[Python-Dev] assignment expressions: an alternative proposal

[Chris Angelico]

On Wed, Apr 25, 2018 at 1:03 AM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> On Tue, Apr 24, 2018 at 10:56 AM, Chris Angelico <rosuav at gmail.com> wrote:
> [..]
>>> A lot of other questions arise though.  PEP 572 proposes:
>>>
>>> a = 1  # assignment
>>> a := 1  # also assignment
>>> (a := 1)  # also assignment
>>> (a = 1)  # error, why?
>>
>> Your third example is just the same as the second, with parentheses
>> around it. In most of Python, parentheses (if legal) have no effect
>> other than grouping; "a + b * c" is the same thing as "(a + b) * c",
>> just done in the other order. The last one is a clear demonstration
>> that "=" is a statement, not an expression. Are people confused by
>> this sort of thing:
>>
>> if x > 1:
>>     print("x is more than 1")
>> (if x > 1:)
>>     print("SyntaxError")
>
> This is a very far-fetched example :)

Heh, yes it is. But my point is that the parens are not creating a
weird situation here. They're just showcasing a distinction: one of
these is a statement, the other an expression. Which is the entire
point of the different operator - one is a syntactic feature of a
statement that creates one or more name bindings, the other is a
binary operator which results in a name binding as well as a value.

ChrisA