I wish I had more time to make my case, but with the PEP 572 pronouncement imminent, let me make an attempt to save Python from having two assignment operators.
I've re-read the PEP, and honestly I am warming up to the idea of allowing a limited form of assignment in expressions. It looks like in the current form, the PEP supports only well-motivated cases where the return value of the assignment expression is non-controversial. It also appears that there are no cases where = can be substituted for := and not cause a syntax error. This means that ":" in ":=" is strictly redundant. Interestingly, Python already has a precedent for using redundant ":" - the line-ending ":" in various statements is redundant, but it is helpful both when reading and writing the code. On the other hand, ':' in ':=' looks like an unnecessary embellishment. When we use ':=', we already know that we are inside an expression and being inside an expression is an obvious context for the reader, the writer and the interpreter. I also believe, allowing a limited form of assignment in expressions is a simpler story to tell to the existing users than an introduction of a new operator that is somewhat like '=', but cannot be used where you currently use '=' and only in places where '=' is currently prohibited.
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