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

Ethan Furman ethan at stoneleaf.us
Mon Apr 23 16:01:21 EDT 2018

On 04/22/2018 10:44 PM, Tim Peters wrote:

[Guido]

In reality there often are other conditions being applied to the match for which if expr as name is inadequate. The simplest would be something like

if ...: elif (m := re.match('(.):(.)', line)) and m.group(1) == m.group(2): And the match() call may not even be the first thing to check -- e.g. we could have elif line is not None and (m := re.match('(.):(.)', line)) and m.group(1) == m.group(2): I find myself warming more to binding expressions the more I keep them in mind while writing new code. And I think it may be helpful to continue showing real examples where they would help. Today's example: I happened to code this a few hours ago: diff = x - xbase if diff: g = gcd(diff, n) if g > 1: return g It's not really hard to follow, but two levels of nesting "feels excessive", as does using the names "diff" and "g" three times each. It's really an "and" test: if the diff isn't 0 and gcd(diff, n) > 1, return the gcd. That's how I thought of it from the start. Which this alternative expresses directly: if (diff := x - xbase) and (g := gcd(diff, n)) > 1: return g

So I really like being able to make the assignment in the expression, but I have a really hard time parsing it with the name first.

Attempting to read just the names first:

if diff scan for ending right paren found and g scan for ending right paren oops, found opening left paren scan for ending right paren found resume scanning for final right paren found > 1: return g

Attempting to read expressions first:

if x - x_base and gcd(diff, n) what's diff? scan backwards diff is x - x_base > 1: return g what's g? scan up and backwards g is gcd(diff, n)

Attempting to read interleaved:

if skip diff x - x_base back to diff as diff and skip g gcd(diff, n) back to g as g

1: return g

On the other hand, if it were using the "as" keyword:

if (x - xbase as diff) and (gcd(diff, n) as g) > 1: return g

I would parse as:

if x - x_base as diff and gcd(diff, n) as g

1: return g

For me at least, the last is much more readable. Thinking about it some more, the problem (or maybe just my problem) is that I see an "if" or "while" and the I look for the thing that is True or False, and using the ":=" syntax the first thing I see is a placeholder for a result that doesn't exist yet, making me constantly scan backwards and forwards to put all the pieces in the correct place.

With "as", it just flows forwards.

-- Ethan

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