PEP 572: Python assignment expressions has been accepted

https://en.m.wikipedia.org/wiki/Zen_of_Python

For anyone who hasn't read the original PEP link, what do you suppose this does? Guess its meaning:

1.

    if (row := cursor.fetchone()) is None:
          raise NotFound
      return row

Next guess the meaning of this - what does it look like it does?

  row = cursor.fetchone()
  if row is None:
      raise NotFound
  return row

The first does the second. The second doesn't need explanation. I literally don't need to tell you what it does.

(The two examples are from the link.)

Now you might think that this means I'm against this PEP.

But actually [EDIT: because I know there is a VERY high bar to being accepted into Python] to me it means this is going to save an incredible amount of time - it must be very good indeed to be accepted.

So if they decided to adopt it - sure, I'll use it. And I bet it's great in practice.

It's also super explicit. If you don't know what := does you will go look it up.

If Python is still driven by Python design philosophy and all that discussion, then this will be very helpful. It surely had a very high threshold to meet.

So, my reasoning is that I surely must end up liking it, if it ended up accepted despite breaking every rule in the book. They wouldn't accept it for something that wasn't incredibly useful. My first impression is that it's incredibly ugly, unpythonic, and unreadable (what does it even do?), and this is pretty "obvious".

So arguing based on what I know about Python design philosophy, and the fact that this has been so deeply discussed, it must be absolutely fantastic.

I wouldn't think it's great (if you gave me the syntax), I'd recite all the Python Values it breaks -

https://en.m.wikipedia.org/wiki/Zen_of_Python :

- Beautiful is better than ugly.

Nope, this is very ugly.

- Simple is better than complex.

Nope, this isn't simple.

- Flat is better than nested.

Nope, this explicitly exists to let you nest something.

- Sparse is better than dense.

Nope, this explicitly exists to make code denser.

- Readability counts.

Nope: this is literally not readable. You don't know what the first code example I gave does. Normally with Python you just "write pseudocode and make sure you indent it properly."

- Special cases aren't special enough to break the rules.

This is a special case.

but here we get to the next rule, which trumps ALL of the above:

Although practicality beats purity.

- There should be one—and preferably only one—obvious way to do it.

This is broken, as now you can do it on two lines or one line.

- If the implementation is hard to explain, it's a bad idea.

It's kind of hard to explain, look at all our comments.

- If the implementation is easy to explain, it may be a good idea.

This isn't the case here.

I will grant that this is extremely explicit syntax. In this sense it is MUCH better than overloading = based on different contexts or something. As I mentioned above, anyone who sees := knows that it's a "known unknown" -- WTF is this?

So we are left with a single solitary Pythonic value:

- Practicality beats purity.

Based on this alone, I reason that it must be great to have been accepted. It must be extremely practical. It must save a lot of time.

I am quite disposed to thinking (honestly!) that it must be pretty darn great. It will save a lot of time.

There's no way that it made it into the language if it didn't, since it's so obviously ugly and unpythonic.

But practicality beats purity. I'm sure I'll love it!

    if reductor := dispatch_table.get(cls):
        rv = reductor(x)
    elif reductor := getattr(x, "__reduce_ex__", None):
        rv = reductor(4)
    elif reductor := getattr(x, "__reduce__", None):
        rv = reductor()
    else:
        raise Error("un(shallow)copyable object of type %s" % cls)

    reductor = dispatch_table.get(cls)
    if reductor:
        rv = reductor(x)
    else:
        reductor = getattr(x, "__reduce_ex__", None)
        if reductor:
            rv = reductor(4)
        else:
            reductor = getattr(x, "__reduce__", None)
            if reductor:
                rv = reductor()
            else:
                raise Error("un(shallow)copyable object of type %s" % cls)

    def f(F, *args):
      f.reductor = F(*args)
      return bool(f.reductor)

    if f(dispatch_table.get, cls)):
        rv = f.reductor(x)
    elif f(getattr, x, "__reduce_ex__", None):
        rv = f.reductor(4)
    elif f(getattr, x, "__reduce__", None):
        rv = f.reductor()
    else:
        raise Error("un(shallow)copyable object of type %s" % cls)

Once you abstract the patterns you can drive towards data-driven code:

    K = (
      (dispatch_table.get, (cls,), (x,)),
      (getattr, (x, "__reduce_ex__", None), (4,)),
      (getattr, (x, "__reduce__", None), ()),
    )

    for F, a, b in K:
      reductor = F(*a)
      if reductor:
        rv = reductor(*b)
        break
    else:
        raise Error("un(shallow)copyable object of type %s" % cls)

At runtime, your second example is actually significantly less explicit because so much is hidden away in mutable state.