[Python] Dict merge in a dict comprehension

In python 3.5, we can merge dicts by using double-splat unpacking

>>> d1 = {1: 'one', 2: 'two'}
>>> d2 = {3: 'three'}
>>> {**d1, **d2}
{1: 'one', 2: 'two', 3: 'three'}


Cool. It doesn't seem to generalise to dynamic use cases, though:

>>> ds = [d1, d2]
>>> {**d for d in ds}
SyntaxError: dict unpacking cannot be used in dict comprehension


Instead we have to do reduce(lambda x,y: {**x, **y}, ds, {}), which seems a lot uglier. Why the "one obvious way to do it" is not allowed by the parser, when there doesn't seem to be any ambiguity in that expression?

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