python pickle 模塊的使用

用於序列化的兩個模塊
　　json：用於字符串和Python數據類型間進行轉換
　　pickle: 用於python特有的類型和python的數據類型間進行轉換
　　json提供四個功能：dumps,dump,loads,load
　　pickle提供四個功能：dumps,dump,loads,load

pickle可以存儲什么類型的數據呢？

1. 所有python支持的原生類型：布爾值，整數，浮點數，復數，字符串，字節，None。
2. 由任何原生類型組成的列表，元組，字典和集合。
3. 函數，類，類的實例

pickle模塊中常用的方法有：

    1. pickle.dump(obj, file, protocol=None,)

    必填參數obj表示將要封裝的對象

    必填參數file表示obj要寫入的文件對象，file必須以二進制可寫模式打開，即“wb”

    可選參數protocol表示告知pickler使用的協議，支持的協議有0,1,2,3，默認的協議是添加在Python 3中的協議3。　　　

• Protocol version 0 is the original “human-readable” protocol and is backwards compatible with earlier versions of Python.
• Protocol version 1 is an old binary format which is also compatible with earlier versions of Python.
• Protocol version 2 was introduced in Python 2.3. It provides much more efficient pickling of new-style classes. Refer to PEP 307 for information about improvements brought by protocol 2.
• Protocol version 3 was added in Python 3.0. It has explicit support for bytes objects and cannot be unpickled by Python 2.x. This is the default protocol, and the recommended protocol when compatibility with other Python 3 versions is required.
• Protocol version 4 was added in Python 3.4. It adds support for very large objects, pickling more kinds of objects, and some data format optimizations. Refer to PEP 3154 for information about improvements brought by protocol 4. 

    2. pickle.load(file,*,fix_imports=True, encoding="ASCII", errors="strict")

    必填參數file必須以二進制可讀模式打開，即“rb”，其他都為可選參數

    3. pickle.dumps(obj)：以字節對象形式返回封裝的對象，不需要寫入文件中

    4. pickle.loads(bytes_object): 從字節對象中讀取被封裝的對象，並返回

 pickle模塊可能出現三種異常：

    1. PickleError：封裝和拆封時出現的異常類，繼承自Exception

    2. PicklingError: 遇到不可封裝的對象時出現的異常，繼承自PickleError

    3. UnPicklingError: 拆封對象過程中出現的異常，繼承自PickleError

應用：

1 # dumps功能
2 import pickle
3 data = ['aa', 'bb', 'cc']  
4 # dumps 將數據通過特殊的形式轉換為只有python語言認識的字符串
5 p_str = pickle.dumps(data)
6 print(p_str)            
7 b'\x80\x03]q\x00(X\x02\x00\x00\x00aaq\x01X\x02\x00\x00\x00bbq\x02X\x02\x00\x00\x00ccq\x03e.

1 # loads功能
2 # loads  將pickle數據轉換為python的數據結構
3 mes = pickle.loads(p_str)
4 print(mes)
5 ['aa', 'bb', 'cc']

1 # dump功能
2 # dump 將數據通過特殊的形式轉換為只有python語言認識的字符串，並寫入文件
3 with open('D:/tmp.pk', 'w') as f:
4     pickle.dump(data, f)

1 # load功能
2 # load 從數據文件中讀取數據，並轉換為python的數據結構
3 with open('D:/tmp.pk', 'r') as f:
4     data = pickle.load(f)