Question

请注意，我在Windows操作系统上使用以下版本的Python：

(venv) C:\>python
    Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 16:07:46) [MSC v.1900 32 bit (Intel)] on win32
    Type "help", "copyright", "credits" or "license" for more information.

*运行下面的Python脚本，并在下面的目录中显示输出。该脚本大部分已从以下来源使用：

https://pynative.com/python-mysql-select-query-to-fetch-data/

脚本1

import mysql.connector
from mysql.connector import Error
try:
    cnx = mysql.connector.connect(user='root', password='coldplay123',
                                  host='localhost',
                                  database='nathan_test_1')

    cursor_1 = cnx.cursor()
    s1="select * from dataframe"
    cursor_1.execute(s1)
    data1 = cursor_1.fetchall()

    print("Total number of dataframes: ", cursor_1.rowcount)

    for i1 in data1:
        print(i1)
    cursor_1.close()

except Error as e1:
    print("Failure to connect ... ", e1)

cnx.close()

SCRIPT 1的输出

Total number of dataframes:  1
('a', 'b', 'c', 699)

*现在，我只更改脚本1的两行在中间，只需将它们注释掉并生成以下输出：

脚本2

import mysql.connector
from mysql.connector import Error
try:
    cnx = mysql.connector.connect(user='root', password='coldplay123',
                                  host='localhost',
                                  database='nathan_test_1')

    cursor_1 = cnx.cursor()
    #s1="select * from nathan"
    #cursor_1.execute(s1)
    data1 = cursor_1.fetchall()

    print("Total number of dataframes: ", cursor_1.rowcount)

    for i1 in data1:
        print(i1)
    cursor_1.close()

except Error as e1:
    print("Failure to connect ... ", e1)

cnx.close()

SCRIPT 2的输出

Failure to connect ...  No result set to fetch from.

*很容易看出是什么导致了此错误，但是为什么却没有允许'cursor_1'执行给定的SQL查询导致错误？

Answer 1

根据PEP 249 - Python Database API Specification v2.0 ，fetchone, fetchmany, fetchall documentation，

如果先前对.execute *（）的调用未产生任何结果集或尚未发出任何调用，则会引发错误（或子类）异常。

fetch*()呼叫后应进行execute呼叫。

Answer 2

我想向您分享我的预定义类以连接MySQL。您将为此使用PyMySQL。

import pymysql


class MySQL:

    """Connect the application to MySQL database."""

    def __init__(self):
        host = 'localhost'
        user = 'root'
        password = 'coldplay123'
        database = 'nathan_test_1'
        self.connection = pymysql.connect(
            host=host,
            user=user,
            password=password,
            db=database,
            charset='utf8mb4',
            cursorclass=pymysql.cursors.DictCursor,
            autocommit=True,
        )
        self.cursor = self.connection.cursor()

    def fetchone(self, sql, data=[]):
        """Select a single row."""
        self.cursor.execute(sql, data)
        result = self.cursor.fetchone()
        return result

    def fetchall(self, sql, data=[]):
        """Select all rows."""
        self.cursor.execute(sql, data)
        result = self.cursor.fetchall()
        return result

    def commit(self, sql, data=[]):
        """Make changes using Insert, Update, or Delete"""
        self.cursor.execute(sql, data)
        self.connection.commit()
        return '200'

    def __del__(self):
        """Closing the connection after a transaction."""
        self.connection.close()


db = MySQL()
# samples
db.fetchone("select * from table where id = %s;", 2)
db.fetchall("select * from table")
db.commit("insert into table (column1, column2) values (%s, %s);",
          ('cell_value1', 'cell_value2'))

Python和MySQL连接器Python

2 个答案: