def _export_corpus(self):
        """
        Calls MALLET's `import-file` method.
        """
        # bin/mallet import-file --input /Users/erickpeirson/mycorpus_docs.txt
        #     --output mytopic-input.mallet --keep-sequence --remove-stopwords

        if not os.path.exists(self.mallet_bin):
            raise IOError("MALLET path invalid or non-existent.")
        self.input_path = os.path.join(self.temp, "input.mallet")

        exit = subprocess.call([
                self.mallet_bin,
                'import-file',
                '--input', self.corpus_path,
                '--output', self.input_path,
                '--keep-sequence',          # Required for LDA.
                '--remove-stopwords'])      # Probably redundant.

        if exit != 0:
            msg = "MALLET import-file failed with exit code {0}.".format(exit)
            raise RuntimeError(msg)
Calls MALLET's `import-file` method.