[Python-Dev] PEP 572: Assignment Expressions (original) (raw)

Tim Peters tim.peters at gmail.com
Tue Apr 17 21:35:35 EDT 2018

[Guido, makes peace with identifier := expression]

... I am fine with this, it certainly seems the easiest to implement, with the fewest corner cases, and the easiest restriction to explain.

(I was thinking there would be a use case for basic tuple unpacking, like seen a lot in for-loop, but the only examples I tried to come up with were pretty sub-optimal, so I don't worry about that any more.)

Chris's pain threshold appears to be higher than ours ;-)

So I would really like to see if anyone has plausibly realistic uses for fancier forms of assignment expression.

I have plenty of code that does stuff like this:

while True:
    x, y = func_returning_tuple()
    if y is None:
        break
    ...

Maybe it's just that I'm used to it, but I find that very easy to understand now. If we had fancy assignment expressions, my first thought was I could write it like so instead:

while ((x, y) := func_returning_tuple()) and y is not None:
    ...

and pray that I put in enough parens to get the intended meaning.

And maybe it's just that I'm not used to that, but I do find it harder to understand. Contributing factor: I don't really want "and" there - what the context requires is really more like C's comma operator (take only the last value from a sequence of expressions). As is, I'm relying on that a 2-tuple is truthy regardless of its content (so that and always goes on to evaluate its RHS).

And, for some reason, I find this even worse:

while ((x, y) := func_returning_tuple())[1] is not None:
    ...

The rub there: I gave y a name but can't use it in the test?!

And those are the same kinds of headaches I saw over & over in my own "fancier" code: stuff that's already perfectly clear would become more obscure instead.

Tuple unpacking works great in for-loops because the only effect there is to give names to the tuple components, none of which are needed in the for statement itself. But in a while" or ifstatement, I would typically _also_ want to use the names _in_ thewhileorif` tests. But, as in C, that's what the comma operator is for, not the assignment operator.

while (s = function_returning_struct_with_x_and_y_members(), s.y != NULL) {
    ...
}

In contrast, ,many plausible uses I saw for identifier := expression in a while or if statement would have been improvements, and most of the rest neutral: I'm still wondering whether this one is better or worse ;-):

def newton(f, fprime, x):
    import math
    while not math.isclose((next_x := x - f(x) / fprime(x)), x):
        x = next_x
    return next_x

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