Python Dynamic Typing – Does This Code Follow Duck Typing?

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The principle of duck typing says that you shouldn't care what type of object you have – just whether or not you can do the required action with your object. For this reason the isinstance keyword is frowned upon. – –Definition

In below snippet(function) group_tweets_by_state, following definition of duck typing, action setdefault is performed on the object tweets_by_state by appending object tweet

def group_tweets_by_state(tweets):
    tweets_by_state = {}
    USA_states_center_position = {n: find_center(s) for n, s in us_states.items()}
    for tweet in tweets:                     # tweets are list of dictionaries 
        if hassattr(tweet, 'setdefault'):    # tweet is a dictionary 
            state_name_key = find_closest_state(tweet, USA_states_center_position)
            tweets_by_state.setdefault(state_name_key, []).append(tweet)
    return tweets_by_state

My understanding is, function hasattr(tweet, 'setdefault') is type checking tweet to be of <class 'dict'> type in duck typing style, before append.

Is my understanding correct? Does function group_tweets_by_state follow duck typing?

Best Answer

A test like hassattr(tweet, 'setdefault') to make sure tweet is a dictionary is not a good one, since it obviously does not assure tweet provides all methods/properties of a dictionary. So as long tweet.setdefault is not the only method called by find_closest_state (which I think is unlikely), this test is not strict enough. On the other hand, a test like isinstance(tweet, dict) is too strict, because it forbids the usage of other, dictionary-like structures, which is exactly the idea of duck typing.

In your example the requirement is not really that tweet is a dictionary, the requirement is that find_closest_state can process the tweet, whatever methods it calls from a tweet, independently of the real type. The following solution will handle this in a generic manner, without the need of knowing exactly what methods inside find_closest_state are used:

def group_tweets_by_state(tweets):
    tweets_by_state = {}
    USA_states_center_position = {n: find_center(s) for n, s in us_states.items()}
    for tweet in tweets:              # a  tweet should behave like a dictionary
        try: 
            state_name_key = find_closest_state(tweet, USA_states_center_position)
            tweets_by_state.setdefault(state_name_key, []).append(tweet)
        except (AttributeError, TypeError):
            pass
    return tweets_by_state

The code checks for an AttributeError because that is the exception you get when find_closest_state calls a method not provided by tweet. It also checks for a TypeError, because that is what you get when you call tweet["abc"] on a non-dictionary. You may need to add some other exceptions, depending on how find_closest_state is implemented internally, but you should not add any artificial constraints.

And that's how duck typing should really be applied - by not making assumptions about the type of the object passed, only by testing whether or not you can do the required action (here: call find_closest_state without getting one of the above exceptions).

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