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