Incorrect scope of list comprehension variables

Alf P. Steinbach alfps at start.no
Sat Apr 17 03:43:53 CEST 2010


* Steven D'Aprano:
> On Fri, 16 Apr 2010 08:48:03 -0700, Aahz wrote:
> 
>> In article <4bb92850$0$8827$c3e8da3 at news.astraweb.com>, Steven D'Aprano 
>> <steve at REMOVE-THIS-cybersource.com.au> wrote:
>>> Nevertheless, it is a common intuition that the list comp variable
>>> should *not* be exposed outside of the list comp, and that the for-loop
>>> variable should. Perhaps it makes no sense, but it is very common --
>>> I've never heard of anyone being surprised that the for-loop variable is
>>> exposed, but I've seen many people surprised by the fact that list-comps
>>> do expose their loop variable.
>> I've definitely seen people surprised by the for-loop behavior.
> 
> What programming languages were they used to (if any)?
> 
> I don't know of any language that creates a new scope for loop variables, 
> but perhaps that's just my ignorance...

MRAB has mentioned Ada, let me mention C++ ...


<code language="C++">
     #include <assert.h>

     int main()
     {
         int const   i = 42;

         for( int i = 0; i < 10; ++i )
         {
             // blah blah
         }
         assert( i == 42 );
     }
</code>


Java and C# take a slightly different approach where code analogous to the above 
won't compile. But it's still a nested scope. E.g. ...


<code language="Java">

     class App
     {
         static public void main( String[] args )
         {
             for( int i = 0; i < 10; ++i )
             {
                 // blah blah
             }

             // Uncomment statement below to get compilation error:
             //System.out.println( i );
         }
     }
</code>


So, yes, considering Ada, C++, Java and C#  --  and so on. ;-)


Cheers & hth.,

- Alf



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