序列化 (Serialization)是將對象的狀態(tài)信息轉(zhuǎn)換為可以存儲或傳輸?shù)男问降倪^程。在序列化期間，對象將其當前狀態(tài)寫入到臨時或持久性存儲區(qū)。以后，可以通過從存儲區(qū)中讀取或反序列化對象的狀態(tài)，重新創(chuàng)建該對象。

在scrapy_redis中，一個Request對象先經(jīng)過DupeFilter去重，然后遞交給scheduler調(diào)度儲存在Redis中，這就面臨一個問題，Request是一個對象，Redis不能存儲該對象，這時就需要將request序列化儲存。

scrapy中序列化模塊如下：

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)

當然python3直接使用pickle模塊， 已經(jīng)沒有cPickle，該模塊最為重要的兩個方法，序列化與反序列化如上，通過序列化后的對象我們可以存儲在數(shù)據(jù)庫、文本等文件中，并快速恢復。

同時模式設(shè)計中的備忘錄模式通過這種方式達到最佳效果《python設(shè)計模式（十九）：備忘錄模式》；可序列化的對象和數(shù)據(jù)類型如下：

• None, True,False
• 整數(shù)，長整數(shù)，浮點數(shù)，復數(shù)
• 普通字符串和Unicode字符串
• 元組、列表、集合和字典，只包含可選擇的對象。
• 在模塊頂層定義的函數(shù)
• 在模塊頂層定義的內(nèi)置函數(shù)
• 在模塊的頂層定義的類。
• 這些類的實例

嘗試對不可序列化對象進行操作，將引發(fā)PicklingError異常；發(fā)生這種情況時，可能已經(jīng)將未指定的字節(jié)數(shù)寫入基礎(chǔ)文件。嘗試選擇高度遞歸的數(shù)據(jù)結(jié)構(gòu)可能會超過最大遞歸深度，RuntimeError在這種情況下會被提起。

模塊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.

至于應用場景，比較常見的有如下幾種：

程序重啟時恢復上次的狀態(tài)、會話存儲、對象的網(wǎng)絡傳輸。

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