Python PostgreSQL – 使用提交和回滚的事务管理

在本文中，我们将了解如何使用 Commit 和 Rollback 进行事务管理。

在 pyscopg 中，psycopg 中的连接类负责处理事务。当您使用游标对象向 PostgreSQL 数据库发出第一条 SQL 语句时，psycopg 会生成一个新事务。从那时起，Psycopg 将在同一事务中执行所有后续语句。如果任何语句失败，Psycopg 将停止事务。 Commit() 和 rollback() 是连接类的两种方法，可用于停止事务。 commit()函数用于永久提交对 PostgreSQL 数据库的所有更改。您还可以使用 rollback()函数撤消所做的任何修改。

犯罪：

connection.commit()

回滚：

connection.rollback()

以下是如何在 psycopg 中处理事务的示例，这是处理它的最基本方法。

我们正在处理的数据库中的表如下所示：

单击此处查看和下载 CSV 文件。

在 PostgreSQL 中创建 sales 表并导入 CSV 文件：

Python3

# import packages
import psycopg2
import pandas as pd
from sqlalchemy import create_engine
 
# establish connections
conn_string = 'postgres://postgres:pass@127.0.0.1/SuperMart'
 
db = create_engine(conn_string)
conn = db.connect()
conn1 = psycopg2.connect(
  database="SuperMart", user='postgres',
  password='pass', host='127.0.0.1', port='5432'
)
 
conn1.autocommit = True
cursor = conn1.cursor()
 
# drop table if it already exists
cursor.execute('drop table if exists sales')
 
sql = '''CREATE TABLE sales(Order_Line int,\
Order_ID char(20),Order_Date Date,Ship_Date Date,\
Ship_Mode char(20) ,Customer_ID char(20),Product_ID char(20),\
Sales decimal,Quantity int,Discount decimal,Profit decimal);'''
 
cursor.execute(sql)
 
# import the csv file to create a dataframe
data = pd.read_csv("Sales.csv")
 
# converting data to sql
data.to_sql('sales', conn, if_exists='replace')
 
# fetching all rows
sql1 = '''select * from sales;'''
cursor.execute(sql1)
for i in cursor.fetchall():
    print(i)
 
conn1.commit()
conn1.close()

示例 1：成功交易的示例

代码从导入包开始，建立与数据库的连接，我们必须确保 connection.autocommit 为 false，因为默认情况下它为 True，它预先提交所有更改，我们不能使用 rollback() 回到以前的状态.游标是使用 connection.cursor() 方法创建的。查询和获取数据。计算特定 order_id 的销售额。由于 SQL 语句没有任何错误，因此事务成功完成。

Python3

# import packages
import psycopg2
 
try:
    # establish a connection to the database
    connection = psycopg2.connect(
        database="SuperMart", user='postgres',
        password='PASS', host='127.0.0.1', port='5432')
     
    # disable autocommit mode
    connection.autocommit = False
     
    # creating a cursor object
    cursor = connection.cursor()
 
    # querying data
    query = """select sales from sales where\
    Order_ID = 'CA-2016-152156'"""
    cursor.execute(query)
    record = cursor.fetchall()
    sum = 0
    for i in record:
        sum = sum+i[0]
    print("total sales from the order id CA-2016-152156 is : " + str(sum))
 
    # commiting changes
    connection.commit()
    print("successfully finished the transaction ")
 
except (Exception, psycopg2.DatabaseError) as error:
    print("Error in transaction, reverting all changes using rollback ", error)
    connection.rollback()
 
finally:
 
    # closing database connection.
    if connection:
       
        # closing connections
        cursor.close()
        connection.close()
        print("PostgreSQL database connection is closed")

输出：

total sales from the order id CA-2016-152156 is : 993.9000000000001

successfully finished the transaction 

PostgreSQL database connection is closed

示例 2：不成功事务的示例

代码与前面的代码类似，只是在 SQL 语句中给出了错误的表名，因为它不正确，所有更改都使用 rollback() 方法恢复或撤消并关闭连接。

Python3

# import packages
import psycopg2
 
try:
    # establish a connection to the database
    connection = psycopg2.connect(database="SuperMart",
                                  user='postgres',
                                  password='sherlockedisi',
                                  host='127.0.0.1',
                                  port='5432')
    # disable autocommit mode
    connection.autocommit = False
     
    # creating a cursor object
    cursor = connection.cursor()
 
    # querying data
    query = """select sales from sales1 \
    where Order_ID = 'CA-2016-1521591'"""
    cursor.execute(query)
     
    # fetching data
    record = cursor.fetchall()
    sum = 0
    for i in record:
        sum = sum+i[0]
    print("total sales from the order\
    id CA-2016-152159 is : " + str(sum))
 
    # commiting changes
    connection.commit()
    print("successfully finished the transaction ")
 
except (Exception, psycopg2.DatabaseError) as error:
    print("Error in transaction, reverting all\
    changes using rollback, error is : ", error)
    connection.rollback()
 
finally:
 
    # closing database connection.
    if connection:
       
        # closing connections
        cursor.close()
        connection.close()
        print("PostgreSQL database connection is closed")

输出：

Error in transaction, reverting all changes using rollback, error is :  relation “sales1” does not exist

LINE 1: select sales from sales1 where order_id = ‘CA-2016-1521591’

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