default behavior

[Steven D'Aprano]

On Sat, 31 Jul 2010 01:02:47 -0400, wheres pythonmonks wrote:


>> Hint -- what does [].append(1) return?
>>
>>
> Again, apologies from a Python beginner.  It sure seems like one has to
> do gymnastics to get good behavior out of the core-python:
> 
> Here's my proposed fix:
> 
>  m['key'] = (lambda x: x.append(1) or x)(m.get('key',[]))
> 
> Yuck!  

Yuk is right. What's wrong with the simple, straightforward solution?

L = m.get('key', [])
L.append(1)
m['key'] = L


Not everything needs to be a one-liner. But if you insist on making it a 
one-liner, that's what setdefault and defaultdict are for.



> So I guess I'll use defaultdict with upcasts to dict as needed.

You keep using that term "upcast". I have no idea what you think it 
means, so I have no idea whether or not Python does it. Perhaps you 
should explain what you think "upcasting" is.


> On a side note:  does up-casting always work that way with shared
> (common) data from derived to base?  (I mean if the data is part of
> base's interface, will  b = base(child) yield a new base object that
> shares data with the child?)

Of course not. It depends on the implementation of the class.


-- 
Steven