The meaning of 'dict' can be overwritten, but '{}' can not.

NIH.

Personally I hate the new {} for set syntax.

Is {} an empty set, or an empty dict?

Did you try your example? It never worked in Python2 and it doesn't work in Python3 by design.

  Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
  [GCC 5.4.0 20160609] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> x = {'foo': 5, 'bar': 6}
  >>> y = {'foo': 7, 'baz': 9}
  >>> dict(x, **{'fo'+'o': 'bar'}, **y)
    File "<stdin>", line 1
      dict(x, **{'fo'+'o': 'bar'}, **y)
                                 ^
  SyntaxError: invalid syntax

  Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
  [GCC 5.4.0 20160609] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> x = {'foo': 5, 'bar': 6}
  >>> y = {'foo': 7, 'baz': 9}
  >>> dict(x, **{'fo'+'o': 'bar'}, **y)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: type object got multiple values for keyword argument 'foo'
  >>> {**x, 'fo'+'o': 'bar', **y}
  {'foo': 7, 'baz': 9, 'bar': 6}

This is the most unreadable code I've seen so far.

Just skip all this and use Perl instead. You can write far more idiomatic, succinct readable code in Perl than you can in Python.

The whole point of Python is to not write code this way.

On the face of it, sure, but really it's some new bytecode that gets rid of the limitations of using kwargs like that, namely it's slow, impossible to really optimize and doesn't support duplicate keys.

Except that does not necessarily work with non-string keys (it'd depend on version and implementation IIRC).

The expanded unpacking works in all cases.

And people say perl is unreadable ;)

Not really, and it's not getting most pythonic way of doing things IMO. With the syntax above (I think) you can work with any iterable, whereas with a dictionary union operator you'd have to define it on every class you'd want to use, and you'd be out of luck with generators.