params = dict(username = self.user.username,
AttributeError: 'NoneType' object has no attribute 'username'

class User(ndb.Model):
    username = ndb.StringProperty(required = True)
    nickname = ndb.StringProperty(required = True)
    pw_hash = ndb.StringProperty(required = True)
    profile_img_key = ndb.BlobKeyProperty()

    @classmethod
    def by_id(cls, id):
        return cls.get_by_id(id)

class Handler(webapp2.RequestHandler):
    def initialize(self, *a, **kw):
        webapp2.RequestHandler.initialize(self, *a, **kw)
        uid = self.read_secure_cookie('user_id')
        # all handlers get self.user
        self.user = uid and User.by_id( int(uid) )

    def read_secure_cookie(self, cookie_name):
        cookie_val = self.request.cookies.get(cookie_name)
        return cookie_val and check_secure_val(cookie_val)

class MyInfoHandler(Handler, blobstore_handlers.BlobstoreUploadHandler):
    def get(self):
        if not self.user:
            self.redirect('/blog/login')

        upload_url = blobstore.create_upload_url('/blog/myinfo')

        # required variables
        params = dict(username = self.user.username,
                      upload_url = upload_url
                      )

        # check for optional variables
        if self.user.nickname:
            params['nickname'] = self.user.nickname
        if self.user.profile_img_key:
            params['profile_img_url'] = '/blog/serve/%s' % self.user.profile_img_key

        # fill in input fields with params
        self.render('myinfo.html', **params)

    def post(self):
        if not self.user:
            self.redirect('/blog/login')

        self.nickname = self.request.get('nickname')

        params = dict(username = self.user.username,
                      nickname = self.nickname,
                    )

        upload_files = self.get_uploads('profile_img')
        if upload_files:
            blob_key = upload_files[0].key()
            params['profile_img_url'] = '/blog/serve/%s' % blob_key

        error_found = False
        if not error_found:
            update_entity(self.user, **params)
            params['help'] = "Successfully updated!"
        else:
            params['help'] = "Check your information again"

        self.render('myinfo.html', **params)

def update_entity(ent, **params):
    for name, value in params.items():
        setattr(ent, name, value)
        ent.put()

class ServeHandler(Handler, blobstore_handlers.BlobstoreDownloadHandler):
    def get(self, resource):
        resource = str(urllib.unquote(resource))
        blob_info = blobstore.BlobInfo.get(resource)
        self.send_blob(blob_info)

<form class="form-horizontal" action="{{ upload_url }}" role="form" method="post" enctype="multipart/form-data">
    <h2>My info</h2>

    <div class="form-group">
        <div class="col-xs-offset-3">
            <img src="{{ profile_img_url }}" />
        </div>
    </div>
    <!-- profile image upload -->
    <div class="form-group">
        <label class="control-label col-xs-3">Profile image:</label>
        <div class="col-xs-3">
            <input class="btn btn-default btn-file" type="file" name="profile_img">
        </div>
    </div>
    <div class="form-group">
        <label class="control-label col-xs-3">Username:</label>
        <label class="col-xs-3">{{username}}</label>
        <span class="help-block error">{{username_error}}</span>
    </div>
    <div class="form-group">
        <label class="control-label col-xs-3">Nickname:</label>
        <div class="col-xs-3">
            <input class="form-control" type="text" name="nickname" value="{{nickname}}">
        </div>
        <span class="help-block error">{{nickname_error}}</span>
    </div>
    <div class="form-group">
        <div class="col-xs-offset-3 col-xs-2">
            <input class="btn btn-lg btn-primary" type="submit" name="submit" value="Update">
            <div class="help-block">{{help}}</div>
        </div>
    </div>
</form>