seeking deeper (language theory) reason behind Python design choice

[Steven D'Aprano]

On Tue, 15 May 2018 22:21:15 +0200, Peter J. Holzer wrote:

> On 2018-05-15 00:52:42 +0000, Steven D'Aprano wrote:
[...]
>> By 1991 there had already been *decades* of experience with C
> 
> About one and a half decades.

That would still be plural decades.

C's first release was 1972, so more like 19 years than 15. That's just 
under two 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'm using "proving" informally above, not in the sense of actual legal or 
scientific standards of proof.

If you are going to object to that, remember that neither is there 
scientific data proving that assignment as an expression is useful, so we 
can always just reject the construct as "useless until proven otherwise" 
:-)


> 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 [...]
> 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.)

You were using version control repositories in the 1980s, and still have 
access to them? I'm impressed.


> 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.

I'm not a C coder, but I think that specific example would be immune to 
the bug we are discussing, since (I think) you can't chain assignments in 
C. Am I right?

    fd = open(...)) = -1

would be a syntax error. But:

    fd = -1

would not be.



-- 
Steve