On 2/5/2021 2:51 AM, Paul Sokolovsky wrote:
https://www.python.org/dev/peps/pep-0572/
...
((a, b) := (1, 2))
File "<stdin>", line 1
SyntaxError: cannot use assignment expressions with tuple
----
Why this accidental syntactic gap?
As should be clear from reading "Differences between assignment
expressions and assignment statements", this 'gap' in entirely
intentional, not accidental. *All* elaborations of 'name := expression'
are listed and rejected as outside the scope of the proposal, which was
to keep one reference to the expression value for later use. At least
some of these elaborations were suggested and rejected during the
voluminous discussion.
The principal a.e. use in conditional expressions is testing for
non-nullness. Your
> while ((a1, b1) := phi([a0, a2], [b0, b2]))[0] < 5:
> a2 = a1 + 1
> b2 = b1 + 1
is an unusual and very specific use. You want to have your tuple (for
subscripting for testing) and eat it too (by unpacking). One can
instead separate unpacking from testing a couple of ways
while (tup := phi([a0, a2], [b0, b2]))[0] < 5:
a2, b2 = tup
a2 = a1 + 1
b2 = b1 + 1
while True:
a1, b1 = phi([a0, a2], [b0, b2])
if a1 >= 5: break
a2 = a1 + 1
b2 = b1 + 1
--
Terry Jan Reedy
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