[Python-Dev] [PEP] += on return of function call result

[Luke Kenneth Casson Leighton]

On Wed, Apr 02, 2003 at 09:54:35AM -0500, Andrew Koenig wrote:
> Luke> example code:
> Luke> log = {}
> 
> Luke> 	for t in range(5):
> Luke> 		for r in range(10):
> Luke> 			log.setdefault(r, '') += "test %d\n" % t
> 
> Luke> pprint(log)
> 
> Luke> instead, as the above is not possible, the following must be used:
> 
> Luke> from operator import add
> 
> Luke>  ...
> Luke>       ...
> Luke> 	      ...
> 
> Luke> 			add(log.setdefault(r, ''), "test %d\n" % t)
> 
> Luke> ... ARGH!  just checked - NOPE!  add doesn't work.
> Luke> and there's no function "radd" or "__radd__" in the
> Luke> operator module.
> 
> Why can't you do this?
> 
>         for t in range(5):
>                 for r in range(10):
>                         foo = log.setdefault(r,'')
>                         foo += "test %d\n" % t
 
 after running this code,

 log = {0: '', 1: '', 2:'', 3: '' ... 9: ''}

 and foo equals "test 5".

 if, however, you do this:

         for t in range(5):
                 for r in range(10):
                         foo = log.setdefault(r,[])
                         foo.append("test %d\n" % t)
 
 then empirically i conclude that you DO end up with the
 expected results (but is this true all of the time?)

 the reason why your example, andrew, does not work, is
 because '' is a string - a basic type to which a pointer is
 NOT returned i presume that the foo += "test %d"... returns a
 DIFFERENT result object such that the string in the dictionary
 is DIFFERENT from the string result of foo being updated.

 if that makes absolutely no sense whatsoever then think of it
 being the difference between integers and pointers-to-integers
 in c.

 
 can anyone tell me if there are any PARTICULAR circumstances where

                         foo = log.setdefault(r,[])
                         foo.append("test %d\n" % t)

 will FAIL to work as expected?



 andrew, sorry it took me so long to respond: i initially
 thought that under all circumstances for all types of foo,
 your example would work.

 l.


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