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Python Library Reference |  |
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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.
# illustrate __setstate__ and __getstate__ methods
# used in pickling.
class TextReader:
"Print and number lines in a text file."
def __init__(self,file):
self.file = file
self.fh = open(file,'r')
self.lineno = 0
def readline(self):
self.lineno = self.lineno + 1
line = self.fh.readline()
if not line:
return None
return "%d: %s" % (self.lineno,line[:-1])
# return data representation for pickled object
def __getstate__(self):
odict = self.__dict__ # get attribute dictionary
del odict['fh'] # remove filehandle entry
return odict
# restore object state from data representation generated
# by __getstate__
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
dict['fh'] = fh # create filehandle entry
self.__dict__ = dict # make dict our attribute dictionary
A sample usage might be something like this:
>>> import TextReader
>>> obj = TextReader.TextReader("TextReader.py")
>>> obj.readline()
'1: #!/usr/local/bin/python'
>>> # (more invocations of obj.readline() here)
... obj.readline()
'7: class TextReader:'
>>> import pickle
>>> pickle.dump(obj,open('save.p','w'))
(start another Python session)
>>> import pickle
>>> reader = pickle.load(open('save.p'))
>>> reader.readline()
'8: "Print and number lines in a text file."'