[Python-Dev] smarter temporary file object (SF #415692)

Wed Jan 3 06:27:40 CET 2007

I came across a complaint that PEP 0042 had become a graveyard of
neglected ideas, and decided to have a look through and implement
something. Creating a smarter temporary file object seemed simple
enough.
Oddly, even after GvR re-opened it, I can't post an attachment to that
tracker item (it's under "Feature Requests" -- does it need to get moved
to "Patches" first?), but the implementation is short, so it's included
below. This is intended to be appended to Lib/tempfile.py (and thus
assumes that module's globals are present).
I would appreciate it if the gurus of python-dev could take a peek and
let me know if this is unsuitable or incorrect for any reason. It's not
the most straightforward implementatio -- I used the optimization
techniques I found in TemporaryFile.
If this looks good, I'll prepare a patch against trunk, including an
additional chunk of documentation and a unit test.
Dustin
 -----cut-here-----
try:
 from cStringIO import StringIO
except:
 from StringIO import StringIO
class SpooledTemporaryFile:
 """Temporary file wrapper, specialized to switch from
 StringIO to a real file when it exceeds a certain size or
 when a fileno is needed.
 """
 _rolled = False
 def __init__(self, max_size=0, mode='w+b', bufsize=-1,
 suffix="", prefix=template, dir=None):
 self._file = StringIO()
 self._max_size = max_size
 self._TemporaryFileArgs = (mode, bufsize, suffix, prefix, dir)
 def _check(self, file):
 if self._rolled: return
 if file.tell() > self.__dict__['_max_size']:
 self._rollover(file)
 def _rollover(self, file):
 args = self.__dict__['_TemporaryFileArgs']
 self.__dict__.clear() # clear attributes cached by __getattr__
 newfile = self._file = TemporaryFile(*args)
 newfile.write(file.getvalue())
 newfile.seek(file.tell(), 0)
 self._rolled = True
 
 # replace patched functions with the new file's methods
 self.write = newfile.write
 self.writelines = newfile.writelines
 self.fileno = newfile.fileno
 def write(self, s):
 file = self.__dict__['_file']
 rv = file.write(s)
 self._check(file)
 return rv
 def writelines(self, iterable):
 file = self.__dict__['_file']
 rv = file.writelines(iterable)
 self._check(file)
 return rv
 def fileno(self):
 self._rollover(self.__dict__['_file'])
 return self.fileno()
 def __getattr__(self, name):
 file = self.__dict__['_file']
 a = getattr(file, name)
 if type(a) != type(0):
 setattr(self, name, a)
 return a