python使用pickle,json等序列化dict详解编程语言

PersistentDict从dict继承，实现了持久化。初始化时自动加载文件到内存，关闭时将dict内容序列化到文件，支持csv，json，pickle格式。

import pickle, json, csv, os, shutil 
 
class PersistentDict(dict): 
    ''' Persistent dictionary with an API compatible with shelve and anydbm. 
 
    The dict is kept in memory, so the dictionary operations run as fast as 
    a regular dictionary. 
 
    Write to disk is delayed until close or sync (similar to gdbm's fast mode). 
 
    Input file format is automatically discovered. 
    Output file format is selectable between pickle, json, and csv. 
    All three serialization formats are backed by fast C implementations. 
 
    ''' 
 
    def __init__(self, filename, flag='c', mode=None, format='pickle', *args, **kwds): 
        self.flag = flag                    # r=readonly, c=create, or n=new 
        self.mode = mode                    # None or an octal triple like 0644 
        self.format = format                # 'csv', 'json', or 'pickle' 
        self.filename = filename 
        if flag != 'n' and os.access(filename, os.R_OK): 
            fileobj = open(filename, 'rb' if format=='pickle' else 'r') 
            with fileobj: 
                self.load(fileobj) 
        dict.__init__(self, *args, **kwds) 
 
    def sync(self): 
        'Write dict to disk' 
        if self.flag == 'r': 
            return 
        filename = self.filename 
        tempname = filename + '.tmp' 
        fileobj = open(tempname, 'wb' if self.format=='pickle' else 'w') 
        try: 
            self.dump(fileobj) 
        except Exception: 
            os.remove(tempname) 
            raise 
        finally: 
            fileobj.close() 
        shutil.move(tempname, self.filename)    # atomic commit 
        if self.mode is not None: 
            os.chmod(self.filename, self.mode) 
 
    def close(self): 
        self.sync() 
 
    def __enter__(self): 
        return self 
 
    def __exit__(self, *exc_info): 
        self.close() 
 
    def dump(self, fileobj): 
        if self.format == 'csv': 
            csv.writer(fileobj).writerows(self.items()) 
        elif self.format == 'json': 
            json.dump(self, fileobj, separators=(',', ':')) 
        elif self.format == 'pickle': 
            pickle.dump(dict(self), fileobj, 2) 
        else: 
            raise NotImplementedError('Unknown format: ' + repr(self.format)) 
 
    def load(self, fileobj): 
        # try formats from most restrictive to least restrictive 
        for loader in (pickle.load, json.load, csv.reader): 
            fileobj.seek(0) 
            try: 
                return self.update(loader(fileobj)) 
            except Exception: 
                pass 
        raise ValueError('File not in a supported format') 
 
if __name__ == '__main__': 
    import random 
 
    # Make and use a persistent dictionary 
    with PersistentDict('/tmp/demo.json', 'c', format='json') as d: 
        print(d, 'start') 
        d['abc'] = '123' 
        d['rand'] = random.randrange(10000) 
        print(d, 'updated') 
 
    # Show what the file looks like on disk 
    with open('/tmp/demo.json', 'rb') as f: 
        print(f.read())