code: def add_key_val(keyname, keyval, keytype, filename, extnum):
      
  
      funtype = {'int': int, 'float': float, 'str': str, 'bool': bool}
      if keytype not in funtype:
          raise ValueError('Undefined keyword type: ', keytype)
      with fits.open(filename, "update") as hdulist:
          hdulist[extnum].header[keyname] = funtype[keytype](keyval)
          print('>>> Inserting ' + keyname + '=' + keyval + ' in ' + filename)
docs: Add/replace FITS key
  
      Add/replace the key keyname with value keyval of type keytype in filename.
  
      Parameters:
      ----------
      keyname : str
          FITS Keyword name.
      keyval : str
          FITS keyword value.
      keytype: str
          FITS keyword type: int, float, str or bool.
      filaname : str
          FITS filename.
      extnum : int
          Extension number where the keyword will be inserted. Note that
          the first extension is number 1 (and not zero).