Evaluation of variable as f-string

[Peter J. Holzer]

On 2023-01-27 21:43:09 +0100, Johannes Bauer wrote:
> x = { "y": "z" }
> s = "-> {x['y']}"
> print(s.format(x = x))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> KeyError: "'y'"
> 
> This. Does. Not. Work.
> 
> I want to pass a single variable as a dictionary and access its members
> inside the expression.

You can do that (see other responses), but you can't have arbitrary
Python expressions inside a format string.

It works with f-strings because f-strings are a compiler construct. The
f-string never exists at run-time. Instead the compiler transforms 
    f"-> {x['y']}"
into the equivalent of
    "-> " + format_value(x["y"]) + ""

So either you need to pass it to the Python compiler (via eval), or you
need to implement enough of a Python parser/interpreter to cover the
cases you are interested in. The latter might be an interesting
exercise, but I would suggest looking at existing template engines like
Jinja2 for production purposes.

        hp

-- 
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"
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