[Python-Dev] PEP 572: Why not := as standard assignment operator?

Terry Reedy tjreedy at udel.edu
Thu Apr 26 16:00:38 EDT 2018

On 4/26/2018 11:38 AM, Gustavo Carneiro wrote:

>     On Thu, Apr 26, 2018 at 11:13 PM, Martin Teichmann
>     <lkb.teichmann at gmail.com <mailto:lkb.teichmann at gmail.com>> wrote:
>     > when reading PEP 572 I actually liked it a lot - I think it's actually
>     > a cool idea. I think it's actually that cool an idea that it should be
>     > made the default way of doing an assignment, over time phasing out the
>     > good ole =.

Please no, I really, really appreciate not having to see add ':' all the 
time.  Plus, it already has two other uses: end of header and dict 
key-item separator.  Plus, see below.

> In the interest of that, do you think := can be made illegal, by the 
> grammar, if used outside an expression?
> a = 1  # legal
> a := 1  # Syntax error
> if a := 1:  # legal

Please no.

PEP 572 does not discuss top-level interactive assignments.  But it 
should, because having two forms of assignment that lets one choose to 
echo or not is a plus.

Sometimes one wants to see the result of an assignment.  After

 >>> a, b = 14387, 344

it would be nice to be able to write

 >>> diff := a - b

instead of

 >>> diff = a - b
 >>> diff

I consider this a significant positive feature of the proposal.

On the other hand, sometimes one does not want the see the result.  A 
long result is often useless to view and may scroll previous results out 
of the window.  A very long result may scroll previous results out of a 
finite console buffer altogether.  (For Windows Command Prompt, the 
default is 300 lines, with a max of 9999.)

And sometimes the result must not be printed.  Tk text widgets (and 
hence IDLE's Shell) do not have a particular limit on the number of 
lines.  On the other hands, long lines drasticly slow down a text 
widget, and a long enough line will freeze it.  (This is supposed to be 
fixed in Tcl/Tk 8.7, now in a1 stage.)  For instance, ">>> 
[None]*1000000" in IDLE freezes the process.

Terry Jan Reedy

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