[Python-Dev] PEP 572: Assignment Expressions

[Antoine Pitrou]

On Tue, 24 Apr 2018 01:55:13 -0500
Tim Peters <tim.peters at gmail.com> wrote:
> [Antoine]
> > Constructs like "with ..." or "try / except / finally" make the
> > language easier to learn compared to the dances they are meant to
> > replace.  
> 
> They nevertheless need to be taught & learned (and try/except/finally
> was essentially always in the language),  You snipped the parts
> pointing out that binding expressions are already familiar to people
> coming from most other languages

Yes... I think most will agree that Python is generally easy to take up
for people coming from C++ etc., so my "easier to learn and teach" was
mostly about non-programmers.

> even for raw beginners the
> semantics are the tiniest part of what they need to learn anyway about
> Python's assignment expressions.

I'm not sure what you mean by that.  If it's the tiniest part, what's
the overwhelming part?  Is the new assigment expression that delicate
to use that it requires reading a long and intimidating design
document ? I didn't get that impression, so it seems you may be making
a stronger point than me for rejeting the PEP :-)

> > "await" is a more readable and less confusing improvement
> > over "yield from".  
> 
> Heh.  Not to me.  I have literally have no idea what to with "await"
> (I use generators heavily, but have had no use yet for coroutines),
> but use
> 
>     yield from an_iterable
> 
> routinely.

Yeah... "yield from" is fine for that, except that it was explicitly
meant for the coroutine use case as well (I'm not sure what the
timeline is, but probably Guido was already thinking/dreaming about
tulip/asyncio back then). And trying to shoehorn both in a single
construct made it confusing and inadequate.

When you want to express two abstractly different concepts (generating
a stream of values, or suspending a task until some asynchronous subtask
finishes), it makes things easier if those two concepts have two
different concrete expressions. Hence "await" making the language
easier to learn for those whose use cases benefit from it.

To bring another example: the fact that Python has separate syntax to
declare classes and functions makes it easier to learn "Python with
classes" than "Javascript with classes", even though the raw _grammar_
is made slightly more complex by it.  In Javascript you have to learn
the weird (ab)use of functional notation for the purpose of declaring
object behaviour, and learn to recognize it when you read it.  In
Python there's the "class" notation which, despite adding a keyword to
remember, makes class declaration easier to learn and master.

(^^ note this example is about a potentially obsolete dialect of
Javascript; perhaps it has class notation nowadays? ^^)

> It's simply impossible that, whatever "await" does, it
> could be more readable or less confusing than what I use "yield from"
> for.

Probably because "await" wouldn't work at all for you, then :-)

> >  Format strings dispense from the older, more convoluted formulations.  
> 
> But they didn't _replace_ them.  That made teaching/learning harder,
> not easier,

Intuitively, it sounds easier to teach f'some {value}' rather than
either the .format() or %-formatting alternatives.  The whole goal of
f-strings, after all, is to make string formatting more approachable.

Learning a language is not learning the whole spec.  When you learn C,
you don't need to learn the oddities of pre-ANSI function
declarations :-)

However, assignment a special case in this regard, since traditional
assignment is so omnipresent in online resources, that people _will_
encounter it even if they make a very focused use of Python.

> > Iteration is much simpler than the longer forms we would have to write
> > if generalized iterators didn't exist.  
> 
> I'll buy that one.  Now go through the HISTORY file and count all the
> changes you didn't name ;-)

You claimed that """almost no addition has ever made a language easier
to learn for raw beginners""".  I claim that several additions did
(for Python alone), but I don't need to prove that most of them did ;-)

Regards

Antoine.