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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _execute_select_commands(self, source, commands):
 """Execute select queries for all of the tables from a source database."""

        rows = {}
        for tbl, command in tqdm(commands, total=len(commands), desc='Executing {0} select queries'.format(source)):
            # Add key to dictionary
            if tbl not in rows:
                rows[tbl] = []
            rows[tbl].extend(self.fetch(command, commit=True))
        self._commit()
        return rows