[Python-Dev] assignment expressions: an alternative proposal

[Nick Coghlan]

On 24 April 2018 at 23:50, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> On Tue, Apr 24, 2018 at 9:46 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> On 24 April 2018 at 23:38, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
>>> I propose to use the following syntax for assignment expressions:
>>>
>>>     ( NAME = expr )
>>>
>>> I know that it was proposed before and this idea was rejected, because
>>> accidentally using '=' in place of '==' is a pain point in
>>> C/C++/JavaScript.
>>>
>>> That said, I believe we can still use this syntax as long as we impose
>>> the following three restrictions on it:
>>>
>>> 1. Only NAME token is allowed as a single target.
>>>
>>> 2. Parenthesis are required.
>>>
>>> 3. Most importantly: it is *not* allowed to mask names in the current
>>> local scope.
>>
>> While I agree this would be unambiguous to a computer, I think for
>> most humans it would be experienced as a confusing set of arcane and
>> arbitrary rules about what "=" means in Python.
>
> I respectfully disagree.  There are no "arcane and confusing rules"
> about "=", it's rather simple:
>
> "=" is always an assignment.
> "==" is always an equality check.

That's not the distinction I meant, I meant the difficulty of
explaining the discrepancies in this list:

a = 1 # Assignment
(a = 1) # Also assignment

a, b = 1, 2 # Tuple assignment
(a, b = 1, 2) # SyntaxError. Why?

a.b = 1 # Attribute assignment
(a.b = 1) # SyntaxError. Why?

a[b] = 1 # Subscript assignment
(a[b] = 1) # SyntaxError. Why?


(a=1), (b=2) # Two assignments
a=1, b=2 # SyntaxError. Why?

f(a=1, b=2) # Function call with keyword args
(a=1, b=2) # SyntaxError. Why?

if (a=1): pass # Assignment
if a=1: pass # SyntaxError. Why?

Whereas if binding expressions use a different symbol, the question is
far less likely to arise, and if it does come up, then the answer is
the same as the one for def statements vs lambda expressions: because
one is a statement, and the other is an expression.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia