Twelfth day: object persistence

A flat file

• Text File

scores = [88, 99, 77, 55]
def write_scores():
    with open('datafile.txt', 'w', encoding = 'utf8') as f: #文件仅含一个列表
        f.write(str(scores))
    print('文件写入完成...')
def read_scores():
    with open('datafile.txt', 'r', encoding='utf8') as f:
        lst = eval(f.read()) #转换数据类型        
    lst[0] = 99
    print(lst)
if __name__ == '__main__':
    #write_scores() #先运行写入函数
    read_scores()

[99, 99, 77, 55]

Two, pickle

1, the sequence to a string

• The target sequence into strings pickle.dumps (obj)
• As objects pickle.loads (s) from the string deserialized

import pickle
person = {'name':'Tom', 'age':20}
s = pickle.dumps(person) #把任意类型对象序列化成字符串
s
p = pickle.loads(s) #反向转换
p

b'\x80\x03}q\x00(X\x04\x00\x00\x00nameq\x01X\x03\x00\x00\x00Tomq\x02X\x03\x00\x00\x00ageq\x03K\x14u.'
{'name': 'Tom', 'age': 20}

2, the sequence object to a file

• Serialize objects into documents pickle.dump (obj, file)
• File deserialize objects pickle.load (file)

pickle.dump(person, open('pickle_db', 'wb'))
p = pickle.load(open('pickle_db', 'rb'))
p
type(p)

{'name': 'Tom', 'age': 20}
dict

Three, shelve

• .open('dbfile')
• .close()
• db['key']=obj
• len (db)

import shelve
scores = [99, 88, 77]
student = {'name':'Mike', 'age':20}
db = shelve.open('shelve_student') #创建数据库文件
db['s'] = student
db['scores'] = scores
len(db) #有两个键值
temp_student = db['s']
temp_student
type(temp_student)

2
{'name': 'Mike', 'age': 20}
dict

• del db['key']

del db['scores']
len(db)

1

class Student:
    def __init__(self, name, age):
        self.name = name 
        self.age = age
    def __str__(self):
        return self.name
def write_shelve():
        s = Student('Yom', 20)
        db = shelve.open('shelve_student_db')
        db['s'] = s
        db.close()
def read_shelve():
    db = shelve.open('shelve_student_db')
    st = db['s']
    print(st)
    print(st.name)
    print(st.age)
    db.close()
if __name__ == '__main__':
    #write_shelve() #先写入再读取
    read_shelve()

Yom
Yom
20