Question

I’m having trouble using python function decorators in Google’s AppEngine. I’m not that familiar with decorators, but they seem useful in web programming when you might want to force a user to login before executing certain functions.

Anyway, I was following along with a flickr login example here that uses django and decorates a function to require the flickr login. I can’t seem to get this type of decorator to work in AppEngine.

I’ve boiled it down to this:

def require_auth(func):
    def check_auth(*args, **kwargs):
        print "Authenticated."
        return func(*args, **kwargs)
    return check_auth

@require_auth
def content():
    print "Release sensitive data!"

content()

This code works from the commandline, but when I run it in GoogleAppEngineLauncher (OS X), I get the following error:

check_auth() takes at least 1 argument (0 given) 

And I’m not really sure why…

EDIT to include actual code:
@asperous.us I changed the definition of content() to include variable arguments, is that what you meant?
@Alex Martelli, ‘print’ does work within AppEngine, but still a completely fair criticism.
Like I said, I’m trying to use the flickr login from the link above. I tried to put it into my app like so:

def require_flickr_auth(view):
    def protected_view(request,*args, **kwargs):
        if 'token' in request.session:
            token = request.session['token']
            log.info('Getting token from session: %s' % token)
        else:
            token = None
            log.info('No token in session')

        f = flickrapi.FlickrAPI(api_key, api_secret,
                                token=token, store_token=False)

        if token:
            # We have a token, but it might not be valid
            log.info('Verifying token')
            try:
                f.auth_checkToken()
            except flickrapi.FlickrError:
                token = None
                del request.session['token']

        if not token:
            # No valid token, so redirect to Flickr
            log.info('Redirecting user to Flickr to get frob')
            url = f.web_login_url(perms='read')
            print "Redirect to %s" % url

        # If the token is valid, we can call the decorated view.
        log.info('Token is valid')
        return view(request,*args, **kwargs)

    return protected_view

@require_flickr_auth
def content(*args, **kwargs):
    print 'Welcome, oh authenticated user!'

def main():
    print 'Content-Type: text/plain'
    content()

if __name__ == "__main__":
    main()

When I remove the @require_flickr_auth decoration, the string ‘Welcome …’ prints out just fine. Otherwise I get a big ugly AppEngine exception page with

type 'exceptions.TypeError': protected_view() takes at least 1 argument (0 given) 

at the bottom.

Answer 1

You’re calling content() without any arguments, but the decorated version protected_view requires the request argument. Either add the argument to content or remove it from protected_view.

If you’re getting that error with your simple version then I’d suspect that content is a class method as Alex suggested. Otherwise it looks like you’re telling it to expect at least one argument, then not supplying it any.