Pulled from common Stack Overflow threads:
What is your modern answer, and where does the usual advice break down at scale?
BobaMilk
Use a | b on 3.9+ for a new dict and a |= b when mutation is fine, but the production trap is that both are shallow so nested state still aliases and big merges can spike memory.
merged = defaults | env | runtime
Sarah
{**a, **b} is the pre-3.9 fallback, but in production I care more about key collisions than syntax because silent overwrite can hide a bad config layer.
Ellen 
