Hello,

Everyone knows how hard to find a compelling usecase for the assignment
expression operator (":=", colloquially "walrus operator").
https://www.python.org/dev/peps/pep-0572/ examples never felt
compelling and we all remember the split it caused.

I finally found a usecase where *not* using assignment expression is
*much* worse than using it. I'm working on SSA (Static Single
Assignment,
https://en.wikipedia.org/wiki/Static_single_assignment_form) conversion
of Python programs, where there's a need to "join" dataflow of values
from different control flow paths using a special function (Phi
function). This "joining" itself creates a new variable, and of course,
the original variable was used in an expression. We've got
assignment in expression, assignment expression operator to the rescue!

With it, a simple loop like:

----
a = 0
while a < 5:
    a += 1
----

becomes:

----
a0 = 0
while (a1 := phi(a0, a2)) < 5:
    a2 = a1 + 1
----

So far, so good. But semantics of Phi function is parallel assignment.
No problem with Python either, "a, b = b, c" is exactly parallel
assignment. So, let's try example with 2 variables:

----
a = 0
b = 10
while a < 5:
    a += 1
    b += 1
----

becomes:

----
a0 = 0
b0 = 10
while ((a1, b1) := phi([a0, a2], [b0, b2]))[0] < 5:
    a2 = a1 + 1
    b2 = b1 + 1
----

But oops:

> SyntaxError: cannot use assignment expressions with tuple

To reproduce in the REPL:

----
>>> ((a, b) := (1, 2))
  File "<stdin>", line 1
SyntaxError: cannot use assignment expressions with tuple
----

Why this accidental syntactic gap? Why assignment statement can do
parallel assignment with a tuple on LHS, and assignment operator
suddenly can't?

Why the adhoc naming and conceptual shift on the AST level, when PEP572
explicitly talks about *assignment operator*, but corresponding node on
the AST level is called NamedExpr? Why look at assignment expression as
"name of expression" instead of assignment expression per se?

It's of course not a problem to recast:

NamedExpr(expr target, expr value)

to

NamedExpr(expr* target, expr value)

in the ASDL (and it works out of the box), the point is that it should
have been ExprAssign from the start (folloing the AugAssign and
AnnAssign tradition).


--
Best regards,
 Paul                          mailto:pmiscml@gmail.com
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