[Python-Dev] assignment expressions: an alternative proposal

[Chris Angelico]

On Wed, Apr 25, 2018 at 12:54 AM, Anthony Flury via Python-Dev
<python-dev at python.org> wrote:
> As discussed previously by others on this exact proposals, you now have the
> issue of  confusion when using keyword arguments : *my_func(a = b)* :
> clearly that is a call to `my_func' where argument a has the value of b, but
> if you want to do an assigment expression when calling the function you now
> have to do *my_func((a=b)) -* which frankly looks messy in my opinion; you
> get the same issue when you are wanting to do assignment expressions in
> tuples.

To be fair, function arguments already follow "practicality beats
purity" in many ways. Let's look at tuples:

x = 1, 2 # fine
x = (1, 2) # fine
x = 1, # fine, though not advisable
x = (1,) # fine

But if you're going to use a tuple literal as a function parameter,
you have to give it extra parens:

f((1, 2)) # one arg, a tuple
f(1, 2) # two args

The comma has multiple meanings, and it has to be disambiguated. The
equals sign would be the same.

I'm still strongly -1 on any proposal to have "=" mean assignment in
any expression context, though. It is WAY too easy for a comparison to
sneakily become an assignment, or to get bizarre syntax errors:

x = 1
if (x = 2): ...

This, according to your proposal, raises SyntaxError - not because a
comparison was wanted and an assignment was made, but because the name
already had a value. And, even worse, this is NOT an error:

x = 1
def f():
    if (x = 2):
        ...

That's a bizarre distinction.

ChrisA