scrapy_redis中序列化源码及其在程序设计中的应用

序列化 (Serialization)是将对象的状态信息转换为可以存储或传输的形式的过程。在序列化期间，对象将其当前状态写入到临时或持久性存储区。以后，可以通过从存储区中读取或反序列化对象的状态，重新创建该对象。

在scrapy_redis中，一个Request对象先经过DupeFilter去重，然后递交给scheduler调度储存在Redis中，这就面临一个问题，Request是一个对象，Redis不能存储该对象，这时就需要将request序列化储存。

scrapy中序列化模块如下：

from scrapy_redis import picklecompat

"""A pickle wrapper module with protocol=-1 by default."""

try:
    import cPickle as pickle  # PY2
except ImportError:
    import pickle

def loads(s):
    return pickle.loads(s)

def dumps(obj):
    return pickle.dumps(obj, protocol=-1)

当然python3直接使用pickle模块， 已经没有cPickle，该模块最为重要的两个方法，序列化与反序列化如上，通过序列化后的对象我们可以存储在数据库、文本等文件中，并快速恢复。

同时模式设计中的备忘录模式通过这种方式达到最佳效果《python设计模式（十九）：备忘录模式》；可序列化的对象和数据类型如下：

• None, True,False
• 整数，长整数，浮点数，复数
• 普通字符串和Unicode字符串
• 元组、列表、集合和字典，只包含可选择的对象。
• 在模块顶层定义的函数
• 在模块顶层定义的内置函数
• 在模块的顶层定义的类。
• 这些类的实例

尝试对不可序列化对象进行操作，将引发PicklingError异常；发生这种情况时，可能已经将未指定的字节数写入基础文件。尝试选择高度递归的数据结构可能会超过最大递归深度，RuntimeError在这种情况下会被提起。

模块API

pickle.dump(obj, file[, protocol])

• Write a pickled representation of obj to the open file object file. This is equivalent to Pickler(file,``protocol).dump(obj).
  If the protocol parameter is omitted, protocol 0 is used. If protocol is specified as a negative value or HIGHEST_PROTOCOL, the highest protocol version will be used.
  *Changed in version 2.3: *Introduced the protocol parameter.
  file must have a write() method that accepts a single string argument. It can thus be a file object opened for writing, a StringIO object, or any other custom object that meets this interface.
• pickle.load(file)
• Read a string from the open file object file and interpret it as a pickle data stream, reconstructing and returning the original object hierarchy. This is equivalent to Unpickler(file).load().
  file must have two methods, a read() method that takes an integer argument, and a readline() method that requires no arguments. Both methods should return a string. Thus file can be a file object opened for reading, a StringIO object, or any other custom object that meets this interface.
  This function automatically determines whether the data stream was written in binary mode or not.
• pickle.dumps(obj[, protocol])
• Return the pickled representation of the object as a string, instead of writing it to a file.
  If the protocol parameter is omitted, protocol 0 is used. If protocol is specified as a negative value or HIGHEST_PROTOCOL, the highest protocol version will be used.
  *Changed in version 2.3: *The protocol parameter was added.
• pickle.loads(string)
• Read a pickled object hierarchy from a string. Characters in the string past the pickled object’s representation are ignored.

至于应用场景，比较常见的有如下几种：

程序重启时恢复上次的状态、会话存储、对象的网络传输。

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