PEP 572: Python assignment expressions has been accepted

What is it? The link points to a discussion more deep than I’m willing to read.

It's a controversial PEP https://www.python.org/dev/peps/pep-0572/ which allows you to write Python like this:

    def foo():
        if n := randint(0, 3):
            return n ** 2
        return 1337


    [(x, y, x/y) for x in input_data if (y := f(x)) > 0]

Can somebody comment on why is this PEP controversial?

I don't think the controversy here is with the feature itself, more with the implementation. Many, me included, would have preferred to seen a different implementation of solutions to the same problems.

Code starts becoming a lot harder to reason about when more than one state is mutated on the same line. The good design of Python makes this harder than in say C and I think this is a step in the wrong direction in that regard.

The two real things this solves are checking for truthyness in an if and reusing values in a filterting comprehension. Instead of the syntax we have now that can be used anywhere, adds a whole new concept and feels kind of out-of-place, I would have much preferred a solution that can only be used in vetted places, doesn't add a new thing people need to learn and follows the style of the language

For example, my preferred solution for `if` would have been:

    if thing() as t:
        print(t)

Usage of `as` is already established by the `with` block

    [value for x in y
     if value
     where value = x * 2]

The order is unfortunately a bit weird here, but there is no need to add the whole concept of a different type of assignment and this syntax will feel instantly recognizable to people familiar mathematical notation, which is where the existing list comprehension syntax comes from and so has been established as well.