dict.get(key, default) evaluates default even if key exists

[2QdxY4RzWzUUiLuE at potatochowder.com]

On 2020-12-16 at 12:01:01 +1300,
dn via Python-list <python-list at python.org> wrote:

> > On Tue, Dec 15, 2020 at 9:57 AM Mark Polesky via Python-list <
> > python-list at python.org> wrote:
> >
> >> Hi.
> >>
> >> # Running this script....
> >>
> >> D = {'a':1}
> >> def get_default():
> >>      print('Nobody expects this')
> >>      return 0
> >> print(D.get('a', get_default()))
> >>
> >> # ...generates this output:
> >>
> >> Nobody expects this
> >> 1
> >>
> >> ###
> >>
> >> Since I'm brand new to this community, I thought I'd ask here first... Is
> >> this worthy of a bug report?  This behavior is definitely unexpected to
> me,
> >> and I accidentally coded an endless loop in a mutual recursion situation
> >> because of it.  Calling dict.get.__doc__ only gives this short sentence:
> >> Return the value for key if key is in the dictionary, else default.
> >> Nothing in that docstring suggests that the default value is evaluated
> even
> >> if the key exists, and I can't think of any good reason to do so.
> 
> 
> Hey this is fun! A quick search reveals that I've (mea culpa) always thought
> of dict.get() as providing a default *value* and not used a function in
> there to provide said value.
> 
> That said, the function's name says it all: get_default(). Is it 'getting' a
> default value? No, it's (also) reporting-back. Thus a "side-effect".
> Undesirable, as you say.
> 
> Were it appropriate to report that a default value was necessary, maybe
> something like:
> 
>     try:
>         print( d[ 'a' ] )
>     except KeyError:
>         print( "Nobody expects this" )
>         d[ 'a' ] = 0

Be careful:  that mutates d in the except clause, whereas calling
d.get('a', 0) doesn't, which may or may not be okay.