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Comment by dstola | Hacker News Reader

In case the values match you could supply a collision callback to define what to do, eg to add the values,

  d1 = {'a': 1}
  d2 = {'a': 2}

  d3 = {**d1, **d2, add_func)

  def add_func(a, b):
      return a+b

Or something along those lines

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Why not raise a ValueError and let the programmer figure out what The Right Thing To Do is when you add two dicts that have the same key with a different value?

I assume the same key with the same value would be OK, but I'm not really sure it's a good idea for it to be OK.

You can't do value comparison without making dict item comparison a pissed in function or making dict values immutable. If you're doing something that really looks like a mathematical Union that will raise if there's any overlap then it's a really confusing abuse of notation. I don't think there's a way out.

That is one thing you could do to merge dicts. To expand on my last paragraph above, I think I would imagine the following operations (stupid syntax):

  a & b = { k: (a[k], b[k]) for k in a.keys() | b.keys() }
  a | b = { k: (a.get(k, None), b.get(k, None)) for k in a.keys() | b.keys() }
  a |& b = { k: (a.get(k,None), v) for k, v in b.items() }
  a &| b = { k: (v, b.get(k,None)) for k, v in a.items() }
  a |_| b = { k: only(a,b,k) for k in a.keys() | b.keys() }
  def only(a,b,k):
    if k in a && k in b:
      throw DuplicateKey(a,b,k)
    elseif k not in a && k not in b:
      assert(false)
    elseif k in a:
      return a[k]
    else:
      return b[k]

This doesn’t work well if values can be None so maybe instead of pairs there should be objects Left(x), Right(y), and Both(x,y)

That syntax doesn't make sense. The

  {**d1, **d2}

idiom is just a clever mashup of Python's dictionary construction literal {}, and * * unpacking. That's why it only works with string-valued keys (which is a major limitation).

Adding a third item to the dictionary literal would require special-casing the {} dictionary construction literal.

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