1384 class filestat(object):

  1385     """help to exactly detect change of a file

  1386 

  1387     'stat' attribute is result of 'os.stat()' if specified 'path'

  1388     exists. Otherwise, it is None. This can avoid preparative

  1389     'exists()' examination on client side of this class.

  1390     """

  1391     def __init__(self, path):

  1392         try:

  1393             self.stat = os.stat(path)

  1394         except OSError as err:

  1395             if err.errno != errno.ENOENT:

  1396                 raise

  1397             self.stat = None

  1398 

  1399     __hash__ = object.__hash__

  1400 

  1401     def __eq__(self, old):

  1402         try:

  1403             # if ambiguity between stat of new and old file is

  1404             # avoided, comparision of size, ctime and mtime is enough

  1405             # to exactly detect change of a file regardless of platform

  1406             return (self.stat.st_size == old.stat.st_size and

  1407                     self.stat.st_ctime == old.stat.st_ctime and

  1408                     self.stat.st_mtime == old.stat.st_mtime)

  1409         except AttributeError:

  1410             return False

  1411 

  1412     def isambig(self, old):

  1413         """Examine whether new (= self) stat is ambiguous against old one

  1414 

  1415         "S[N]" below means stat of a file at N-th change:

  1416 

  1417         - S[n-1].ctime  < S[n].ctime: can detect change of a file

  1418         - S[n-1].ctime == S[n].ctime

  1419           - S[n-1].ctime  < S[n].mtime: means natural advancing (*1)

  1420           - S[n-1].ctime == S[n].mtime: is ambiguous (*2)

  1421           - S[n-1].ctime  > S[n].mtime: never occurs naturally (don't care)

  1422         - S[n-1].ctime  > S[n].ctime: never occurs naturally (don't care)

  1423 

  1424         Case (*2) above means that a file was changed twice or more at

  1425         same time in sec (= S[n-1].ctime), and comparison of timestamp

  1426         is ambiguous.

  1427 

  1428         Base idea to avoid such ambiguity is "advance mtime 1 sec, if

  1429         timestamp is ambiguous".

  1430 

  1431         But advancing mtime only in case (*2) doesn't work as

  1432         expected, because naturally advanced S[n].mtime in case (*1)

  1433         might be equal to manually advanced S[n-1 or earlier].mtime.

  1434 

  1435         Therefore, all "S[n-1].ctime == S[n].ctime" cases should be

  1436         treated as ambiguous regardless of mtime, to avoid overlooking

  1437         by confliction between such mtime.

  1438 

  1439         Advancing mtime "if isambig(oldstat)" ensures "S[n-1].mtime !=

  1440         S[n].mtime", even if size of a file isn't changed.

  1441         """

  1442         try:

  1443             return (self.stat.st_ctime == old.stat.st_ctime)

  1444         except AttributeError:

  1445             return False

  1446