SERVER_TIMESTAMP should survive deep copies (dataclasses.asdict)

[mgraczyk]

Problem

Currently SERVER_TIMESTAMP does not work if it has been copied via copy.deepcopy.
This is especially important for dataclasses.

Here is an example script to demonstrate the problem

from google.cloud.firestore_v1.transforms import Sentinel
from google.cloud.firestore_v1.transforms import SERVER_TIMESTAMP
import dataclasses

@dataclasses.dataclass
class Example:
  timestamp: type[SERVER_TIMESTAMP] = SERVER_TIMESTAMP

example = Example()


assert example.timestamp is SERVER_TIMESTAMP

asdict = dataclasses.asdict(example)
assert asdict['timestamp'] is SERVER_TIMESTAMP

Currently the second assertion fails because the Sentinel has been deep copied.

Traceback (most recent call last):
  File "/Users/michael/dev/rfp-tool/test.py", line 15, in <module>
    assert asdict['timestamp'] is SERVER_TIMESTAMP
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Solution

The Sentinel class should define dunder __copy__ and __deepcopy__ methods.

diff --git a/google/cloud/firestore_v1/transforms.py b/google/cloud/firestore_v1/transforms.py
index f1361c9..ef0def8 100644
--- a/google/cloud/firestore_v1/transforms.py
+++ b/google/cloud/firestore_v1/transforms.py
@@ -26,6 +26,15 @@ class Sentinel(object):
     def __repr__(self):
         return "Sentinel: {}".format(self.description)

+    def __copy__(self):
+        # Sentinel identity should be preserved across copies.
+        return self
+
+    def __deepcopy__(self):
+        # Sentinel identity should be preserved across deep copies.
+        return self
+
+

 DELETE_FIELD = Sentinel("Value used to delete a field in a document.")

I will send a PR attached to this issue.