Some people wrote:
Function calls in Python are indicated by () following the function name.
I contend that "x )= f" captures some perfume of the invariant you mention,
But not enough of it. A syntax of "x ()= f" would seem to have more chance of being accepted. But I would still give it no more than 0.1% chance, based on the potential confusion between it and "x() = f"...
In conclusion, I guess I'm warranted to take this to mean "we can dream up no appropriate syntax".
If I were you, I would take it more as "that suggestion is too Functional (or perhaps just too confusing) for Python." (If you're looking for a language that has filed all the corners off, might I suggest Scheme. No, seriously, I'm not making a parenthesis joke here.)