dict slice in python (translating perl to python)

Nick Craig-Wood nick at craig-wood.com
Fri Sep 12 18:36:42 CEST 2008


Steven D'Aprano <steven at REMOVE.THIS.cybersource.com.au> wrote:
>  On Thu, 11 Sep 2008 03:36:35 -0500, Nick Craig-Wood wrote:
> 
> > As an ex-perl programmer and having used python for some years now, I'd
> > type the explicit
> > 
> >   v1,v2,v3 = mydict['one'], mydict['two'], mydict['two'] # 54 chars
> > 
> > Or maybe even
> > 
> >   v1 = mydict['one'] # 54 chars
> >   v2 = mydict['two']
> >   v3 = mydict['two']
> > 
> > Either is only a couple more characters to type.
> 
>  But that's an accident of the name you have used. Consider:
> 
>  v1,v2,v3 = section_heading_to_table_index['one'], \
>   section_heading_to_table_index['two'], \
>   section_heading_to_table_index['two']  # 133 characters
> 
>  versus:
> 
>  v1,v2,v3 = [section_heading_to_table_index[k] for k in
>   ['one','two','two']]  # 75 characters
> 
>  It also fails the "Don't Repeat Yourself" principle, and it completely 
>  fails to scale beyond a handful of keys.

If you have more than a handful of keys then you have a different
problem (far too many local variables) with your code I think!

DRY is a good principle.  I still prefer the 3 explicit assignments
though ;-)

>  Out of interest, on my PC at least the list comp version is significantly 
>  slower than the explicit assignments. So it is a micro-optimization that 
>  may be worth considering if needed -- but at the cost of harder to 
>  maintain code.
> 
> > It is completely
> > explicit and comprehensible to everyone, in comparison to
> > 
> >   v1,v2,v3 = [ mydict[k] for k in ['one','two','two']] # 52 chars
> >   v1,v2,v3 = [ mydict[k] for k in 'one two two'.split()] # 54 chars
> 
>  That's a matter for argument. I find the list comprehension perfectly 
>  readable and comprehensible, and in fact I had to read your explicit 
>  assignments twice to be sure I hadn't missed something. But I accept that 
>  if you aren't used to list comps, they might look a little odd.

A matter of taste certainly!

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick



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