """
         _, ext = os.path.splitext(filename)
-        if ext in (".pickle", ".pkl"):
-            with open(filename, "wb") as ff:
-                pickle.dump(self, ff)
-        elif ext == ".yaml":
-            with open(filename, "w") as ff:
-                yaml.dump(self, ff)
-        else:
-            raise ValueError(f"Unexpected file format requested: {ext}")
+        if ext not in self.formats:
+            raise ValueError(f"Format from {ext} extension in file {filename} not recognized")
+        with open(filename, "wb") as ff:
+            ff.write(self.toBytes(self.formats[ext]))
 
     def __len__(self):
         return len(self._packages)

tests/test_packages.py

@@ -134,6 +134,18 @@ def testPackages(self):
         self.assertDictEqual(packages.extra(new), {})
         self.assertEqual(len(packages), len(new))
 
+        # Serialize via bytes
+        for format in ("pickle", "yaml"):
+            asbytes = new.toBytes(format)
+            from_bytes = lsst.base.Packages.fromBytes(asbytes, format)
+            self.assertEqual(from_bytes, new)
+
+        with self.assertRaises(ValueError):
+            new.toBytes("unknown_format")
+
+        with self.assertRaises(ValueError):
+            lsst.base.Packages.fromBytes(from_bytes, "unknown_format")
+
 
 if __name__ == "__main__":
     unittest.main()