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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 _ReadN(self, n):
 """Reads n characters from the input stream, or until EOF. This is equivalent to the current CPython implementation of read(n), but not guaranteed by the docs. Args: n: int Returns: string """

    ret = ""
    while True:
      chunk = self._read_file.read(n - len(ret))
      ret += chunk

      if len(ret) == n or not chunk:
        return ret