actually, only

  dict(d1, **d2)

has that problem. it works fine if you unpack into a dict literal:

  >>> d1 = {1: 'a'}
  >>> d2 = {2: 'b'}
  >>> {**d1, **d2}
  {1: 'a', 2: 'b'}

iirc the pep mostly just says that it's suboptimal because it's syntactically heavy/noisy, non-obvious and can't be overloaded in dict subclasses

---

i was curious why the two double-stars behave differently despite syntactic similarity. so i went and checked the bytecode, and it turns out they compile down to different opcodes! `{××d1, ××d2}` yields a BUILD_MAP_UNPACK, while `dict(d1, ××d2)` yields a CALL_FUNCTION_EX/CALL_FUNCTION_KW (depending on the CPython version)

This seems pretty straightforward. When doing

`dict(d1, d2)`

You are calling the dict function, and using the normal syntax for unpacking a dictionary into kwargs. In this case, the name for kwargs must be strings.

yeah, that side of the (in)equation was pretty obvious, i was mostly interested in the `{...}` one. i admit that a bytecode listing probably isn't the best exposition, i just like digging into VM stuff :)

  dict(d1, **d2)

only works with string keys

  {**d1,**d2} or dict({**d1,**d2}) 

works with all key types it seems