[Python-Dev] Announcing PEP 3136

[Michael Foord]

Yuvgoog Greenle wrote:
> I like how python has a minimalistic and powerful syntax (-1 for the 
> break ___ PEP).
>
> Also, I really dislike the for/else ambiguity "butterflies".
>
> When is the else after a loop executed?
> 1. When the loop isn't entered at all.
If you mean because the list / iterator is empty then yes. If you mean 
something else then explain.

> 2. When the loop terminates through exhaustion of the list (does this 
> include when the list was empty?)
Yes.
> 3. When the loop didn't exit because of a break statement.
>

No.

This is pretty much the definition of the else statement semantics and 
are clearly defined.

> HINTS: The way django does it is opposite the way python does it and 
> there may be more than one correct answer.
>
If you mean the Django template language does it differently from Python 
then you'll have to speak to the Django guys. The Django template 
language may be inspired by Python and written in Python but it isn't 
Python and has no obligation to use the same semantics. Python is very 
clear.

> Any chances of cleaning this one up for python 4? I'm not sure how 
> though I have a few ideas.
>
See above. The else statement semantics for both loops and exception 
handling are extremely useful and don't need cleaning up. If you have 
further ideas then the Python-ideas list is the right place to discuss 
potential language changes or additions.

All the best,


Michael Foord

> On Tue, Sep 29, 2009 at 11:47 PM, Guido van Rossum <guido at python.org 
> <mailto:guido at python.org>> wrote:
>
>     On Tue, Sep 29, 2009 at 1:20 PM, Hatem Nassrat <hnassrat at gmail.com
>     <mailto:hnassrat at gmail.com>> wrote:
>     > Tue Jul 3 10:14:17 CEST 2007, Guido van Rossum wrote:
>     >> On 6/30/07, Matt Chisholm <matt-python at theory.org
>     <http://theory.org>> wrote:
>     >> > I've created and submitted a new PEP proposing support for
>     labels in
>     >> > Python's break and continue statements.  Georg Brandl has
>     graciously
>     >> > added it to the PEP list as PEP 3136:
>     >> >
>     >> > http://www.python.org/dev/peps/pep-3136/
>     >>
>     >> I think this is a good summary of various proposals that have been
>     >> floated in the past, plus some new ones. As a PEP, it falls short
>     >> because it doesn't pick a solution but merely offers a large
>     menu of
>     >> possible options. Also, there is nothing about implementation yet.
>     >>
>     >> However, I'm rejecting it on the basis that code so complicated to
>     >> require this feature is very rare. In most cases there are existing
>     >> work-arounds that produce clean code, for example using 'return'.
>     >
>     > I agree that this feature will only serve as a quick hack and in
>     many
>     > cases it would be misused and ugly code will be the result. However,
>     > it seems that there are some shortcuts that have sneaked into python
>     > (I am a fairly new Python programmer, only since late 2.4, so don't
>     > shoot me). The specific one of which I speak about is:
>     >
>     > while_stmt ::=  "while" expression ":" suite
>     >                ["else" ":" suite]
>     >
>     > for_stmt ::=  "for" target_list "in" expression_list ":" suite
>     >              ["else" ":" suite]
>     >
>     > try1_stmt ::=  "try" ":" suite
>     >               ("except" [expression ["as" target]] ":" suite)+
>     >               ["else" ":" suite]
>     >               ["finally" ":" suite]
>     >
>     > All these else's seem like shortcuts to me. I did find a use for
>     them,
>     > once I found out they existed, but I get butterflies whenever I do.
>
>     In English, butterflies are usually a good thing (they mean you'e
>     in love).
>
>     These else clauses (assuming you're talking about that) have been in
>     the language pretty much forever. The combined except/finally clause
>     is newer, it was added because Java allows it and it was actually a
>     pretty common usage.
>
>     --
>     --Guido van Rossum (home page: http://www.python.org/~guido/
>     <http://www.python.org/%7Eguido/>)
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