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The problem isn't that simple use case. Although even in that case, they already had '=' as an assignment operator, and they could've easily kept it like the majority of other languages do instead of introducing an inconsistency.

The more major problem with the walrus operator is more complicated expressions they made legal with it. Like, could you explain to me why making these legal was a good thing?

  def foo()
      return ...
  def bar():
      yield ...
  while foo() or (w := bar()) < 10:
      # w is in-scope here, but possibly nonexistent!
      # Even in C++ it would at least *exist*!
      print(w)

  # The variable is still in-scope here, and still *nonexistent*
  # Ditto as above, but even worse outside the loop
  print(w := w + 1)

If they just wanted your use case, they could've made only expressions of the form 'if var := val' legal, and maybe the same with 'while', not full-blown assignments in arbitrary expressions, which they had (very wisely) prohibited for decades for the sake of readability. And they would've scoped the variable to the 'if', not made it accessible after the conditional. But nope, they went ahead and just did what '=' does in any language, and to add insult to injury, they didn't even keep the existing syntax when it has exactly the same meaning. And it's not like they even added += and -= and all those along with it (or +:= and -:= because apparently that's their taste) to make it more useful in that direction, if they really felt in-expression assignments were useful, so it's not like you get those benefits either.

In your example, if you leave out the parentheses around w := bar(), you get "SyntaxError: cannot use assignment expressions with operator" which makes me think it's a bug in the interpreter and not intentionally designed to allow it.

I am baffled to learn that it's kept in scope outside of the statement it's assigned, and I agree it would have a negative impact on readability if used outside of the if statement.

> if you leave out the parentheses around w := bar(), you get "SyntaxError: cannot use assignment expressions with operator" which makes me think it's a bug in the interpreter and not intentionally designed to allow it.

No, I'm pretty sure that's intentional. You want the left-hand side of an assignment to be crystal clear, which "foo() or w := bar()" is not. It looks like it's assigning to (foo() or w).

To be clear:

    def thing(): return True
    
    if thing() or w:= "ok": # SyntaxError: cannot use assignment expressions with operator
        pass
    print(w)

    . . .

    if thing() or (w := "ok"):
        pass
    print(w) # NameError: name 'w' is not defined

The first error makes me think your concern (that w is conditionally undefined) was anticipated and supposed to be guarded against with the SyntaxError. I believe the fact you can bypass it with parentheses is a bug and not an intentional design decision.

Oh I see, you're looking at it from that angle. But no, it's intentional. Check out PEP 572 [1]:

> The motivation for this special case is twofold. First, it allows us to conveniently capture a "witness" for an any() expression, or a counterexample for all(), for example:

  if any((comment := line).startswith('#') for line in lines):
    print("First comment:", comment)
  else:
    print("There are no comments")

I have a hard time believing even the authors (let alone you) could tell me with a straight face that that's easy to read. If they really believe that, I... have questions about their experiences.

The beauty of Python...

[1] https://www.python.org/dev/peps/pep-0572/

Your new example makes me wonder: if I can intentionally conditionally bring variables into existence with the walrus operator, what's the motivation behind the SyntaxError in my statement above? I maintain my belief that the real issue here is, readability aside, if blocks do not implement a new scope, which has always been a problem in the language. The walrus operator just gives you new ways to trip over that problem.

From the PEP:

> An assignment expression does not introduce a new scope. In most cases the scope in which the target will be bound is self-explanatory: it is the current scope. If this scope contains a nonlocal or global declaration for the target, the assignment expression honors that. A lambda (being an explicit, if anonymous, function definition) counts as a scope for this purpose.

I find this particularly strange and inconsistent:

    lines = ["1"]

    [(comment := line).startswith('#') for line in lines]

    print(comment) # 1

    [x for x in range(3)]
    print(x) # NameError: name 'x' is not defined

> what's the motivation behind the SyntaxError in my statement above?

I'm pretty sure it's what I explained here: https://news.ycombinator.com/item?id=28899404

I did not understand what you meant.

I'm saying it's the same reason why (x + y = z) should be illegal even if (x + (y = z)) is legal in any language. It's not specific to Python by any means. The target of an assignment needs to be obvious and not confusing. You don't want x + y to look like it's being assigned to.

I see. It has low precedence in the operator hierarchy [1] so

    False or w := 1:

Is grouped like so:

    (False or w) := 1

Which is a SyntaxError. That's... not a smart place for it to be in the operator hierarchy. I expected it to be near the very top, like await.

1. https://docs.python.org/3/reference/expressions.html#operato...

Edit: 20 minutes later, can't respond.

