Python基础及语法（十三）

目录

• 数据库开发
  • pymysql与MySQLdb模块
• 元编程
  • 类构建
  • 构建元类

数据库开发

比较有名的模块有：
MySQLdb 支持Python2，不知道Python3，已停止更新
mysqlclient 在MySQLdb的基础上支持Python3
MYSQL官方Connector 网址：https://dev.mysql.com/downloads/
pymysql 语法兼容MySQLdb，使用纯Python写的Mysql客户端库，支持Python3

pymysql与MySQLdb模块

MySQLdb的安装命令是：pip install mysqlclient

import pymysql
import MySQLdb
conn = None
cursor = None
try:
    # conn = pymysql.connect(host='localhost', user='user', password='password', database='test', port=3306)  # 用pymysql建立mysql连接
    conn = MySQLdb.connect(host='localhost', user='user', password='password', database='db_test', port=3306)  # 用MySQLdb建立mysql连接
    print(type(conn), conn)
    conn.ping(False)  # ping不通抛异常，True则重连
    # with conn as cursor:  # pymysql支持上下文获得游标
    cursor = conn.cursor()  # 获得游标
    sql = """select * from tb_test"""
    rows = cursor.execute(sql)  # 执行sql语句
    print(rows)  # 返回行数
    print('1', cursor.rownumber, cursor.rowcount)  # 游标当前行数，总行数
    print(cursor.fetchone())  # 返回一行
    print('2', cursor.rownumber, cursor.rowcount)
    print(cursor.fetchmany(2))  # 返回指定若干行
    print('3', cursor.rownumber, cursor.rowcount)
    cursor.rownumber = 0  # 设置游标行数，支持正负数
    print(cursor.fetchall())  # 返回游标后的所有行
    print('4', cursor.rownumber, cursor.rowcount)
    conn.commit()  # 进行增删改时，需要提交事务，查不用
except:
    conn.rollback()  # 事务回滚
finally:
    if cursor:
        cursor.close()  # 关闭游标
    if conn:
        conn.close()  # 关闭数据库连接

元编程

写代码来生成需要的代码，这就是元编程

类构建

class Test(object):
    a = 1

    def __init__(self, b):
        self.b = b


def init(self, b):
    self.b = b


test = type('test', (object,), {'__init__': init})
test.a = 1

print(type(Test), Test, Test.mro(), Test.__dict__)
print(type(test), test, test.mro(), Test.__dict__)
t1 = Test(3)
t2 = test(3)
print(t1.a)
print(t2.a)
print(type(t1), t1, t1.__dict__)
print(type(t2), t2, t2.__dict__)
# test与Test结构一样

构建元类

class A(type):  # 元类
    def __new__(cls, name, bases, dicts):
        print(cls)
        print(name)
        print(bases)
        print(dicts)
        return super().__new__(cls, name, bases, dicts)


class B(A):  # 继承父类
    pass


class C(metaclass=A):  # 使用元类A
    pass


class D(C):  # 继承父类C，使用元类A
    pass


E = A('E', (C,), {})

print(type(A), A.__bases__)  # <class 'type'> (<class 'type'>,)
print(type(B), B.__bases__)  # <class 'type'> (<class '__main__.A'>,)
print(type(C), C.__bases__, C.mro())  # <class '__main__.A'> (<class 'object'>,)
print(type(D), D.__bases__, D.mro())  # <class '__main__.A'> (<class '__main__.C'>,)
print(type(E), E.__bases__, E.mro())  # <class '__main__.A'> (<class '__main__.C'>,)