[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Antoine Pitrou]

On Thu, 26 Apr 2018 19:19:05 +1000
Chris Angelico <rosuav at gmail.com> wrote:
> >
> > If such were the need, you could very well make it part of the language
> > specification.  We are talking about a trivial optimization that any
> > runtime could easily implement (e.g. if a sequence `DUP_TOP, STORE_FAST,
> > POP_TOP` occurs, replace it with `STORE_FAST`).  
> 
> Not at the REPL, no. At the REPL, you need to actually print out that
> value. It's semantically different.

The REPL compiles expressions in a different mode than regular modules,
so that's entirely a strawman.  The REPL doesn't care that it will
spend a fraction of microseconds executing two additional bytecodes
before presenting something to the user.

> > Any runtime already has to implement a set of performance properties
> > that's far less trivial than that.  For example, any decent runtime is
> > expected to provide amortized O(1) list append or dict insertion.  You
> > are breaking user expectations if you don't.  
> 
> You assume that, but it isn't always the case.

Yeah, so what?

> Did you know, for instance, that string subscripting (an O(1) operation in CPython) is
> allowed to be O(n) in other Python implementations?

Why would I give a sh*t?  Here, we are not talking about a non-trivial
design decision such as how to represent string values internally
(utf-8 vs. fixed-width, etc.).  We are talking about optimizing a
trivial bytecode sequence into another more trivial bytecode sequence.
CPython can easily do it.  PyPy can easily do it.  Other runtimes can
easily do it.

If some implementation is significantly slower because it can't
optimize away pointless DUP_TOPs *and* it implements DUP_TOP
inefficiently enough to have user-noticeable effect, then it's either
1) a deliberate toy, a proof-of-concept not meant for serious use, or
2) a pile of crap produced by incompetent people.

So there's zero reason to bother about efficiency issues here.

Regards

Antoine.