    def testzip(self):
        """Read all the files and check the CRC."""
        chunk_size = 2 ** 20
        for zinfo in self.filelist:
            try:
                # Read by chunks, to avoid an OverflowError or a
                # MemoryError with very large embedded files.
                f = self.open(zinfo.filename, "r")
                while f.read(chunk_size):     # Check CRC-32
                    pass
            except BadZipfile:
                return zinfo.filename