Python 和 MySQL-Connector-Python

Question

note I am using the following version of Python on a Windows OS:

(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.

*The following Python script below is run and the output is shown directory below it. Most of this script has been used from the following source:

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

SCRIPT 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()

Output of SCRIPT 1

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

*Now, I change just two lines from Script 1 in the middle by simply commenting them out and the following output is generated:

SCRIPT 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()

Output of SCRIPT 2

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

*It is easy to see what causes this error, but why does not allowing 'cursor_1' to execute a given SQL query cause this error?

Answer 1

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

An Error (or subclass) exception is raised if the previous call to .execute*() did not produce any result set or no call was issued yet.

fetch*() call should be followed by execute call.