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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 open(self, mode=MODE_READ):
 """ Opens this repo in the specified mode. TODO: figure out the correct semantics of this and document the intended future behaviour as well as the current transitional behaviour. """

        if mode not in [MODE_READ, MODE_WRITE]:
            error = "Open mode must be '{}' or '{}'".format(
                MODE_READ, MODE_WRITE)
            raise ValueError(error)
        self._openMode = mode
        if mode == MODE_READ:
            self.assertExists()
        if mode == MODE_READ:
            # This is part of the transitional behaviour where
            # we load the whole DB into memory to get access to
            # the data model.
            self.load()