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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 unzip_file_if_necessary(self, source_file):
 """ Unzip file if zipped. """

        if source_file.endswith(".gz"):
            self.print_message("Decompressing '%s'" % source_file)
            subprocess.check_call(["gunzip", "--force", source_file])
            source_file = source_file[:-len(".gz")]
        return source_file