Question

Suppose we have the following code. I want to swap out the inner parse_place function. How do you do this without replacing the whole method?

class GoogleV3Place(GoogleV3):
    """Simply extends the GoogleV3 to bucket the object into a place"""

    def parse_json(self, page, exactly_one=True):
        """Returns location, (latitude, longitude) from json feed."""
        if not isinstance(page, basestring):
            page = util.decode_page(page)
        self.doc = json.loads(page)
        places = self.doc.get('results', [])

        if not places:
            check_status(self.doc.get('status'))
            return None
        elif exactly_one and len(places) != 1:
            raise ValueError(
                "Didn't find exactly one placemark! (Found %d)" % len(places))

        def parse_place(place):
            """This returns an object how we want it returned."""
            location.formatted_address = place.get('formatted_address')
            location.latitude = place['geometry']['location']['lat']
            location.longitude = place['geometry']['location']['lng']

            latitude = place['geometry']['location']['lat']
            longitude = place['geometry']['location']['lng']
            return (location, (latitude, longitude))

        if exactly_one:
            return parse_place(places[0])
        else:
            return [parse_place(place) for place in places]

Answer 1

If you have control over the GoogleV3Place code (ie it’s code you’ve written and isn’t in a library), then I’d refactor parse_json to take a “parse_place_fn” argument, and move parse_place to be a top-level function (if accessibility is an issue you can always prefix it with double-underscore):

def parse_place(place):
    """This returns an object how we want it returned."""
    location.formatted_address = place.get('formatted_address')
    location.latitude = place['geometry']['location']['lat']
    location.longitude = place['geometry']['location']['lng']

    latitude = place['geometry']['location']['lat']
    longitude = place['geometry']['location']['lng']
    return (location, (latitude, longitude))

class GoogleV3Place(GoogleV3):
    """Simply extends the GoogleV3 to bucket the object into a place"""

    def parse_json(self, page, exactly_one=True, parse_place_fn = parse_place):
        """Returns location, (latitude, longitude) from json feed."""
        if not isinstance(page, basestring):
            page = util.decode_page(page)
        self.doc = json.loads(page)
        places = self.doc.get('results', [])

        if not places:
            check_status(self.doc.get('status'))
            return None
        elif exactly_one and len(places) != 1:
            raise ValueError(
                "Didn't find exactly one placemark! (Found %d)" % len(places))

        if exactly_one:
            return parse_place_fn(places[0])
        else:
            return [parse_place_fn(place) for place in places]

Now any function you create which takes a place and returns a tuple of the form (location, (latitude, latitude)) can be passed to parse_json, and if no function is specified it uses the default of parse_place.