On Sat, 24 Oct 2009 02:02:48 pm Ben Finney wrote:
Steven D'Aprano <steve@pearwood.info> writes:
On Sat, 24 Oct 2009 10:26:27 am Ben Finney wrote:
Steven D'Aprano <steve@pearwood.info> writes:
The lack of get() in sets and frozensets is sounding more and more to me like the victory of purity over practicality.
What would be the input to ‘set.get’?
It wouldn't take any input.
That is even less obvious. I would expect a parameter-less ‘set.get’ to get the set. Not terribly useful, but the name and function signature doesn't suggest anything else.
You already have the set. Why would you need a method that you call that returns the set you already have? A method called "get" obviously gets something, and if it takes no arguments the only thing is has access to is the set. The obvious things it could get are the set as a whole or some part of the set. Since getting the set as a whole would be pointless, and we're allowed to assume that the language designers aren't lunatics who would waste time creating pointless methods, the obvious answer is that it gets some part of the set. Since there's no obvious reason to choose one subset over another subset, the only useful thing it could return is a single arbitrary item. But if you're not willing to guess what it gets, you are permitted to read the docstring.
"get" is such a generic term that I don't believe that is a problem.
The problem above is made less problematic by the fact that the function signature (e.g. ‘foo_dict.get(key)’) clarifies the answer to the question “get what?”. Whereas ‘foo_set.get()’ doesn't communicate much at all to the reader.
You are demanding a level of intuitiveness that few, if any, functions in the standard library would be able to meet. Consider list.index(x): would you expect it to return the value at index x, or the index of value x? Either description is compatible with the name, and in fact people sometimes mix them up. Or sorted() -- from the name alone, I'd expect this to work, but it doesn't:
sorted(1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'int' object is not iterable
That's not a problem with the built-in function. I took a guess about the API, and guessed wrong. I'll learn from this, and get it right next time.
If we want a method that gets one item from a set, perhaps the name can make it clearer: name it ‘set.getitem’. But which item should it get? The ‘__getitem__’ special method of lists and dictionaries requires an index or key as parameter.
Neither of which is appropriate for sets -- sets are unordered, so elements don't have indexes, and they don't map keys to values. They just have elements. Sets are not lists, and they're not dicts. Analogies only go so far, and you can't reason about set methods by considering how lists or dicts will behave. -- Steven D'Aprano