[Python-ideas] Add .= as a method return value assignment operator

[Chris Angelico]

On Wed, Sep 26, 2018 at 9:14 PM Jasper Rebane <rebane2001 at gmail.com> wrote:
> When using Python, I find myself often using assignment operators, like 'a += 1' instead of 'a = a + 1', which saves me a lot of time and hassle
>
> Unfortunately, this doesn't apply to methods, thus we have to write code like this:
> text = "foo"
> text = text.replace("foo","bar")
> # "bar"
>
> I propose that we should add '.=' as a method return value assignment operator so we could write the code like this instead:
> text = "foo"
> text .= replace("foo","bar")
> # "bar"
> This looks cleaner, saves time and makes debugging easier
>

All the other augmented operators are of the form:

target #= value

with some sort of valid assignment target, and any value at all on the
right hand side. You can take the value and put it in a variable, you
can replace it with a function returning that value, etc, etc, etc, as
it is simply a value. There's no difference between "x += 123" and "y
= 123; x += y". (Putting it another way: "x *= y + z" is not
equivalent to "x = x * y + z", but to "x = x * (y + z)".) With your
proposed ".=" operator, it's quite different: the RHS is textually
concatenated with the LHS. This creates odd edge cases. For example:

items = ["foo", "bar", "quux"]
items[randrange(3)] .= upper()

Is this equivalent to:

items[randrange(3)] = items[randrange(3)].upper()

? That would call randrange twice, potentially grabbing one element
and dropping it into another slot. If it isn't equivalent to that, how
is it defined?

I'm sure something can be figured out that will satisfy the
interpreter, but will it work for humans?

ChrisA