pythonpickle类库介绍(对象序列化和反序列化)

一、pickle

pickle模块用来实现python对象的序列化和反序列化。通常地pickle将python对象序列化为二进制流或文件。

python对象与文件之间的序列化和反序列化:

代码如下:

pickle.dump()
pickle.load()

如果要实现python对象和字符串间的序列化和反序列化,则使用:

代码如下:

pickle.dumps()
pickle.loads()

可以被序列化的类型有:
* none,true 和 false;
* 整数,浮点数,复数;
* 字符串,字节流,字节数组;
* 包含可pickle对象的tuples,lists,sets和dictionaries;
* 定义在module顶层的函数:
* 定义在module顶层的内置函数;
* 定义在module顶层的类;
* 拥有__dict__()或__setstate__()的自定义类型;

注意:对于函数或类的序列化是以名字来识别的,所以需要import相应的module。

二、pickle的运行过程

在大部分情况下,要是的对象picklable,我们不需要额外的代码。默认地pickle将智能地检查类和实例的属性,当一个类实例反序列化的时候,它的__init__()方法通常不被调用。而是首先创建一个未初始化的实例,然后再回复存储的属性。

但是可以通过实现下列的方法来修改默认的行为:

代码如下:

object.__getstate__() :默认地序列化对象的__dict__,但是如果你实现了__getstate__(),则__getstate__()函数返回的值将被序列化。
object.__setstate__(state) :如果类型实现了此方法,则在反序列化的时候,此方法用来恢复对象的属性。
object.__getnewargs__() : 如果实例构造的时候(__new__())需要参数,则需要实现此函数。

注意:如果__getstate__()返回false,则在反序列化的时候__setstate__()则不被调用。

有的时候为了效率,或上面的3个函数不能满足需求时,需要实现__reduce__()函数。

三、实例

代码如下:

import pickle

# an arbitrary collection of objects supported by pickle.
data = {
‘a’: [1, 2.0, 3, 4+6j],
‘b’: (“character string”, b”byte string”),
‘c’: set([none, true, false])
}

with open(‘data.pickle’, ‘wb’) as f:
# pickle the ‘data’ dictionary using the highest protocol available.
pickle.dump(data, f, pickle.highest_protocol)

with open(‘data.pickle’, ‘rb’) as f:
# the protocol version used is detected automatically, so we do not
# have to specify it.
data = pickle.load(f)
print(str(data))

四、修改picklable类型的默认行为

代码如下:

class textreader:
“””print and number lines in a text file.”””

def __init__(self, filename):
self.filename = filename
self.file = open(filename)
self.lineno = 0

def readline(self):
self.lineno += 1
line = self.file.readline()
if not line:
return none
if line.endswith(‘\n’):
line = line[:-1]
return “%i: %s” % (self.lineno, line)

def __getstate__(self):
# copy the object’s state from self.__dict__ which contains
# all our instance attributes. always use the dict.copy()
# method to avoid modifying the original state.
state = self.__dict__.copy()
# remove the unpicklable entries.
del state[‘file’]
return state

def __setstate__(self, state):
# restore instance attributes (i.e., filename and lineno).
self.__dict__.update(state)
# restore the previously opened file’s state. to do so, we need to
# reopen it and read from it until the line count is restored.
file = open(self.filename)
for _ in range(self.lineno):
file.readline()
# finally, save the file.
self.file = file

reader = textreader(“hello.txt”)
print(reader.readline())
print(reader.readline())
s = pickle.dumps(reader)
#print(s)
new_reader = pickle.loads(s)
print(new_reader.readline())

# the output is
# 1: hello
# 2: how are you
# 3: goodbye

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