[Python-ideas] kwargs for return

[Eric V. Smith]

On 1/26/2019 12:30 PM, David Mertz wrote:
> On Sat, Jan 26, 2019 at 10:31 AM Steven D'Aprano <steve at pearwood.info 
> <mailto:steve at pearwood.info>> wrote:
> 
>     In what way is it worse, given that returning a namedtuple with named
> 
>     fields is backwards compatible with returning a regular tuple? We can
>     have our cake and eat it too.
>     Unless the caller does a type-check, there is no difference. Sequence
>     unpacking will still work, and namedtuples unlike regular tuples can
>     support optional attributes.
> 
> 
> I suppose the one difference is where someone improperly relies on tuple 
> unpacking.
> 
> Old version:
> 
>     def myfun():
>          # ...
>          return a, b, c
> 
>     # Call site
>     val1, val2, val3 = myfun()
> 
> 
> New version:
> 
>     def myfun():
>          # ...
>          return a, b, c, d
> 
> 
> Now the call site will get "ValueError: too many values to unpack".  
> Namedtuples don't solve this problem, of course.  But they don't make 
> anything worse either.
> 
> The better approach, of course, is to document the API as only using 
> attribute access, not positional.  I reckon dataclasses from the start 
> could address that concern... but so can documentation alone.  E.g.:
> 
> Old version (improved):
> 
>     def myfun():
> 
>          mydata = namedtuple("mydata", "a b c")
> 
>          # ...
>          return mydata(a, b, c)
> 
>     # Call site
>     ret = myfun()
> 
>     val1, val2, val3 = ret.a, ret.b, ret.c
> 
> 
> New version (improved)
> 
>     def myfun():
> 
>          mydata = namedtuple("mydata", "a b c d e")
> 
>          # ...
>          return mydata(a, b, c, d, e)
> 
> Now the call site is completely happy with no changes (assuming it 
> doesn't need to care about what values 'ret.d' or 'ret.e' contain... but 
> presumably those extra values are optional in some way.
> 
> Moreover, we are even perfectly fine if we had created 
> namedtuple("mydata", "e d c b a") for some reason, completely changing 
> the positions of all the named attributes in the improved namedtuple.

Preventing this automatic unpacking (and preventing iteration in 
general) was one of the motivating factors for dataclasses: 
https://www.python.org/dev/peps/pep-0557/#id47

Eric