open mode	describe
b	binary mode.
+	Open a file for update (readable and writable).
r	Open the file read-only . The file pointer will be placed at the beginning of the file. This is the default mode.
rb	Open a file in binary format for reading only . The file pointer will be placed at the beginning of the file. This is the default mode. Generally used for non-text files such as pictures, etc.
r+	Open a file for reading and writing . The file pointer will be placed at the beginning of the file.
rb+	Open a file in binary format for reading and writing . The file pointer will be placed at the beginning of the file. Generally used for non-text files such as pictures, etc.
w	Open a file for writing only . If the file already exists, the file will be opened and edited from the beginning, ie the original content will be deleted . If the file does not exist, create a new file .
wb	Open a file in binary format for writing only . If the file already exists, the file will be opened and edited from the beginning , ie the original content will be deleted . If the file does not exist, create a new file .
w+	Open a file for reading and writing . If the file already exists, the file will be opened and edited from the beginning, ie the original content will be deleted . If the file does not exist, create a new file .
wb+	Open a file in binary format for reading and writing . If the file already exists, the file will be opened and edited from the beginning, ie the original content will be deleted . If the file does not exist, create a new file . Generally used for non-text files such as pictures, etc.
a	Open a file for appending . If the file already exists, the file pointer will be placed at the end of the file. That is, the new content will be written after the existing content . If the file does not exist, a new file is created for writing.
ab	Open a file in binary format for appending . If the file already exists, the file pointer will be placed at the end of the file. That is, the new content will be written after the existing content. If the file does not exist, a new file is created for writing.
a+	Open a file for reading and writing . If the file already exists, the file pointer will be placed at the end of the file . The file will be opened in append mode. If the file does not exist, create a new file for reading and writing .
ab+	Open a file in binary format for appending . If the file already exists, the file pointer will be placed at the end of the file. If the file does not exist, create a new file for reading and writing.

Reference from: https://www.runoob.com/python3/python3-file-methods.html

1-1) Two ways to open the file:

# 打开指定的文件，默认以读的方式打开，并指定编码的格式为utf-8
f = open('./test.txt','r', encoding = 'utf-8')

# 读取所有的内容
content = f.read()

# 输出内容
print(content)

# 使用完时手动关闭文件
f.close()

# 使用 with 自动处理资源关闭
with open('test.txt', 'r', encoding='utf-8') as f:
    content = f.read()
    # 到 with 缩进内容的最后，文件会被关闭
print(content)          # 执行这句前，文件已被关闭

1-2) Simple example of file operations:

test.txt
Yesterday was Monday, the weather was fine and I was very happy;
Today is Tuesday, it is raining heavily, I am very sad;
Tomorrow is Wednesday, I heard that it will rain heavily, I don't think I will be happy;
The day after tomorrow is Thursday, I don't know what will happen;
Always believe that good things are about to happen!

write method:

# 'a' 用于追加，文件指针放到文件内容的结尾
f = open('test.txt', 'a', encoding = 'utf-8')

# 换行追加指定的内容
f.write("\n Hello. goodbye!")

f.close()

Yesterday was Monday, the weather was fine and I was very happy;
Today is Tuesday, it is raining heavily, I am very sad;
Tomorrow is Wednesday, I heard that it will rain heavily, I don't think I will be happy;
The day after tomorrow is Thursday, I don't know what will happen;
Always believe that good things are about to happen!
 Hello. goodbye!

read method:

# 读取 指定读取的长度
f = open('test.txt', 'r', encoding='utf-8')
content = f.read(10)              # 读取前10个字符，如果没有指定，则是读入全部内容
print(list)
f.close()

with open('test.txt', 'r', encoding='utf8') as f:
    res = f.read(3)      # 默认读取第一行的内容，读取前3个字符
    print(res)
    res = f.read(3)      # 接着读入后面3个字符
    print(res)

readline method:

with open('test.txt', 'r', encoding='utf8') as f:
    res = f.readline()      # 默认读取第一行的内容，包括最后的换行符
    print(res)
    res = f.readline()      # 读取第二行的内容，包括最后的换行符
    print(res)

readlines method:

with open('test.txt', 'r', encoding='utf-8') as f:
    line = f.readlines()        # 返回一个列表，其中的每一个元素是每一行的内容，包括最后的换行符
    print(line)

