Python 持久性管理

什么是持久性？

对象持久性

一些经过 pickle 的 Python

>>> import cPickle as pickle

清单 1. dumps() 和 loads() 的演示

>>> import cPickle as pickle  >>> t1 = ('this is a string', 42, [1, 2, 3], None)  >>> t1  ('this is a string', 42, [1, 2, 3], None)  >>> p1 = pickle.dumps(t1)  >>> p1  "(S'this is a string'/nI42/n(lp1/nI1/naI2/naI3/naNtp2/n.">>> print p1  (S'this is a string'I42  (lp1  I1  aI2  aI3  aNtp2  .  >>> t2 = pickle.loads(p1)  >>> t2  ('this is a string', 42, [1, 2, 3], None)  >>> p2 = pickle.dumps(t1, True)  >>> p2  '(U/x10this is a stringK*]q/x01(K/x01K/x02K/x03eNtq/x02.'>>> t3 = pickle.loads(p2)  >>> t3  ('this is a string', 42, [1, 2, 3], None)

清单 2. dump() 和 load() 示例

>>> a1 = 'apple'>>> b1 = {1: 'One', 2: 'Two', 3: 'Three'}  >>> c1 = ['fee', 'fie', 'foe', 'fum']  >>> f1 = file('temp.pkl', 'wb')  >>> pickle.dump(a1, f1, True)  >>> pickle.dump(b1, f1, True)  >>> pickle.dump(c1, f1, True)  >>> f1.close()  >>> f2 = file('temp.pkl', 'rb')  >>> a2 = pickle.load(f2)  >>> a2  'apple'>>> b2 = pickle.load(f2)  >>> b2  {1: 'One', 2: 'Two', 3: 'Three'}  >>> c2 = pickle.load(f2)  >>> c2  ['fee', 'fie', 'foe', 'fum']  >>> f2.close()

Pickle 的威力

可移植性

清单 3. 检索所支持的格式

>>> pickle.format_version
'1.3'
>>> pickle.compatible_formats
['1.0', '1.1', '1.2']

多个引用，同一对象

清单 4. 对象引用的维护

>>> a = [1, 2, 3]
>>> b = a
>>> a
[1, 2, 3]
>>> b
[1, 2, 3]
>>> a.append(4)
>>> a
[1, 2, 3, 4]
>>> b
[1, 2, 3, 4]
>>> c = pickle.dumps((a, b))
>>> d, e = pickle.loads(c)
>>> d
[1, 2, 3, 4]
>>> e
[1, 2, 3, 4]
>>> d.append(5)
>>> d
[1, 2, 3, 4, 5]
>>> e
[1, 2, 3, 4, 5]

循环引用和递归引用

>清单 5. 递归引用

>>> l = [1, 2, 3]
>>> l.append(l)
>>> l
[1, 2, 3, [...]]
>>> l[3]
[1, 2, 3, [...]]
>>> l[3][3]
[1, 2, 3, [...]]
>>> p = pickle.dumps(l)
>>> l2 = pickle.loads(p)
>>> l2
[1, 2, 3, [...]]
>>> l2[3]
[1, 2, 3, [...]]
>>> l2[3][3]
[1, 2, 3, [...]]

清单 6. 循环引用

>>> a = [1, 2]
>>> b = [3, 4]
>>> a.append(b)
>>> a
[1, 2, [3, 4]]
>>> b.append(a)
>>> a
[1, 2, [3, 4, [...]]]
>>> b
[3, 4, [1, 2, [...]]]
>>> a[2]
[3, 4, [1, 2, [...]]]
>>> b[2]
[1, 2, [3, 4, [...]]]
>>> a[2] is b
1
>>> b[2] is a
1
>>> f = file('temp.pkl', 'w')
>>> pickle.dump((a, b), f)
>>> f.close()
>>> f = file('temp.pkl', 'r')
>>> c, d = pickle.load(f)
>>> f.close()
>>> c
[1, 2, [3, 4, [...]]]
>>> d
[3, 4, [1, 2, [...]]]
>>> c[2]
[3, 4, [1, 2, [...]]]
>>> d[2]
[1, 2, [3, 4, [...]]]
>>> c[2] is d
1
>>> d[2] is c
1

