python使用pickle,json等序列化dict

来源：http://www.open-open.com/code/view/1420520368937

1. import pickle, json, csv, os, shutil
   
2. 
   
3. class PersistentDict(dict):
   
4.     ''' Persistent dictionary with an API compatible with shelve and anydbm.
   
5. 
   
6.     The dict is kept in memory, so the dictionary operations run as fast as
   
7.     a regular dictionary.
   
8. 
   
9.     Write to disk is delayed until close or sync (similar to gdbm's fast mode).
   
10. 
    
11.     Input file format is automatically discovered.
    
12.     Output file format is selectable between pickle, json, and csv.
    
13.     All three serialization formats are backed by fast C implementations.
    
14. 
    
15.     '''
    
16. 
    
17.     def __init__(self, filename, flag='c', mode=None, format='pickle', *args, **kwds):
    
18.         self.flag = flag                    # r=readonly, c=create, or n=new
    
19.         self.mode = mode                    # None or an octal triple like 0644
    
20.         self.format = format                # 'csv', 'json', or 'pickle'
    
21.         self.filename = filename
    
22.         if flag != 'n' and os.access(filename, os.R_OK):
    
23.             fileobj = open(filename, 'rb' if format=='pickle' else 'r')
    
24.             with fileobj:
    
25.                 self.load(fileobj)
    
26.         dict.__init__(self, *args, **kwds)
    
27. 
    
28.     def sync(self):
    
29.         'Write dict to disk'
    
30.         if self.flag == 'r':
    
31.             return
    
32.         filename = self.filename
    
33.         tempname = filename + '.tmp'
    
34.         fileobj = open(tempname, 'wb' if self.format=='pickle' else 'w')
    
35.         try:
    
36.             self.dump(fileobj)
    
37.         except Exception:
    
38.             os.remove(tempname)
    
39.             raise
    
40.         finally:
    
41.             fileobj.close()
    
42.         shutil.move(tempname, self.filename)    # atomic commit
    
43.         if self.mode is not None:
    
44.             os.chmod(self.filename, self.mode)
    
45. 
    
46.     def close(self):
    
47.         self.sync()
    
48. 
    
49.     def __enter__(self):
    
50.         return self
    
51. 
    
52.     def __exit__(self, *exc_info):
    
53.         self.close()
    
54. 
    
55.     def dump(self, fileobj):
    
56.         if self.format == 'csv':
    
57.             csv.writer(fileobj).writerows(self.items())
    
58.         elif self.format == 'json':
    
59.             json.dump(self, fileobj, separators=(',', ':'))
    
60.         elif self.format == 'pickle':
    
61.             pickle.dump(dict(self), fileobj, 2)
    
62.         else:
    
63.             raise NotImplementedError('Unknown format: ' + repr(self.format))
    
64. 
    
65.     def load(self, fileobj):
    
66.         # try formats from most restrictive to least restrictive
    
67.         for loader in (pickle.load, json.load, csv.reader):
    
68.             fileobj.seek(0)
    
69.             try:
    
70.                 return self.update(loader(fileobj))
    
71.             except Exception:
    
72.                 pass
    
73.         raise ValueError('File not in a supported format')
    
74. 
    
75. if __name__ == '__main__':
    
76.     import random
    
77. 
    
78.     # Make and use a persistent dictionary
    
79.     with PersistentDict('/tmp/demo.json', 'c', format='json') as d:
    
80.         print(d, 'start')
    
81.         d['abc'] = '123'
    
82.         d['rand'] = random.randrange(10000)
    
83.         print(d, 'updated')
    
84. 
    
85.     # Show what the file looks like on disk
    
86.     with open('/tmp/demo.json', 'rb') as f:
    
87.         print(f.read())

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