Python pickle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()

代码如下:

pickle.dumps()
pickle.loads()

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。

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