Unsupported format character ";". Can't figure this out

Unsupported format character ";". Can't figure this out

I've been stumped on what is probably a very stupid mistake for a while
now. Searching google has not helped me.
I am learning python and trying to create a simple signup page. When I try
to use string substitution I seems to be failing with this error:
'Unsupported format character ";"'
This is the python code.
def write_form(self, username=""):
self.response.out.write(signupform %{'usr': username})
Here is the html:
<form method="post">
<label>Username:</label>
<input type="text" name="username" value="">%(usr)s<br>