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

Greg Ewing greg.ewing at canterbury.ac.nz
Mon Apr 23 19:54:13 EDT 2018

Tim Peters wrote:

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

My problem with this is -- how do you read such code out loud?

From my Pascal days I'm used to reading ":=" as "becomes". So this says:

"If diff becomes x - base and g becomes gcd(diff, n) is
 greater than or equal to 1 then return g."

But "diff becomes x - base" is not what we're testing! That makes it sound like the result of x - base may or may not get assigned to diff, which is not what's happening at all.

The "as" variant makes more sense when you read it as an English sentence:

if ((x - x_base) as diff) and ...

"If x - x_base (and by the way, I'm going to call that
 diff so I can refer to it later) is not zero ..."

-- Greg

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