Mailing List Archive

Parenthesize.

On Mon, Mar 28, 2022 at 3:50 PM Ethan Furman <ethan@stoneleaf.us> wrote:

> In the following bit of code:
>
>
> while s := input.read(MAXBINSIZE):
> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
> s += ns
> line = binascii.b2a_base64(s)
> output.write(line)
>
> I'm getting this error on the second line:
>
> cannot use assignment expressions with expression
>
> Can somebody explain why that isn't working?
>
> Many thanks.
>
> --
> ~Ethan~
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--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>

On Tue, 29 Mar 2022 at 09:53, Ethan Furman <ethan@stoneleaf.us> wrote:
>
> In the following bit of code:
>
>
> while s := input.read(MAXBINSIZE):
> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
> s += ns
> line = binascii.b2a_base64(s)
> output.write(line)
>
> I'm getting this error on the second line:
>
> cannot use assignment expressions with expression
>
> Can somebody explain why that isn't working?
>

I'm getting a good hint from the exception in 3.11:

>>> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
File "<stdin>", line 1
while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: cannot use assignment expressions with expression

Looks like it's counting "len(s) < MAXBINSIZE and ns" as the
assignment target. Parens around the second half would solve that.

ChrisA
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On 2022-03-28 23:59, Chris Angelico wrote:
> On Tue, 29 Mar 2022 at 09:53, Ethan Furman <ethan@stoneleaf.us> wrote:
>>
>> In the following bit of code:
>>
>>
>> while s := input.read(MAXBINSIZE):
>> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
>> s += ns
>> line = binascii.b2a_base64(s)
>> output.write(line)
>>
>> I'm getting this error on the second line:
>>
>> cannot use assignment expressions with expression
>>
>> Can somebody explain why that isn't working?
>>
>
> I'm getting a good hint from the exception in 3.11:
>
>>>> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
> File "<stdin>", line 1
> while len(s) < MAXBINSIZE and ns := input.read(MAXBINSIZE-len(s)):
> ^^^^^^^^^^^^^^^^^^^^^^^^^^
> SyntaxError: cannot use assignment expressions with expression
>
> Looks like it's counting "len(s) < MAXBINSIZE and ns" as the
> assignment target. Parens around the second half would solve that.
>
I think that's because assignment has low precedence, lower than that of
the RHS, because the RHS needs to be evaluated before the assignment,
but the consequence of that it that the preceding expression will also
get evaluated first, consuming the identifier, leaving a trailing ':='.
It's easier just to complain and require on parentheses than to back up
while parsing.
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