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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 read_string(self, size):
 """Reads up to 'size' bytes from the stream, stopping early only if we reach the end of the stream. Returns the bytes read as a string. """

        if size < 0:
            raise errors.DecodeError('Negative size %d' % size)
        s = self._input.read(size)
        if len(s) != size:
            raise errors.DecodeError('String claims to have %d bytes, but read %d' % (size, len(s)))
        self._pos += len(s)  # Only advance by the number of bytes actually read.
        return s