Here's a simple example of how to modify pickling behavior for a
class. The TextReader class opens a text file, and returns
the line number and line contents each time its readline()
method is called. If a TextReader instance is pickled, all
attributes except the file object member are saved. When the
instance is unpickled, the file is reopened, and reading resumes from
the last location. The __setstate__() and
__getstate__() methods are used to implement this behavior.
class TextReader:
"""Print and number lines in a text file."""
def __init__(self, file):
self.file = file
self.fh = open(file)
self.lineno = 0
def readline(self):
self.lineno = self.lineno + 1
line = self.fh.readline()
if not line:
return None
if line.endswith("\n"):
line = line[:-1]
return "%d: %s" % (self.lineno, line)
def __getstate__(self):
odict = self.__dict__.copy() # copy the dict since we change it
del odict['fh'] # remove filehandle entry
return odict
def __setstate__(self,dict):
fh = open(dict['file']) # reopen file
count = dict['lineno'] # read from file...
while count: # until line count is restored
fh.readline()
count = count - 1
self.__dict__.update(dict) # update attributes
self.fh = fh # save the file object
If you want to see that pickle works across Python
processes, start another Python session, before continuing. What
follows can happen from either the same process or a new process.
>>> import pickle
>>> reader = pickle.load(open('save.p'))
>>> reader.readline()
'8: "Print and number lines in a text file."'
