What is the purpose of passing "globals()" as web.py fvars?

Question

web.application accepts an undocumented fvars argument to which the web.py tutorial passes globals() like so:

import web

class index:
    def GET(self):
        return "Hello, world!"

urls = (
  '/', 'index'
)

if __name__ == "__main__": 
    app = web.application(urls, globals())

I've seen at least one application that passes locals(). What is this variable used for, and why would you want to pass it locals() or globals()?

Answer 1

It's used by application.handle() (which in turn calls application._delegate()) to convert the handler class from a string to the class object itself. Source code here.

For example, in your code snippet above, urls = ('/', 'index') is the URL-to-class-string mapping. So web.application needs your globals() dict to be able to look up the string 'index' and get the class itself.

I actually think this is a somewhat non-Pythonic design. Why not just pass in the class directly? I think web.py supports that approach too. However, I believe the classes were done as strings so autoreload is simpler. The autoreload code uses fvars heavily.

Re locals(): at the module level, locals() doesn't really make sense, but it returns the same dictionary as globals(), which is why that would work.