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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 _check_content(self, content_str):
 """Check if the content is likely to be successfully read."""

        if self.do_content_check:
            space_ratio = float(content_str.count(' '))/len(content_str)
            if space_ratio > self.max_space_ratio:
                return "space-ratio: %f > %f" % (space_ratio,
                                                 self.max_space_ratio)
            if len(content_str) > self.input_character_limit:
                return "too long: %d > %d" % (len(content_str),
                                              self.input_character_limit)
        return None