[Python-Dev] How about integrating "as" semantics and postpone PEP 572 to Python 4.0

[Rin Arakaki]

Hi,

I bring a strong proposal that is prematurely rejected but indeed the
most likely to be accepted to our community.
That is to select `as` spelling not `:=` and modify `with` and
`except` statements. Here is this.

* Below is copy-pasted from my gist:
https://gist.github.com/rnarkk/cdabceaedbdb498d99802a76cc08b549


# `as` assignment expression

```python
# This
STATEMENT(EXPR_0 as NAME_0, EXPR_1 as NAME_2, ..., EXPR_n as NAME_n)

# can always be replaced by
NAME_0 = EXPR_0
NAME_1 = EXPR_1
...
NAME_n = EXPR_n
STATEMENT(NAME_0, NAME_1, ..., NAME_n)

# except `import` statement since it's special from the beginning
which means no `EXPR` cannot be the left hand of `as` in its statement
but simply `NAME`: `import NAME_0 as NAME_1`

# This interpretation above is valid no matter how `EXPR_i` uses
`NAME_j` (i > j).

# When you write
EXPR as NAME_0, NAME_1

# it's interpreted as
(EXPR as NAME_0), NAME_1

# TODO Should unpacking is allowed by this?
EXPR as (NAME_0, NAME_1)

# `EXPR as NAME` itself can be `EXPR` and it returns just `EXPR` in
`EXPR as NAME` which means you can write
((EXPR as NAME_0) as NAME_1) ... as NAME_n

# or simply like this even at the top level since it's determininable.
EXPR as NAME_0 as NAME_1 ... as NAME_n

NAME_0 = EXPR as NAME_1 ... as NAME_n

# Also you can write
f(x=EXPR as NAME)

# since this is valid and `EXPR as NAME` can be `EXPR`.
f(x=EXPR)

# And also this is passible.
if (EXPR as NAME).method(): ...

# The `EXPR` in `EXPR as NAME` is matched as long as possible which means
if 0 < 1 as x: ...

# is interpreted as
if (0 < 1) as x: ...

# not
if 0 < (1 as x): ...

# but also `EXPR` is separated by `,` which means
EXPR_0, EXPR_1 as NAME

# is interpreted as
EXPR_0, (EXPR_1 as NAME)

# rather than
(EXPR_0, EXPR_1) as NAME

# even when used `as` assignment expression in list comprehension,
# you can apply the same rules above first by putting it to `for` loop form.

# There is no equivalence to type annotation and augmented assignment.
```


# `with` statement

- `with` statement will no longer responsible for passing returned
value from `__enter__` to the right hand of `as` in its statement and
merely call `__enter__` when enter the statement and `__exit__` when
exit from it an
- Context manager can be renamed simply to context since it will no
longer be manager but context itself. Context object has status of the
context and encapsulates it.

# `except` statement

- `except` statement will no longer responsible for passing instance
of the right hand of `as` in its statement.
- Exceptions must be instanciated and also it must be confirmed
otherwise `except` statement could rewrite the class globally.


Thanks,
Rin Arakaki
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