Re: Configuring an object via a dictionary

On 18/03/24 04:11, Peter J. Holzer via Python-list wrote:

On 2024-03-17 17:15:32 +1300, dn via Python-list wrote:

On 17/03/24 12:06, Peter J. Holzer via Python-list wrote:

On 2024-03-16 08:15:19 +0000, Barry via Python-list wrote:

On 15 Mar 2024, at 19:51, Thomas Passin via Python-list <python-list@python.org> wrote:
I've always like writing using the "or" form and have never gotten bit

>
I, on the other hand, had to fix a production problem that using “or” introducted.
I avoid this idiom because it fails on falsy values.

>
Perl has a // operator (pronounced "err"), which works like || (or),
except that it tests whether the left side is defined (not None in
Python terms) instead of truthy. This still isn't bulletproof but I've
found it very handy.

>
>
So, if starting from:
>
     def method( self, name=None, ):
>
  rather than:
>
     self.name = name if name else default_value
>
ie
>
     self.name = name if name is True else default_value

 These two lines don't have the same meaning (for the reason you outlined
below). The second line is also not very useful.
  

the more precise:
>
     self.name = name if name is not None or default_value
>
or:
>
     self.name = default_value if name is None or name

 Those are syntax errors. I think you meant to write "else" instead of
"or".
 Yes, exactly. That's the semantic of Perl's // operator.
 JavaScript has a ?? operator with similar semantics (slightly
complicated by the fact that JavaScript has two "nullish" values).

Thanks Peter!
(yes, sad consequences of suffering a neighbor's party-til-midnight followed by an 0530 meeting-start - children: don't code exhausted!)
Herewith, an illustration of how the corrected version of the above works - and how (in what seem unusual cases) it avoids any truthy/falsy confusion, as raised earlier in this thread:
 >>> default_value = "default"
 >>> name = "Fred Flintstone"
 >>> name if name is not None else default_value
'Fred Flintstone'
 >>> name = None
 >>> name if name is not None else default_value
'default'
 >>> name = False
 >>> name if name is not None else default_value
False
 >>> name = 1
 >>> name if name is not None else default_value
1
 >>> name = 0
 >>> name if name is not None else default_value
0
Personally: I find the above coding more logical, because our primary interest is on 'the happy case', ie where the value has been assigned (to "name"); and the default_value is only applied as a "guard".
On the other hand, I dislike the not-condition because it forces me to think (and maybe dust-off DeMorgan). Accordingly:
 >>> name = "Fred Flintstone"
 >>> default_value if name is None else name
'Fred Flintstone'
 >>> name = None
 >>> default_value if name is None else name
'default'
 >>> name = False
 >>> default_value if name is None else name
False
...
YMMV!
NB your corporate Style Guide may prefer 'the happy path'...
--
Regards,
=dn