Keeping track of things with dictionaries

[Steven D'Aprano]

On Tue, 08 Apr 2014 09:14:39 +0200, Frank Millman wrote:

> It appears that when you use 'setdefault', the default is always
> evaluated, even if the key exists.
> 
>>>> def get_value(val):
> ...   print('getting value', val)
> ...   return val*2
> ...
>>>> my_dict = {}
>>>> my_dict.setdefault('a', get_value('xyz'))
> getting value xyz
> 'xyzxyz'
>>>> my_dict.setdefault('a', get_value('abc'))
> getting value abc
> 'xyzxyz'
>>>> my_dict
> {'a': 'xyzxyz'}
>>>>
>>>>
> It seems odd. Is there a situation where this behaviour is useful?

It's not a feature of setdefault. It's how Python works: arguments to 
functions and methods are always evaluated before the function is called. 
The same applies to most languages.

Only a very few number of syntactic features involve delayed evaluation. 
Off the top of my head:

- the second argument to short-circuit "or" and "and" operators:

    if obj and obj[0]: ...

- ternary if:

    1/x if x != 0 else float("inf")

- generator expressions

- and of course the body of functions and methods don't execute until 
  the function is called.


-- 
Steven