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Unlike to other dbm implementations, a dumpdbm database is always opened for update, and will be created if it does not exist. We can fix this discrepancy and implement common behavior for 'r' and 'w' modes, but this will change current behavior and some deprecation period is needed.
Here is a patch (based on patch by Claudiu Popa, submitted in issue18039), which adds deprecation warnings for cases when current behavior differs from desired: invalid values for the flag parameter (will raise ValueError in future), opening non-existing database in 'r' or 'w' mode (will raise dbm.dumb.error), modifying a database opened for reading only (will raise dbm.dumb.error).
This patch needs approving by other core developer.