SMTP connection and string processing errors and solutions for Python development

introduction

During Python development, developers often encounter various errors and exceptions. Some errors are obvious, such as syntax errors; while others are more concealed, such as network connectivity issues or restrictions on third-party APIs. This article will summarize two typical Python development problems: SMTP server connection failure and f-string string processing errors, analyze its causes, and provide detailed solutions. The article will also include code samples and best practice suggestions to help developers avoid similar issues.

1. SMTP server connection failure problem

1.1 Error phenomenon

When trying to connect to the QQ mailbox SMTP server using Python's smtplib, the following error occurs:

2025-05-15 23:34:58,808 - app - ERROR - SMTP server connection failed: (-1, b'\x00\x00\x00')
Traceback (most recent call last):
File "/doudian-phone-tool/doudian/send_qq_email.py", line 70, in send_email_with_attachment
with smtplib.SMTP_SSL(smtp_server, smtp_port) as server:
File "/usr/local/lib/python3.11/", line 284, in __exit__
raise SMTPResponseException(code, message)
: (-1, b'\x00\x00\x00')

1.2 Possible Causes

Service not enabled: QQ mailbox has SMTP service turned off by default and needs to be enabled manually.

2. Incorrect server configuration:

Incorrect SMTP server address (such as misspelling)

Incorrect port number (465 or 587)

3. Authorization issues:

Email password instead of authorization code (QQ mailbox requires authorization code instead of original password)

The authorization code has expired (the authorization code will be invalid after modifying the QQ email password)

4. Network or firewall restrictions:

The server cannot access SMTP service (such as the cloud server does not open port 465)

Local firewall or security group rules block connections

1.3 Solution

(1) Check and enable the QQ mailbox SMTP service

Find POP3/IMAP/SMTP/Exchange/CardDAV/CalDAV service

Turn on the IMAP/SMTP service and generate a new authorization code (16-bit string)

(2) Verify SMTP configuration

Make sure the SMTP configuration in the code is correct:

smtp_server = ""  # Must be correctsmtp_port = 465  # SSL port, or 587 (STARTTLS)sender_email = "your_email@"
password = "your_authorization_code"  # Not the QQ email password!

(3) Improve error handling

Optimize the code, add more detailed error logs and retry mechanisms:

import smtplib
import time
from socket import gaierror

def send_email_with_retry(sender, password, recipients, msg, max_retries=3):
    for attempt in range(max_retries):
        try:
            with smtplib.SMTP_SSL("", 465) as server:
                (sender, password)
                (sender, recipients, msg.as_string())
                return True
        except :
            print("SMTP authentication failed: Please check the email and authorization code")
            return False
        except (, gaierror) as e:
            print(f"SMTPConnection failed（try {attempt + 1}/{max_retries}）：{str(e)}")
            (2)  # Retry after delaying by 2 seconds    return False

2. Error in nesting quotation marks of f-string strings

2.1 Error phenomenon

When using f-string in Python code, syntax errors are caused by quotation nesting errors:

current_app.(f"【{item['waybillNum']}】way bill number, [{record["Mobile phone number"]}] mobile phone number is processed successfully, save the order to the database")
^^^^^^^^^
SyntaxError: f-string: unmatched '['

2.2 Cause analysis

Quotation conflict: The outer layer of f-string uses double quotes "...", and the internal dictionary key also uses double quotes record["Mobile Number"], causing the Python parser to fail to correctly recognize string boundaries.

Parse rules for f-string: Python's f-string requires that quotes must be paired correctly, otherwise a SyntaxError will be thrown.

2.3 Solution

(1) Use single quotes on the outer layer and double quotes on the inner layer

current_app.(f'【{item["waybillNum"]}】Way bill number，【{record["Phone number"]}】The mobile phone number is successfully processed，Save orders to database')

(2) Use single quotes in a uniform manner

current_app.(f'【{item[\'waybillNum\']}】Way bill number，【{record[\'Phone number\']}】The mobile phone number is successfully processed，Save orders to database')

(3) Use escape characters

current_app.(f"【{item['waybillNum']}】Way bill number，【{record['Phone number']}】The mobile phone number is successfully processed，Save orders to database")

2.4 Best Practices

Single quotes are preferred as string separators (recommended by the Python community).

Avoid mixing quotes, such as outer double quotes + inner double quotes.

Use formatting tools such as black or autopep8 to automatically adjust the code style.

3. Summary and Suggestions

3.1 How to avoid similar errors

Question Type	Preventive measures
SMTP connection failed	1. Check whether the SMTP service is enabled 2. Use the correct authorization code 3. Add error retry mechanism
F-string quotation mark error	1. Uniform quote style 2. Use the code formatting tool 3. Avoid multi-layer nesting

3.2 Debugging skills

Logging: Use the logging module to record detailed error information.

Step by step test: first test the SMTP connection (such as telnet 465), and then debug the code.

Static checking: Use pylint or flake8 to check for syntax problems.

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