[Python-ideas] Jump to function as an an alternative to call function

[Chris Angelico]

On Mon, Aug 20, 2018 at 12:43 AM, Kirill Balunov
<kirillbalunov at gmail.com> wrote:
>
> чт, 16 авг. 2018 г. в 22:37, Chris Barker via Python-ideas
> <python-ideas at python.org>:
>>
>>
>> I wonder why locals doesn't return a Mapping Proxy, or other read-only
>> mapping object?
>>
>> If it's not guaranteed to be THE locals dict, and changes *may* not affect
>> the real one (but may), a read-only object seems like much safer idea.
>>
>> -CHB
>>
>
> I have the same feeling and already have asked the same question before
> https://mail.python.org/pipermail/python-list/2018-February/731241.html. The
> main take away was that it is the CPython implementation detail and that
> `MappingProxy` type was introduced only in 3.3. I still do not find these
> arguments convincing. On the other hand with the current status quo -
> `locals` returns `dict` inside a function, we have only advantages:
>
> a) `locals` always returns a dict and is consistent in all places. (To be
> consistent is always good.)
>
> b) This type of question occurs every 6 months (there is always a topic to
> talk about at ML).
>
> c) When someone encounters this behavior first time, he thinks that he found
> a bug <- There was no error but nothing happened (gives an opportunity to
> study the internal details of the CPython implementation, the average level
> increases. Everyone also tries to find a way, something like "I believe,
> there must be a way, because Python is so dynamic" which theoretically
> allows you to make even more new discoveries.).
>
> d) This is considered by some as a way not to leak the CPython
> implementation details. (Here I have no comments, let it be on the
> conscience of others.)
>
> e) It leaves a room for a future changes (In fact, in some situations I
> would like to have this possibility).

Related to (e) is that there is room for other implementations to
permit changes to locals(), and furthermore, a fully-compliant Python
implementation may use an actual dictionary for locals, and simply
return that. (That CPython doesn't is an implementation detail for the
sake of performance.) Requiring that it be a proxy would impose
unnecessary cost on the implementation, without benefiting any
compliant use.

ChrisA