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0 pointsLatty3y ago0 comments

Dictionary merge is a weird one to single out to me, it's an existing operator (|) and functions the same way as a set union over the keys (which uses that operator) in concept, it's a very natural thing that doesn't feel like an expansion of the language, just behaviour you'd expect from the datatype.

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Someone3y ago

A problem with dictionary merge that doesn’t exist with sets is what to do with the values if you have a key in both dictionaries.

Do you keep the value on the left, the value on the right, error out, try merging the values (if so: how?), add a parameter allowing the user to make that choice, require the user to pass a lambda?

I don’t think there’s a ‘best’ answer here, but if you want to make one of them easier to write as a|b, you can only support one.

LattyOP3y ago

At the end of the day people were doing {**a, **b} or dict(d1.items() | d2.items()) (which is subtly different in that set order is arbitrary so which "wins" in the duplicate key case is arbitrary) in the wild, so I think supporting a natural dict-level union is the better option, even if it can't be all things to all people.

robertlagrant3y ago

Agree. More complex stuff where precedence is tricky can be done in a dictionary comprehension.

duckerude3y ago

That problem also exists in sets because some values that aren't interchangeable still compare equal:

  >>> {1} | {1.0} | {True}
  {1}

But the behavior for dicts is surprising even in that light:

  >>> {1: 1} | {1.0: 1.0} | {True: True}
  {1: True}

wodenokoto3y ago

But that is not really the `|` operators fault. You get the exact same behaviour when you create a dictionary.

    >>> {1: 1, 1.0: 1.0, True: True}
    {1: True}

So `|` between dictionaries behaves just as you'd expect, if your expectations about creating a dictionary are correct. I think that's very reasonable.

kzrdude3y ago

bool as int is sometimes useful and often infuriating

jenny913y ago

Honestly the set operators too are just screwed: I end up using `set` surprisingly much yet I'm always going back to the docs to figure out what the "or" symbol was, etc.

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