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@app.before_request
def check_valid_login():
    login_valid = 'user' in session # or whatever you use to check valid login

    if (request.endpoint and 
        'static' not in request.endpoint and 
        not login_valid and 
        not getattr(app.view_functions[request.endpoint], 'is_public', False) ) :
        return render_template('login.html', next=request.endpoint)

#then created an is_public() decorator for the few places that would need to be accessible 
#without login:

def public_endpoint(function):
    function.is_public = True
    return function