2. Exception handling

2-1) Exception capture and processing that will not interrupt the program

num_1 = 10
num_2 = 0

try:
    res = num_1 / num_2
    print(res)          # 不会执行
except:
    print('除数不能为0')     # 执行
finally:
    print('finally')       # 放到 finally中的内容最后无论如何都会执行

print(1 + 3)               # 会继续往下执行，程序不会中断

try:
    res = num_1 / num_2
    print(res)          # 不会执行
except Exception as e:      # 捕获所有 Exception 类中已定义的错误
    print(e)            # division by zero
finally:
    print('finally')    # 执行

print(1 + 2)            # 会继续执行

2-2) Exception capture and processing of interrupt program raise

try:
    res = num_1 / num_2
    print(res)          # 不会执行
except Exception as e:
    print(e)            # division by zero
    raise Exception('除数不能为0')       # 抛出异常，并提示 除数不能为0
finally:
    print('finally')

print(1 + 2)            # 不会执行，程序会中断

try:
    res = num_1 / num_2
    print(res)          # 不会执行
except ZeroDivisionError as e1:          # 逐层捕获错误
    print(e1)            # division by zero
    raise ZeroDivisionError('除数不能为0')   # 抛出错误并自定义提示内容
except Exception as e2:                  # Exception 是所有异常类的父类，应该放在最后
    print(e2)
    raise               # 抛出错误并给出对应的默认提示
finally:
    print('finally')

print(1 + 2)            # 不会执行

2-3) Exception handling for operation files:

try:
    with open('../Normal/test.txt', 'r', encoding='utf-8') as f:
        res = f.read()
except FileNotFoundError as e:      # 文件不存在时的错误
    raise FileNotFoundError('文件不存在')
except Exception as e:              # 其他错误
    raise Exception('发生错误')
else:                   # 没有异常时执行
    print(res)

3. Object-oriented

class People:
    # 定义类属性
    type = '哺乳动物'

    # 定义私有属性 双下划线开头
    __private = ''

    # 定义构造方法 实例化时会自动调用，并且可以将实例变量或实例属性在此定义
    # self 形参属于类实例的引用，与其他语言中的 this 中的关键字类似
    # 类方法的第一个形参一定是 self （可以用其他的名称），代表实例本身
    def __init__(self, name = '', age = '', gender = ''):
        # 定义实例属性
        self.name = name
        self.age = age
        self.gender = gender

    # 用于释放实例空间，释放内存空间   程序结束后会自动调用，可以处理收尾工作
    def __del__(self):
        print('正在释放实例空间')

    # 打印类的实例使会触发的方法， 相当于 java 中的 toString；下面的操作是重写
    def __str__(self):
        return f'当前类的实例正在被打印'

    # 受保护的方法    子类可以继承
    def _protectFunc(self):
        print('这是一个受保护的方法')

    # 私有的方法     子类不能继承
    def __privateFunc(self):
        print('这是一个私有的方法')

    # 定义三个方法
    def talk(self, str):
        print(f'{self.name}: {str}.')

    def doing(self, str):
        print(f'{self.name}正在{str}.')

    def info(self):
        print(f'name: {self.name},age: {self.age}, gender: {self.gender}')

    # 定义类的静态方法，直接使用类名调用
    @classmethod
    def people_type(cls):       # cls 代表类本身
        print(cls.type)

# 新建一个Man类，继承于 People 类
class Man(People):
    # 重写父类的方法
    def talk(self, str):
        print('子类重写了父类的talk方法')

    # 拓展自己的新方法
    def eat(self):
        print('子类自己的方法 eat')

# 如果执行者是当前文件自身，则会执行以下的内容；被其他的模块导入时，不会执行下面的内容
if __name__ == '__main__':
    # p1 = People()           # 类的实例化，会自动调用构造方法
    # p1.name = '张三'          # 先找实例变量，在找类变量
    # p1.age = 14
    #
    # p2 = People()
    #
    # print(id(p1), id(p2))    # 两个不同的虚拟地址


