During a recent HN discussion about the walrus operator I came to realize yet
another advantage of notation. I used APL professionally for about ten years,
which made it an obvious source of inspiration for an example that, in my
opinion, demonstrates why the Python team missed a very valuable opportunity to
take this wonderful language and start exploring the judicious introduction of
notation as a valuable tool for thought (borrowing from Ken Iverson's APL paper
with that title [0]).
To simplify, I'll define the desire for this walrus operator ":=" as "wanting
to be able to make assignments within syntax where it was previously
impossible":
if x = 5 # was impossible
# and now
if x := 5 # makes is possible
A more elaborate example given in the PEP goes like this:
Current:
reductor = dispatch_table.get(cls)
if reductor:
rv = reductor(x)
else:
reductor = getattr(x, "__reduce_ex__", None)
if reductor:
rv = reductor(4)
else:
reductor = getattr(x, "__reduce__", None)
if reductor:
rv = reductor()
else:
raise Error(
"un(deep)copyable object of type %s" % cls)
Improved:
if reductor := dispatch_table.get(cls):
rv = reductor(x)
elif reductor := getattr(x, "__reduce_ex__", None):
rv = reductor(4)
elif reductor := getattr(x, "__reduce__", None):
rv = reductor()
else:
raise Error("un(deep)copyable object of type %s" % cls)
At first I thought, well, just extend "=" and be done with it. The HN thread
resulted in many comments against this idea. The one that made me think was
this one [1]:
"These two are syntactically equal and in Python there's no
way a linter can distinguish between these two:
if reductor = dispatch_table.get(cls):
if reductor == dispatch_table.get(cls):
A human being can only distinguish them through careful inspection.
The walrus operator not only prevents that problem, but makes
the intent unambiguous."
Which is a perfectly valid point. I get it.
Still, the idea of two assignment operators just didn't sit well with me.
That's when I realized I had seen this kind of a problem nearly thirty years
ago, with the introduction of "J". I won't get into the details unless someone
is interested, I'll just say that J turned APL into ASCII soup. It was and is
ugly and it completely misses the point of the very reason APL has specialized
notation; the very thing Iverson highlighted in his paper [0].
Back to Python.
This entire mess could have been avoided by making one simple change that would
have possibly nudged the language towards a very interesting era, one where a
specialized programming notation could be evolved over time for the benefit of
all. That simple change would have been the introduction and adoption of APL's
own assignment operator: "←"
In other words, these two things would have been equivalent in Python:
a ← 23
a = 23
What's neat about this is that both human and automated tools (linters, etc.)
would have no problem understanding the difference between these:
if reductor ← dispatch_table.get(cls):
if reductor == dispatch_table.get(cls):
And the larger example would become this:
if reductor ← dispatch_table.get(cls):
rv ← reductor(x)
elif reductor ← getattr(x, "__reduce_ex__", None):
rv ← reductor(4)
elif reductor ← getattr(x, "__reduce__", None):
rv ← reductor()
else:
raise Error("un(deep)copyable object of type %s" % cls)
This assignment operator would work everywhere and, for a period of time, the
"=" operator would be retained. The good news is that old code could be updated
with a simple search-and-replace. In fact, code editors could even display "="
as "←" as an option. The transition to only allowing "←" (and perhaps other
symbols) could be planned for Python 4.
Clean, simple and forward-looking. That, to me, is a good solution. Today we
have "=" and ":=" which, from my opinionated perspective, does not represent
progress at all.
[0] http://www.eecg.toronto.edu/~jzhu/csc326/readings/iverson.pdf
[1] https://news.ycombinator.com/item?id=21426338
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