There's a lot of talk in this thread about Python going down-hill and becoming less obvious/simple. I rather like modern python, but I agree that some features (like async/await, whose implementation fractures functions and libraries into two colors [0]) seem like downgrades in "Pythonicity".
That said, I think some things have unquestionably gotten more "Pythonic" with time, and the := operator is one of those. In contrast, this early Python feature (mentioned in an article [1] linked in the main one) strikes me as almost comically unfriendly to new programmers:
> Python vowed to solve [the problem of accidentally assigning instead of comparing variables] in a different way. The original Python had a single "=" for both assignment and equality testing, as Tim Peters recently reminded him, but it used a different syntactic distinction to ensure that the C problem could not occur.
If you're just learning to program and know nothing about the distinction between an expression and a statement, this is about as confusing as shell expansion (another context-dependent syntax). It's way too clever to be Pythonic. The new syntax, though it adds an extra symbol to learn, is at least 100% explicit.
I'll add that := fixes something I truly hate: the lack of `do until` in Python, which strikes me as deeply un-Pythonic. Am I supposed to break out of `while True`? Am I supposed to set the variable before and at the tail of the loop (a great way to add subtle typos that will cause errors)? I think it also introduces a slippery slope to be encouraged to repeat yourself: if assigning the loop variable happens twice, you might decide to do something funny the 2:Nth time to avoid writing another loop, and that subtlety in loop variable assignment can be very easy to miss when reading code. There is no general solution I've seen to this prior to :=. Now, you can write something like `while line := f.readline()` and avoid repetition. I'm very happy to see this.
[0] https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...
[1] https://lwn.net/Articles/757713/
[edit] fixed typos