Evaluation of variable as f-string

[Rob Cliffe]

Whoa! Whoa! Whoa!
I appreciate the points you are making, Chris, but I am a bit taken 
aback by such forceful language.

On 27/01/2023 19:18, Chris Angelico wrote:
> On Sat, 28 Jan 2023 at 05:31, Rob Cliffe via Python-list
> <python-list at python.org> wrote:
>> On 23/01/2023 18:02, Chris Angelico wrote:
>>> Maybe, rather than asking for a way to treat a string as code, ask for
>>> what you ACTUALLY need, and we can help?
>>>
>>> ChrisA
>> Fair enough, Chris, but still ISTM that it is reasonable to ask (perhaps
>> for a different use-case) whether there is a way of evaluating a string
>> at runtime as if it were an f-string.  We encourage people to ask
>> questions on this list, even though the answer will not always be what
>> they're hoping for.
> No, it's not, because that's the "how do I use X to do Y" problem.
> Instead, just ask how to do *what you actually need*. If the best way
> to do that is to eval an f-string, then someone will suggest that.
> But, much much much more likely, the best way to do it would be
> something completely different. What, exactly? That's hard to say,
> because *we don't know what you actually need*. All you tell us is
> what you're attempting to do, which there is *no good way to achieve*.
If the above is addressed to the OP, I can't answer for him.
If it's addressed to me:  How about if I wanted a program (a learning 
tool) to allow the user to play with f-strings?
I.e. to type in a string, and then see what the result would be if it 
had been an f-string?
I suspect there are other use cases, but I confess I can't think of one 
right now.
>
>> I appreciate that the answer may be "No, because it would be a lot of
>> work - and increase the maintenance burden - to support a relatively
>> rare requirement".
> What about: "No, because it's a terrible TERRIBLE idea, requires that
> you do things horribly backwards, and we still don't even know what
> you're trying to do"?
>
>> Perhaps someone will be inspired to write a function to do it. 😎
> See, we don't know what "it" is, so it's hard to write a function
> that's any better than the ones we've seen.
Again, if this is addressed to the OP: I'm not his keeper. 😁
If it's addressed to me: "it" means a function that will take a string 
and evaluate it at runtime as if it were an f-string.  Sure, with 
caveats and limitations.  And indeed Thomas Passim found this partial 
solution on Stack Overflow:
def effify(non_f_str: str):
     return eval(f'f"""{non_f_str}"""')
>   Using eval() to construct
> an f-string and then parse it is TERRIBLE because:
>
> 1) It still doesn't work in general, and thus has caveats like "you
> can't use this type of quote character"
> 2) You would have to pass it a dictionary of variables, which also
> can't be done with full generality
> 3) These are the exact same problems, but backwards, that led to
> f-strings in the first place
> 4) eval is extremely slow and horrifically inefficient.
I understand these limitations.  Nonetheless I can conceive that there 
may be scenarios where it is an acceptable solution (perhaps the 
learning tool program I suggested above).
Addressing your points specifically:
     1) I believe the quote character limitation could be overcome. It 
would need a fair amount of work, for which I haven't (yet) the time or 
inclination.
     2) Yes in general you would have to pass it one dictionary, maybe 
two.  I don't see this as an insuperable obstacle.  I am not sure what 
you mean by "can't be done with full generality" and perhaps that's not 
important.
     3) Not sure I understand this.
     4) On the fairly rare occasions that I have used eval(), I can't 
remember speed ever being a consideration.
>
> For some reason, str.format() isn't suitable, but *you haven't said
> why*, so we have to avoid that in our solutions. So, to come back to
> your concern:
>
>> We encourage people to ask
>> questions on this list, even though the answer will not always be what
>> they're hoping for.
> Well, yes. If you asked "how can I do X", hoping the answer would be
> "with a runtime-evaluated f-string", then you're quite right - the
> answer might not be what you were hoping for. But since you asked "how
> can I evaluate a variable as if it were an f-string", the only
> possible answer is "you can't, and that's a horrible idea".
I hope that I have shown that this is a somewhat dogmatic response.
>
> Don't ask how to use X to do Y. Ask how to do Y.
Good advice.
Best wishes
Rob Cliffe
>
> ChrisA