This function should return current_record or else the documentation is incorrect.

Good catch @Bre77 - I think in this case the documentation needs to be updated, the current_record should be updated with the new field without the need to return since it's essentially pass-by-reference for this usage (I always go back to this article when I need to refresh my memory: https://robertheaton.com/2014/02/09/pythons-pass-by-object-reference-as-explained-by-philip-k-dick/).

So if you all agree can we fix the README.md above to the following?

class CustomStreamingCommand(StreamingCommand):
    def stream(self, records):
        for index, record in enumerate(records):
            if index % 1 == 0:
                self.add_field(record, "odd_record", "true")
            yield record

And to make sure that example we provide is working ^ that's probably a good implementation to add when adding tests for the StreamingCommand / add_field case

I've changed my implementation to using pass by reference (as per the suggested doco update) and everything works as expected.

Good catch @Bre77
I've updated the README accordingly.

Really nice demonstration of the issue!