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Python Classes, Objects and Special Methods, what does print(c.host, c.port) print after class Config?

Block 3, Classes, Objects and Special Methods, question 15 of the Python sample

class Config:
    def __init__(self, **kwargs):
        self._data = kwargs

    def __getattr__(self, name):
        try:
            return self._data[name]
        except KeyError:
            raise AttributeError(name)

c = Config(host='localhost', port=8080)
print(c.host, c.port)

What does this print?

  1. A. None None
  2. B. AttributeError
  3. C. localhost 80
  4. D. localhost 8080
Show the answer and why each option is right or wrong

Answer, D. localhost 8080

A. None None
Assumes the fallback returns nothing usable for keys that were never set as real attributes. Both host and port were passed as keyword arguments and stored in _data, so the fallback finds and returns each of them.
B. AttributeError
Assumes accessing keys that only live inside the internal dictionary always fails with an error. __getattr__ specifically catches the missing attribute case and redirects it to a dictionary lookup, returning the stored value instead of failing, since both keys exist.
C. localhost 80
Truncates the port number as if only part of it were stored or printed. The full integer 8080 is stored in the dictionary and returned exactly as it was passed in.
D. localhost 8080
Correct. host and port are not real attributes, so each access triggers __getattr__, which looks them up in the stored dictionary.

Storing arbitrary keyword arguments in a dictionary and then defining __getattr__ to look names up in that dictionary is a common way to expose dynamic attributes without declaring them individually. Since __getattr__ only runs for names missing from the instance, and host and port were never set directly, both accesses are redirected into the stored dictionary.

The answer above is what the CPython interpreter printed when the snippet was run, not what someone expected it to do.

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