shelve -- 用來(lái)持久化任意的Python對(duì)象
這幾天接觸了Python中的shelve這個(gè)module，感覺(jué)比pickle用起來(lái)更簡(jiǎn)單一些，它也是一個(gè)用來(lái)持久化Python對(duì)象的簡(jiǎn)單工具。當(dāng)我們寫程序的時(shí)候如果不想用關(guān)系數(shù)據(jù)庫(kù)那么重量級(jí)的東東去存儲(chǔ)數(shù)據(jù)，不妨可以試試用shelve。shelf也是用key來(lái)訪問(wèn)的，使用起來(lái)和字典類似。shelve其實(shí)用anydbm去創(chuàng)建DB并且管理持久化對(duì)象的。
創(chuàng)建一個(gè)新的shelf
直接使用shelve.open()就可以創(chuàng)建了
import shelves = shelve.open('test_shelf.db')try: s['key1'] = { 'int': 10, 'float':9.5, 'string':'Sample data' }finally: s.close()如果想要再次訪問(wèn)這個(gè)shelf，只需要再次shelve.open()就可以了，然后我們可以像使用字典一樣來(lái)使用這個(gè)shelf
import shelves = shelve.open('test_shelf.db')try: existing = s['key1']finally: s.close()print existing當(dāng)我們運(yùn)行以上兩個(gè)py，我們將得到如下輸出：
$ python shelve_create.py$ python shelve_existing.py{'int': 10, 'float': 9.5, 'string': 'Sample data'} dbm這個(gè)模塊有個(gè)限制，它不支持多個(gè)應(yīng)用同一時(shí)間往同一個(gè)DB進(jìn)行寫操作。所以當(dāng)我們知道我們的應(yīng)用如果只進(jìn)行讀操作，我們可以讓shelve通過(guò)只讀方式打開DB：
import shelves = shelve.open('test_shelf.db', flag='r')try: existing = s['key1']finally: s.close()print existing當(dāng)我們的程序試圖去修改一個(gè)以只讀方式打開的DB時(shí)，將會(huì)拋一個(gè)訪問(wèn)錯(cuò)誤的異常。異常的具體類型取決于anydbm這個(gè)模塊在創(chuàng)建DB時(shí)所選用的DB。
寫回（Write-back）
由于shelve在默認(rèn)情況下是不會(huì)記錄待持久化對(duì)象的任何修改的，所以我們?cè)趕helve.open()時(shí)候需要修改默認(rèn)參數(shù)，否則對(duì)象的修改不會(huì)保存。
import shelves = shelve.open('test_shelf.db')try: print s['key1'] s['key1']['new_value'] = 'this was not here before'finally: s.close()s = shelve.open('test_shelf.db', writeback=True)try: print s['key1']finally: s.close()上面這個(gè)例子中，由于一開始我們使用了缺省參數(shù)shelve.open()了，因此第6行修改的值即使我們s.close()也不會(huì)被保存。
執(zhí)行結(jié)果如下：
$ python shelve_create.py$ python shelve_withoutwriteback.py{'int': 10, 'float': 9.5, 'string': 'Sample data'}{'int': 10, 'float': 9.5, 'string': 'Sample data'} 所以當(dāng)我們?cè)噲D讓shelve去自動(dòng)捕獲對(duì)象的變化，我們應(yīng)該在打開shelf的時(shí)候?qū)riteback設(shè)置為True。當(dāng)我們將writeback這個(gè)flag設(shè)置為True以后，shelf將會(huì)將所有從DB中讀取的對(duì)象存放到一個(gè)內(nèi)存緩存。當(dāng)我們close()打開的shelf的時(shí)候，緩存中所有的對(duì)象會(huì)被重新寫入DB。
import shelves = shelve.open('test_shelf.db', writeback=True)try: print s['key1'] s['key1']['new_value'] = 'this was not here before' print s['key1']finally: s.close()s = shelve.open('test_shelf.db', writeback=True)try: print s['key1']finally: s.close()writeback方式有優(yōu)點(diǎn)也有缺點(diǎn)。優(yōu)點(diǎn)是減少了我們出錯(cuò)的概率，并且讓對(duì)象的持久化對(duì)用戶更加的透明了；但這種方式并不是所有的情況下都需要，首先，使用writeback以后，shelf在open()的時(shí)候會(huì)增加額外的內(nèi)存消耗，并且當(dāng)DB在close()的時(shí)候會(huì)將緩存中的每一個(gè)對(duì)象都寫入到DB，這也會(huì)帶來(lái)額外的等待時(shí)間。因?yàn)閟helve沒(méi)有辦法知道緩存中哪些對(duì)象修改了，哪些對(duì)象沒(méi)有修改，因此所有的對(duì)象都會(huì)被寫入。
$ python shelve_create.py$ python shelve_writeback.py {'int': 10, 'float': 9.5, 'string': 'Sample data'}{'int': 10, 'new_value': 'this was not here before', 'float': 9.5, 'string': 'Sample data'} {'int': 10, 'new_value': 'this was not here before', 'float': 9.5, 'string': 'Sample data'}最后再來(lái)個(gè)復(fù)雜一點(diǎn)的例子：
#!/bin/env pythonimport timeimport datetimeimport md5import shelveLOGIN_TIME_OUT = 60db = shelve.open('user_shelve.db', writeback=True)def newuser(): global db prompt = "login desired: " while True: name = raw_input(prompt) if name in db: prompt = "name taken, try another: " continue elif len(name) == 0: prompt = "name should not be empty, try another: " continue else: break pwd = raw_input("password: ") db[name] = {"password": md5_digest(pwd), "last_login_time": time.time()} #print '-->', dbdef olduser(): global db name = raw_input("login: ") pwd = raw_input("password: ") try: password = db.get(name).get('password') except AttributeError, e: print "/033[1;31;40mUsername '%s' doesn't existed/033[0m" % name return if md5_digest(pwd) == password: login_time = time.time() last_login_time = db.get(name).get('last_login_time') if login_time - last_login_time < LOGIN_TIME_OUT: print "/033[1;31;40mYou already logged in at: <%s>/033[0m" % datetime.datetime.fromtimestamp(last_login_time).isoformat() db[name]['last_login_time'] = login_time print "/033[1;32;40mwelcome back/033[0m", name else: print "/033[1;31;40mlogin incorrect/033[0m"def md5_digest(plain_pass): return md5.new(plain_pass).hexdigest()def showmenu(): #print '>>>', db global db prompt = """(N)ew User Login(E)xisting User Login(Q)uitEnter choice: """ done = False while not done: chosen = False while not chosen: try: choice = raw_input(prompt).strip()[0].lower() except (EOFError, KeyboardInterrupt): choice = "q" print "/nYou picked: [%s]" % choice if choice not in "neq": print "invalid option, try again" else: chosen = True if choice == "q": done = True if choice == "n": newuser() if choice == "e": olduser() db.close()if __name__ == "__main__": showmenu()感謝閱讀本文，希望能幫助到大家，謝謝大家對(duì)本站的支持！
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