class_docstr: This class allows to capture writes to FD1 and FD2
  and may connect a NULL file to FD0 (and prevent
  reads from sys.stdin).  If any of the 0,1,2 file descriptors
  is invalid it will not be captured.
code: def _readsnapshot(self, f):
      f.seek(0)
      res = f.read()
      enc = getattr(f, "encoding", None)
      if enc:
          res = py.builtin._totext(res, enc, "replace")
      f.truncate(0)
      f.seek(0)
      return res
  
imports: import os
  import sys
  import py
  import tempfile
label_desc: Use image node_id 7 for calling the StdCaptureFD obj's underlying member method code with example usage: obj._readsnapshot(f) and returns: res