Object does not support item assignment error

In my views.py I assign values before saving the form. I used to do it the following way:

projectForm.lat = session_results['lat']
projectForm.lng = session_results['lng']

Now, since the list of variables got a bit long, I wanted to loop over session_results with the following loop (as described by Adam here):

for k,v in session_results.iteritems():
    projectForm[k] = v

But I get the error 'Project' object does not support item assignment for the loop solution. I have trouble to understand why. Project is the model class, which I use for the ModelForm.

Thank you for your help!