There are two aspects I have been thinking about while looking at this: Introduction of non-obvious behavior (foot-guns) and readability. Readability is important, but I have been thinking primarily about the foot-gun bits, and you have been emphasizing the readability bits. I can't really accurately assess readability of something until I encounter it in the wild.

If the precedence was higher then you'd get a situation like

  x := 1 if cond else 2

never resulting in x := 2 which is pretty unintuitive.

And you have to realize, even if the precedence works out, nobody is going to remember the full ordering for every language they use. People mostly remember a partial order that they're comfortable with, and the rest they either avoid or look up as needed. Like in C++, I couldn't tell you exactly how (a << b = x ? c : d) groups (though I could make an educated guess), and I don't have any interest in remembering it either.

Ultimately, this isn't about the actual precedence. Even if the precedence was magically "right", it's about readability. It's just not readable to assign to a compound expression, even if the language has perfect precedence.

While the walrus operator gives a way to see this sort of non-C++ behavior, it's more showing that Python isn't C++ than something special about the operator.

Here's another way to trigger the same NameError, via "global":

    import random
    def foo():
        return random.randrange(2)
    def bar():
        global w
        w = return random.randrange(20)
        return w
        
    while foo() or (bar() < 10):
        print(w)

For even more Python-is-not-C++-fun:

    import re
    def parse_str(s):
        def m(pattern):  # I <3 Perl!
            nonlocal _
            _ = re.match(pattern, s)
            return _ is not None

        if m("Name: (.*)$"):
            return ("name", _[1])
        if m("State: (..) City: (.*)$"):
            return ("city", (_[2], _[1]))
        if m(r"ZIP: (\d{5})(-(\d{4}))?$"):
            return ("zip", _[1] + (_[2] if _[2] else ""))

        return ("Unknown", s)
        del _  # Remove this line and the function isn't valid Python(!)

    for line in (
            "Name: Ernest Hemingway",
            "State: FL City: Key West",
            "ZIP: 33040",
            ):
        print(parse_str(line))

Right, I'm quite well-aware of that, but I'm saying this change has made the situation even worse. If they ensured the variables were scoped and actually initialized it'd have actually been an improvement.

I apologize. I misinterpreted your comment:

      # w is in-scope here, but possibly nonexistent!
      # Even in C++ it would at least *exist*!

because I don't see how bringing up C++'s semantics is relevant when Python has long raised an UnboundLocalError for similar circumstances.

If I understand you correctly, you believe Python should have introduced scoping so the "w" would be valid only in the if, elif, and else clauses, and not after the 'if' ends.

This would be similar to how the error object works in the 'except' clause:

  >>> try:
  ...   1/0
  ... except Exception as err:
  ...   err = "Hello"
  ...
  >>> err
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'err' is not defined

If so, I do not have the experience or insight to say anything meaningful.

The walrus operator provides no benefit here. `my_match` is still in scope in both cases.

Python’s `if` statements do not introduce a new scope.

I know they don't, normally. I really thought that was basically the point of the walrus operator to begin with, that the variable was only in scope for the lifetime of the if statement where it's needed. Huge bummer to find out that's not true.

Looks like now you're seeing why it's an abomination ;)

IMO the real abomination was already present in the language, which is that if blocks do not introduce new scope. My IDE protects me from the bugs this could easily introduce when I try to use a variable that may not yet be in scope, but it should be detected before runtime.

I will readily admit that the walrus operator doesn't do what I thought it did and I have no interest in whatever utility it provides as it exists today.

> IMO the real abomination was already present in the language, which is that if blocks do not introduce new scope.

Definitely. You would think if they're going to undermine decades of their own philosophy, they would instead introduce variable declarations and actually help mitigate some bugs in the process.

Scoop in python is normally defined by functions/methods, not blocks. The same happens with for-loos and with-blocks. So this is consistent. And this good, because it can be very useful. The exception here are try/except-blocks, where the fetched error is cleaned up after leaving the except-block, for reasons.

There's this example from an explanation linked elsewhere in the discussion. The relevant parts of the if statement are now much further separated.

if (percentage := ((student["marks"]/700)*100)) > 70:

I don't know how important this is, but I believe it does make it less readable for "outsiders".

As a non-Python programmer it is usually pretty easy for me to correctly guess what a piece of Python code does. (And once in a while I need to take a look at some Python code).

Walrus operator got me. I tried to guess what it did, but even having simple code examples I could not. My guesses were along the lines of binding versus plain assignment, or some such. None of my guesses were even close. I had to google it to find out (of course I could also read the documentation).