7.11 whichdb -- Guess which DBM module created a database

The single function in this module attempts to guess which of the several simple database modules available-dbm, gdbm, or dbhash-should be used to open a given file.

Returns one of the following values: None if the file can't be opened because it's unreadable or doesn't exist; the empty string ('') if the file's format can't be guessed; or a string containing the required module name, such as 'dbm' or 'gdbm'.
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