    # p3 = People('张三', 23, '男')
    # # print(p3)
    # print(p3.name)
    #
    # People.people_type()        # 使用 类名.静态方法 调用类的静态方法（推荐）


    man = Man()
    man._protectFunc()          # 子类可以调用父类受保护的方法，不能调用父类私有的方法

4. Closure

When an inner function is defined in a function, and the inner function uses the parameters or variables of the outer function, it is called a closure.

def outer(x, y):
    z = x * y
    y = 20
    def inner(a = 0):
        return a + x + y + z
    return inner

# 由外部函数到内部函数依次传参
res = outer(2, 3)(4)        # 4 + 2 + 20 + 6
print(res)

res = outer(2, 3)()         # 0 + 2 + 20 + 6
print(res)

5. Decorator (function decorator)

In the object-oriented above, the People class defines the last line of the static method people_type: @classmethod is a decorator.

The decorator itself is also a function, and the decorator is used by other functions to change the order of program execution.

'''
定义装饰器，用于统计函数的运行时间
函数里面定义了一个函数，称之为内部函数（闭包）
装饰器自带一个参数 —— func，用于获取被装饰函数的地址
内部函数运行结束后，必须要返回其函数名（地址）
'''
def stat(func):             # 装饰器，接收到的第一个参数是要使用该装饰器的函数的地址(函数名)
    def inner():
        start = time.time()
        print('当前时间为：', start)
        func()              # 执行该函数
        end = time.time()
        print('结束时间为：', end)
        print('总用时：', end - start)
    return inner


@stat       # 表示 test2 函数使用 stat 装饰器
def test2():
    result = 99999
    for i in range(100000):
        result = result - i ** 2
    print("result = ",result)

test2()             # 如果该函数有装饰器，则会直接去执行装饰器

def decorate(func):
    def inner(*args, **kwargs):
        print('###',args, kwargs)
        func(*args, **kwargs)
        print('finally')
    return inner

@decorate
def test(name, age):
    print(f'Hello,{name}.your age is {age}.')


test('张三', age=18)

Here, test('Zhang San',age=18) is equivalent to calling inner('Zhang San',age=18) in the decorator

The parameters received by the internal function of the decorator are the parameters passed to the test. In the fourth line, the test function is executed.

6. Data manipulation on JSON

sourceObj = [
    {'name': 'Tom', 'age': 15, 'gender': 'man', 'num': 1001},
    {'name': 'ben', 'age': 14, 'gender': 'woman', 'num': 1002},
    {'name': 'jam', 'age': 15, 'gender': 'woman', 'num': 1003},
    {'name': 'jack', 'age': 16, 'gender': 'man', 'num': 1004},
]
sourceStr = ('[{"name": "Tom", "age": 15, "gender": "man", "num": 1001}, '
             '{"name": "ben", "age": 14, "gender": "woman", "num": 1002}, '
             '{"name": "jam", "age": 15, "gender": "woman", "num": 1003}, '
             '{"name": "jack", "age": 16, "gender": "man", "num": 1004}]')

import json

6-1) dumps method

Convert a json object (python dictionary) to a json string. Single quotes are converted to double quotes.

import json
jsonstr = json.dumps(sourceObj)        # 使用 dumps 方法将其转换成 json 字符串
print(type(jsonstr))
print(jsonstr)              # 单引号会变成双引号

6-2) loads method

Convert json string to json object (Python dictionary)

jsonObj = json.loads(sourceStr)         # 将 json 格式的字符串转成对象列表的形式
print(jsonObj[0]['name'])               # Tom

6-3) dump method

Write out the content in json object format to the specified file as json object

with open('./jsonString.json','w', encoding='utf-8') as f:
    json.dump(json.loads(sourceStr), f)         # 将json对象格式的内容写出

6-4) load method

Read the content in the json object file as a list of dictionaries

with open('./jsonString.json', 'r') as f:
    res = json.load(f)
    print(res[0]['name'])           # Tom
    print(type(res))                # <class 'list'>

Python notes (2) - files, exceptions, object-oriented, decorators, json

1. File operation