[Python-ideas] Null coalescing operators
tjreedy at udel.edu
Mon Sep 21 23:23:42 CEST 2015
On 9/21/2015 1:07 PM, Guido van Rossum wrote:
> I apologize for having misunderstood the status of your PEP. I think it
> would be great if you finished the PEP. As you know the ? operator has
> its share of fans as well as detractors, and I will happily wait until
> more of a consensus appears.
Add me to the detractors of what I have read so far ;-).
In arithmetic, 1/0 and 0/0 both stop the calculation. My hand
calculator literally freezes until I hit 'on' or 'all clear'. Early
computers also stopped, maybe with an instruction address and core dump.
Three orthogonal solutions are: test y before x/y, so one can do
something else; introduce catchable exceptions, so one can do something
else; introduce contagious special objects ('inf' and 'nan'), which at
some point can be tested for, so one can do something else. Python
introduced 'inf' and 'nan' but did not use them to replace
Some languages lacking exceptions introduce a contagious null object.
Call it Bottom. Any operation on Bottom yields Bottom. Python is not
such a language. None is anti-contagious; most operations raise an
I agree with Paul Moore that propagating None is generally a bad idea.
It merely avoids the inevitable exception. Or is it inevitable? Trying
to avoid exceptions naturally leads to the hypergeneralization of
allowing '?' everywhere.
Instead of trying to turn None into Bottom, I think a better solution
would be a new, contagious, singleton Bottom object with every possible
special method, all returning Bottom. Anyone could write such for their
one use. Someone could put it on pypi to see if there how useful it
I agree with Ron Adam that the narrow issue is that bool(x) is False is
sometimes too broad and people dislike of spelling out 'x is not None'.
So abbreviate that with a unary operator; 'is not None', is a property
of objects, not operators. I think 'x!' or 'x?', either meaning 'x is
not None', might be better than a new binary operator. The former, x!,
re-uses ! in something close to its normal meaning: x really exists.
Terry Jan Reedy
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