[Python-ideas] 'where' statement in Python?

Jack Diederich jackdied at gmail.com
Wed Jul 21 18:02:06 CEST 2010

On Wed, Jul 21, 2010 at 7:20 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Wed, Jul 21, 2010 at 12:56 PM, Chris Rebert <pyideas at rebertia.com> wrote:
>> On Tue, Jul 20, 2010 at 3:13 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>> On Wed, Jul 21, 2010 at 6:13 AM, Alex Light <scialexlight at gmail.com> wrote:
>>>> i would use as because this whole where clause acts very similarly to a
>>>> context manager in that it sets a variable to a value for a small block
>>> No, the idea is for the indented suite to be a perfectly normal suite
>>> of Python code. We want to be able to define functions, classes, etc
>>> in there. Inventing a new mini-language specifically for these clauses
>>> would be a bad idea (and make them unnecessarily hard to understand)
>> <snip>
>> Did you not read Nick's reply yet when you wrote this, or...?
> Alex actually has a reasonable point here: break, continue, yield and
> return actually don't make sense in the top-level of the given clause
> (since it is conceptually all one statement).
> For break and continue, they will naturally give a SyntaxError with
> the proposed implementation (for "'break' outside loop" or "'continue'
> not properly in loop", just to be randomly inconsistent).
> yield and return (at the level of the given clause itself) will need
> to be disallowed explicitly by the compiler (similar to the "'return'
> outside function" and "'yield' outside function" errors you get if you
> attempt to use these keywords in a class or module scope).

I'm -sys.maxint on the PEP for a many reasons.

1) I don't want to have to explain this to people.  "It's just like
regular python but you can't read it top-to-bottom and you can't
include control flow statements."
2a) No control flow statements in the block means if you need to
augment the code to do a return/break/continue/yield you then have to
refactor so everything in the "given:" block gets moved to the top and
a 1-line change becomes a 10 line diff.
2b) Allowing control flow statements in the block would be even more confusing.
2c) Is this legal?
     x = b given:
       b = 0
       for item in range(100):
         b += item
         if b > 10:
3) I really don't want to have to explain to people why that is, or isn't valid.
4) decorators and "with" blocks read top-to-bottom even if they change
the emphasis.  This doesn't.
5) There are no compelling use cases.  The two examples in the PEP are toys.


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