Make 'Blob.bucket' a readonly property. (#9113)

Closes #9107.

storage/google/cloud/storage/blob.py

@@ -172,7 +172,7 @@ def __init__(
         super(Blob, self).__init__(name=name)
 
         self.chunk_size = chunk_size  # Check that setter accepts value.
-        self.bucket = bucket
+        self._bucket = bucket
         self._acl = ObjectACL(self)
         if encryption_key is not None and kms_key_name is not None:
             raise ValueError(
@@ -187,6 +187,15 @@ def __init__(
         if generation is not None:
             self._properties["generation"] = generation
 
+    @property
+    def bucket(self):
+        """Bucket which contains the object.
+
+        :rtype: :class:`~google.cloud.storage.bucket.Bucket`
+        :returns: The object's bucket.
+        """
+        return self._bucket
+
     @property
     def chunk_size(self):
         """Get the blob's default chunk size.

storage/tests/unit/test_blob.py

@@ -274,6 +274,14 @@ def test_path_with_non_ascii(self):
         blob = self._make_one(blob_name, bucket=bucket)
         self.assertEqual(blob.path, "/b/name/o/Caf%C3%A9")
 
+    def test_bucket_readonly_property(self):
+        blob_name = "BLOB"
+        bucket = _Bucket()
+        other = _Bucket()
+        blob = self._make_one(blob_name, bucket=bucket)
+        with self.assertRaises(AttributeError):
+            blob.bucket = other
+
     def test_client(self):
         blob_name = "BLOB"
         bucket = _Bucket()