class_docstr: Minimal read-only file object that prepends a string to the contents
  of an actual file. Shouldn't be used outside of gzip.py, as it lacks
  essential functionality.
code: def __init__(self, f, prepend=b""):
      self._buffer = prepend
      self._length = len(prepend)
      self.file = f
      self._read = 0
  
imports: import struct, sys, time, os
  import zlib
  import builtins
  import io
  import _compression
label_desc: Use image node_id 1 to create a new _PaddedFile object  with example: obj = _PaddedFile(f, prepend)