清单 7. 分别 pickle vs. 在一个元组中一起 pickle

>>> f = file('temp.pkl', 'w')
>>> pickle.dump(a, f)
>>> pickle.dump(b, f)
>>> f.close()
>>> f = file('temp.pkl', 'r')
>>> c = pickle.load(f)
>>> d = pickle.load(f)
>>> f.close()
>>> c
[1, 2, [3, 4, [...]]]
>>> d
[3, 4, [1, 2, [...]]]
>>> c[2]
[3, 4, [1, 2, [...]]]
>>> d[2]
[1, 2, [3, 4, [...]]]
>>> c[2] is d
0
>>> d[2] is c
0

相等，但并不总是相同

清单 8. 作为原来对象副本的被恢复的对象

>>> j = [1, 2, 3]
>>> k = j
>>> k is j
1
>>> x = pickle.dumps(k)
>>> y = pickle.loads(x)
>>> y
[1, 2, 3]
>>> y == k
1
>>> y is k
0
>>> y is j
0
>>> k is j
1

清单 9. 维护分别 pickle 的对象间的引用

>>> f = file('temp.pkl', 'w')
>>> pickler = pickle.Pickler(f)
>>> pickler.dump(a)
<cPickle.Pickler object at 0x89b0bb8>
>>> pickler.dump(b)
<cPickle.Pickler object at 0x89b0bb8>
>>> f.close()
>>> f = file('temp.pkl', 'r')
>>> unpickler = pickle.Unpickler(f)
>>> c = unpickler.load()
>>> d = unpickler.load()
>>> c[2]
[3, 4, [1, 2, [...]]]
>>> d[2]
[1, 2, [3, 4, [...]]]
>>> c[2] is d
1
>>> d[2] is c
1

不可 pickle 的对象

清单 10. 试图 pickle 文件对象的结果

>>> f = file('temp.pkl', 'w')
>>> p = pickle.dumps(f)
Traceback (most recent call last):
  File "<input>", line 1, in ?
  File "/usr/lib/python2.2/copy_reg.py", line 57, in _reduce
    raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle file objects

类实例

清单 11. 新型类的定义

class Foo(object):
    def __init__(self, value):
        self.value = value

清单 12. pickle Foo 实例

>>> import cPickle as pickle
>>> from Orbtech.examples.persist import Foo
>>> foo = Foo('What is a Foo?')
>>> p = pickle.dumps(foo)
>>> print p
ccopy_reg
_reconstructor
p1
(cOrbtech.examples.persist
Foo
p2
c__builtin__
object
p3
NtRp4
(dp5
S'value'
p6
S'What is a Foo?'
sb.
>>>

清单 13. 试图装入一个被重命名的 Foo 类的经过 pickle 的实例

>>> import cPickle as pickle
>>> f = file('temp.pkl', 'r')
>>> foo = pickle.load(f)
Traceback (most recent call last):
  File "<input>", line 1, in ?
AttributeError: 'module' object has no attribute 'Foo'

清单 14. 试图装入一个被重命名的 persist.py 模块的经过 pickle 的实例

>>> import cPickle as pickle
>>> f = file('temp.pkl', 'r')
>>> foo = pickle.load(f)
Traceback (most recent call last):
  File "<input>", line 1, in ?
ImportError: No module named persist

特殊的状态方法

清单 15. 处理不能 pickle 的实例属性

class Foo(object):
    def __init__(self, value, filename):
        self.value = value
        self.logfile = file(filename, 'w')
    def __getstate__(self):
        """Return state values to be pickled."""
        f = self.logfile
        return (self.value, f.name, f.tell())
    def __setstate__(self, state):
        """Restore state from the unpickled state values."""
        self.value, name, position = state
        f = file(name, 'w')
        f.seek(position)
        self.logfile = f

模式改进

类名的更改

清单 16. 更改类名：添加到原来类定义的方法

def __setstate__(self, state):
    self.__dict__.update(state)
    self.__class__ = NewClassName

属性的添加和删除

清单 17. 最初的类定义

class Person(object):
    def __init__(self, firstname, lastname):
        self.firstname = firstname
        self.lastname = lastname

清单 18. 新的类定义

class Person(object):
    def __init__(self, fullname):
        self.fullname = fullname
    def __setstate__(self, state):
        if 'fullname' not in state:
            first = ''
            last = ''
            if 'firstname' in state:
                first = state['firstname']
                del state['firstname']
            if 'lastname' in state:
                last = state['lastname']
                del state['lastname']
            self.fullname = " ".join([first, last]).strip()
        self.__dict__.update(state)

模块的修改

结束语

（版权归原作者所有，侵删）

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