seeking deeper (language theory) reason behind Python design choice

[Peter J. Holzer]

On 2018-05-15 00:52:42 +0000, Steven D'Aprano wrote:
> Now remember that in 1991 when Guido made the decision to ban = as an 
> expression, those concepts didn't even exist. There were no Python 
> linters, and no reason to imagine that there ever would be. Guido didn't 
> know that Python would become one of the top 10 most used languages. For 
> all he knew, version 1.0 could be the final release.
> 
> By 1991 there had already been *decades* of experience with C

About one and a half decades.

> proving that the "=" assignment syntax is dangerously confusable with
> == and a total bug magnet when allowed as expressions as well, so it
> was perfectly reasonable to ban it from the language.

Experience? Yes. Data? I doubt it.

I will readily admit that my knowledge of research into the usability
of programming languages is more than spotty, but I have read a few
papers about the topic over the last decade. I don't recall any which
quantified "bug magnets" under realistic conditions (two hour toy
problems for first-year students don't count). I thought there was one
by Google, but that was about compile-time errors.

Language design ist still mostly an art driven by gut feeling, not
engineering driven by data. I doubt that this was better in 1991.

I have been programming in C since the mid-80's and in Perl since the
mid-90's (both languages allow assignment expressions). I accumulated my
fair share of bugs in that time, but AFAIR I made this particular error
very rarely (I cannot confidently claim that I never made it). Clearly
it is not “a total bug magnet” in my experience. There are much bigger
problems in C and Perl (and Python, too). But of course my experience is
just that of a single programmer (or a handful if I include people
whose code I've reviewed or debugged) and in any case just anecdotal.
(Could I quantify even my own experience? I guess I could write a script
which searches through all my repositories for changesets where “=” was
replaced by “==”. Not sure what that would prove.)

OTOH, despite having used C and Perl long before Python, I don't miss
assignment expressions. Every language has its idioms, and just because
you write stuff like “if ((fd = open(...)) == -1)” a lot in C doesn't
mean you have to be able to write that in Python.

        hp

-- 
   _  | Peter J. Holzer    | we build much bigger, better disasters now
|_|_) |                    | because we have much more sophisticated
| |   | hjp at hjp.at         | management tools.
__/   | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/>
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