I am writing a Python program that can login Gmail.

The purpose of this program is to check whether the username/password combination exists and is correct.

Since this program is to test the the username/password combination existence, it's no need to know any mail contents in Gmail.

The input of this program is a username and password.

The output of this program is either

successful login

or

login failure

Login failure could be:

1. existing username+wrong password
2. nonexisting username

My idea is to login Gmail first. Afterward, when login failure, the gmail webpage will show particular message on the login webpage. I can parse the webpage content and check whether it has that particular message.

However, I still have no idea how to login Gmail in Python. Please let me know which module can be used or write me a small piece of sample code.

Here's an idea:

Why don't you try to send an email from the account and see if it sends? You can do this with smtplib in the python standard module. There's code examples here. You'll have to look into the doc of the module, but it looks like an exception is thrown if the login fails, which should contain the details you're interested in.

In edit:

I dug up this bit of code that I wrote to do exactly that. You'll need to put a try/catch around the bit at the bottom to detect erroneous login credentials.

# Subject
now = dt.datetime.now().ctime()
subject = 'Change to system on %s' % now# Body
body = 'Subject: %s,\n' % subject
body += 'On %s, a change to the system was detected. Details follow.\n\n' % nowrelevantFiles = list(set([x.file for x in relevantChunks]))
for file in relevantFiles:fileChunks = [x for x in relevantChunks if x.file == file]for chunk in fileChunks:body += '****** Affected file %s. ' % chunk.file<some other stuff>server = smtp.SMTP(args.host) # host = smtp.gmail.com:<port> look this bit up
server.starttls()
server.login(args.username, args.password)
server.sendmail(args.sender, args.recipient, body)
server.quit()

As an aside, I'm not quite sure why this question was down-voted, or even what it takes to be down-voted other than the fact that you asked the wrong question.