Using 'fields' in Blob.exists() to reduce payload size.

gcloud/storage/blob.py

@@ -28,6 +28,7 @@
 from _gcloud_vendor.apitools.base.py import transfer
 
 from gcloud.credentials import generate_signed_url
+from gcloud.exceptions import NotFound
 from gcloud.storage._helpers import _PropertyMixin
 from gcloud.storage._helpers import _scalar_property
 from gcloud.storage import _implicit_environ
@@ -174,7 +175,15 @@ def exists(self):
         :rtype: boolean
         :returns: True if the blob exists in Cloud Storage.
         """
-        return self.bucket.get_blob(self.name) is not None
+        try:
+            # We only need the status code (200 or not) so we seek to
+            # minimize the returned payload.
+            query_params = {'fields': 'name'}
+            self.connection.api_request(method='GET', path=self.path,
+                                        query_params=query_params)
+            return True
+        except NotFound:
+            return False
 
     def rename(self, new_name):
         """Renames this blob using copy and delete operations.

gcloud/storage/test_blob.py

@@ -216,15 +216,19 @@ def test_generate_signed_url_w_explicit_method(self):
         self.assertEqual(SIGNER._signed, [(EXPECTED_ARGS, EXPECTED_KWARGS)])
 
     def test_exists_miss(self):
+        from six.moves.http_client import NOT_FOUND
         NONESUCH = 'nonesuch'
-        connection = _Connection()
+        not_found_response = {'status': NOT_FOUND}
+        connection = _Connection(not_found_response)
         bucket = _Bucket(connection)
         blob = self._makeOne(NONESUCH, bucket=bucket)
         self.assertFalse(blob.exists())
 
     def test_exists_hit(self):
+        from six.moves.http_client import OK
         BLOB_NAME = 'blob-name'
-        connection = _Connection()
+        found_response = {'status': OK}
+        connection = _Connection(found_response)
         bucket = _Bucket(connection)
         blob = self._makeOne(BLOB_NAME, bucket=bucket)
         bucket._blobs[BLOB_NAME] = 1
@@ -245,8 +249,10 @@ def test_rename(self):
         self.assertTrue(NEW_NAME in bucket._blobs)
 
     def test_delete(self):
+        from six.moves.http_client import NOT_FOUND
         BLOB_NAME = 'blob-name'
-        connection = _Connection()
+        not_found_response = {'status': NOT_FOUND}
+        connection = _Connection(not_found_response)
         bucket = _Bucket(connection)
         blob = self._makeOne(BLOB_NAME, bucket=bucket)
         bucket._blobs[BLOB_NAME] = 1
@@ -1000,7 +1006,12 @@ def __init__(self, *responses):
         self.http = _HTTP(*responses)
 
     def api_request(self, **kw):
-        return self._respond(**kw)
+        from six.moves.http_client import NOT_FOUND
+        from gcloud.exceptions import NotFound
+        result = self._respond(**kw)
+        if result.get('status') == NOT_FOUND:
+            raise NotFound(result)
+        return result
 
     def build_api_url(self, path, query_params=None,
                       api_base_url=API_BASE_URL, upload=False):
@@ -1031,9 +1042,6 @@ def __init__(self, connection):
         self._blobs = {}
         self._deleted = []
 
-    def get_blob(self, blob_name):
-        return self._blobs.get(blob_name)
-
     def copy_blob(self, blob, destination_bucket, new_name):
         destination_bucket._blobs[new_name] = self._blobs[blob.name]
         return blob.__class__(None, bucket=destination_bucket,