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diff --git a/google/api_core/rest_helpers.py b/google/api_core/rest_helpers.py
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+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Helpers for rest transports."""
+
+import functools
+import operator
+
+
+def flatten_query_params(obj):
+    """Flatten a nested dict into a list of (name,value) tuples.
+
+    The result is suitable for setting query params on an http request.
+
+    .. code-block:: python
+
+        >>> obj = {'a':
+        ...         {'b':
+        ...           {'c': ['x', 'y', 'z']} },
+        ...      'd': 'uvw', }
+        >>> flatten_query_params(obj)
+        [('a.b.c', 'x'), ('a.b.c', 'y'), ('a.b.c', 'z'), ('d', 'uvw')]
+
+    Note that, as described in
+    https://github.com/googleapis/googleapis/blob/48d9fb8c8e287c472af500221c6450ecd45d7d39/google/api/http.proto#L117,
+    repeated fields (i.e. list-valued fields) may only contain primitive types (not lists or dicts).
+    This is enforced in this function.
+
+    Args:
+      obj: a nested dictionary (from json), or None
+
+    Returns: a list of tuples, with each tuple having a (possibly) multi-part name
+      and a scalar value.
+
+    Raises:
+      TypeError if obj is not a dict or None
+      ValueError if obj contains a list of non-primitive values.
+    """
+
+    if obj is not None and not isinstance(obj, dict):
+        raise TypeError("flatten_query_params must be called with dict object")
+
+    return _flatten(obj, key_path=[])
+
+
+def _flatten(obj, key_path):
+    if obj is None:
+        return []
+    if isinstance(obj, dict):
+        return _flatten_dict(obj, key_path=key_path)
+    if isinstance(obj, list):
+        return _flatten_list(obj, key_path=key_path)
+    return _flatten_value(obj, key_path=key_path)
+
+
+def _is_primitive_value(obj):
+    if obj is None:
+        return False
+
+    if isinstance(obj, (list, dict)):
+        raise ValueError("query params may not contain repeated dicts or lists")
+
+    return True
+
+
+def _flatten_value(obj, key_path):
+    return [(".".join(key_path), obj)]
+
+
+def _flatten_dict(obj, key_path):
+    items = (_flatten(value, key_path=key_path + [key]) for key, value in obj.items())
+    return functools.reduce(operator.concat, items, [])
+
+
+def _flatten_list(elems, key_path):
+    # Only lists of scalar values are supported.
+    # The name (key_path) is repeated for each value.
+    items = (
+        _flatten_value(elem, key_path=key_path)
+        for elem in elems
+        if _is_primitive_value(elem)
+    )
+    return functools.reduce(operator.concat, items, [])