[Python-Dev] PEP 548: More Flexible Loop Control

[Serhiy Storchaka]

06.09.17 03:11, R. David Murray пише:
> I've written a PEP proposing a small enhancement to the Python loop
> control statements.  Short version: here's what feels to me like a
> Pythonic way to spell "repeat until":
> 
>      while:
>          <do stuff>
>          break if <done condition>
> 
> The PEP goes into some detail on why this feels like a readability
> improvement in the more general case, with examples taken from
> the standard library:
> 
>       https://www.python.org/dev/peps/pep-0548/
> 
> Unlike Larry, I don't have a prototype, and in fact if this idea
> meets with approval I'll be looking for a volunteer to do the actual
> implementation.
> 
> --David
> 
> PS: this came to me in a dream on Sunday night, and the more I explored
> the idea the better I liked it.  I have no idea what I was dreaming about
> that resulted in this being the thing left in my mind when I woke up :)

This looks rather like Perl way than Python way.

"There should be one-- and preferably only one --obvious way to do it."

This proposing saves just a one line of the code. But it makes "break" 
and "continue" statement less visually distinguishable as it is seen in 
your example from uuid.py.

If allow "break if" and "continue if", why not allow "return if"? Or 
arbitrary statement before "if"? This adds PHP-like inconsistency in the 
language.

Current idiom is easier for modification. If you add the second 
condition, it may be that you need to execute different code before "break".

     while True:
         <do stuff>
         if not <condition>:
             <exit code>
             break
         <do stuff 2>
         if not <condition >:
             <exit code 2>
             break

It is easy to modify the code with the current syntax, but the code with 
the proposed syntax should be totally rewritten.

Your example from sre_parse.py demonstrates this. Please note that 
pre-exit code is slightly different. In the first case self.error() is 
called with one argument, and in the second case it is called with two 
arguments. Your rewritten code is not equivalent to the existing one.

Other concern is that the current code is highly optimized for common 
cases. Your rewritten code checks the condition "c is None" two times in 
common case.

I'm -1 for this proposition.