You can in Javascript - but with the caveat that the keys can only be strings.

Without that, if Python allowed it, would it do given:

  a = "xxx"
  test_dict = {a=3, b=2}

would it take test_dict to mean {"xxx":3, "b":2} or {"a":3, "b":2} ?

JS does the latter always, so the variable a is not related to the literal key a which is understood as an unquoted string "a":

  > a = "xxx"
  'xxx'
  > test_dict = {a:3, b:2}
  { a: 3, b: 2 }
  > test_dict["xxx"]
  undefined
  > test_dict["a"]
  3

The assignment operator `=` can never appear in a valid Python expression, so `{a=3, b=2}` should be distinguishable from `{a: 3, b: 2}`. (So does JS, where `{[a]: 3, [b]: 2}` would evaluate a and b.)