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by ali_m 1877 days ago
IMO defaultdicts are kind of dangerous, especially when passed as arguments to other calls that expect a normal dict. Silently returning a default value instead of a raising KeyError can lead to hard-to-find bugs.

I generally prefer to use .setdefault(key, default_value) with regular dicts, as it's much more explicit. If I do use a defaultdict for convenience, I will usually only use it within a limited scope, and if I'm returning it then I'll cast it back to a normal dict to avoid surprising the caller.
1 comments
dragonwriter 1877 days ago
> IMO defaultdicts are kind of dangerous, especially when passed as arguments to other calls that expect a normal dict.

A defaultdict is a normal dict[0], its just a space-efficient way of expressing a large normal dict, most of whose keys won’t be accessed.

The default function itself should throw KeyError on values that are logically not in the dict (including, due to Python’s dynamically typed nature, those which are outside of the key domain because of type.) Though in some uses you can skip out on this because its used in a very lonited scope where you know its not going to be indexed improperly.

> I generally prefer to use .setdefault(key, default_value) with regular dicts, as it's much more explicit.

I’m not sure why one would prefer one of those over the other, as they have very different use cases; certainly defaultdict isn’t a great choice for places where .setdefault makes sense, but that’s true in reverse, too.

[0] well, except for the unfortunate .get() behavior; a ReallyDefaultDict where rdd.get(k, default) works more sensibly, returning rdd[k] unless that throws KeyError, and default otherwise, would be better.

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ali_m 1877 days ago
> The default function itself should throw KeyError on values that are logically not in the dict (including, due to Python’s dynamically typed nature, those which are outside of the key domain because of type.)

That's not how `defaultdict` works - the key isn't passed to the default factory, so there's no opportunity for it to raise a `KeyError` if the key doesn't "logically belong" in the dict. It's possible to get that sort of behaviour by overriding `__missing__`, but I very rarely see this sort of thing in the wild.

A more typical use case is something like `defaultdict(list)` as a convenient way to build a dict of lists. This is fine within a limited scope where it's obvious to the reader that they are dealing with a defaultdict that has special `__getitem__` semantics, however it's a bad idea to return a defaultdict to a caller who might be expecting a normal dict, and would be surprised that missing keys don't result in KeyErrors.

With `.setdefault(key, default_value)` it's unambiguous what we're trying to achieve - the reader doesn't need to know whether they are dealing with a defaultdict.

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dragonwriter 1876 days ago
> That's not how `defaultdict` works - the key isn't passed to the default factory, so there's no opportunity for it to raise a `KeyError` if the key doesn't "logically belong" in the dict. It's possible to get that sort of behaviour by overriding `__missing__`, but I very rarely see this sort of thing in the wild.

You're right. I was thinking of the how Ruby Hash-with-default works (I forget that you have to override __missing__ to get that with Python because .get() not working consistently with __getitem__ leads me to avoid defaultdict in practice.)

> With `.setdefault(key, default_value)` it's unambiguous what we're trying to achieve

Sure, but that’s only useful where you know the key that is going ro be accessed, which isn’t the use case for passing a “dictionary that can generate an appropriate default for keys that aren’t stored” either up or down a call chain. So while I agree that setdefault is ideal foe that use case, I don’t think it substitutes for a well-designed dictionary-with-default-generator. (Unfortunately, defaultdict doesn’t either, but its at least-usable-but-dangerous in that role, whereas setdefault() fundamentally can’t fill it since it involves situations where thr access is nonlocal to the code that wants the special handling.)

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