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[Python-Dev] Is PEP 572 really the most effective way to solve the problems it's targeting?

Tim Peters tim.peters at gmail.com
Thu Apr 26 20:46:02 EDT 2018 
[Mike Miller]
>>     - How are other modern languages solving this issue?

[Greg Ewing <greg.ewing at canterbury.ac.nz>]
> In all the languages I can think of that allow assignments in
> expressions, there is only one assignment operator -- a stand
> alone assignment is just a bare assignment expression.

Pretty much so, but I don't know what "modern" means to Mike.  The R
language may set a record for, umm, innovation here:

"""
There are three different assignment operators: two of them have
leftwards and rightwards forms.[1]
"""

So there are 5 assignment operator spellings in R:

=
<-
->
<<-
->>

Note that the link doesn't tell the whole story either; e.g., they
don't all have the same precedence level.  And, in addition to the 5
infix spellings shown above, there are also prefix (looks like a
2-argument function call) spellings.

Back on Earth ;-) , I think it's worth it to point out that only
languages (with assignment expressions) aping C use "=" for assignment
and "==" for equality.  That was a Really Bad Idea that all other (not
aping C) languages I know of avoided.

But I'm not sure any of this is relevant to what Mike meant by "this issue".


> But those languages were all designed that way from the start.
> I'm not aware of any that began by forbidding assignment in
> expressions and then added it later.

Me neither.  It's certainly the case that Guido would not have
designed a language that aped C's poor decision here.  At its very
start, Python used "=" for both assignment and equality testing (and
== was a syntax error).  So I think it's evident that, at the time, he
didn't envision ever adding assignment expressions.


[1] https://www.rdocumentation.org/packages/base/versions/3.5.0/topics/assignOps
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