Name of the user not getting displayed in python code

I have made a signup page using python. Following is the link to the code :

Signup Page Code

As soon as a user signs up, he should be redirected to /welcome which says :

Welcome user

But I cannot display it by the above mentioned code.

Whereas when I write

self.redirect('/welcome?username = '+user)

I get redirected to a page which says

Welcome user

Why is it so?