I’d vote #3 as well.
--
Eric
> On Mar 28, 2018, at 11:27 AM, Serhiy Storchaka <storch...@gmail.com> wrote:
>
> There is a subtle semantic difference between str.format() and "equivalent"
> f-string.
>
> '{}{}'.format(a, b)
> f'{a}{b}'
>
> In the former case b is evaluated before formatting a. This is equivalent to
>
> t1 = a
> t2 = b
> t3 = format(t1)
> t4 = format(t2)
> r = t3 + t4
>
> In the latter case a is formatted before evaluating b. This is equivalent to
>
> t1 = a
> t2 = format(t1)
> t3 = b
> t4 = format(t3)
> r = t2 + t4
>
> In most cases this doesn't matter, but when implement the optimization that
> transforms the former expression to the the latter one ([1], [2]) we have to
> make a decision what to do with this difference.
>
> 1. Keep the exact semantic of str.format() when optimize it. This means that
> it should be transformed into AST node different from the AST node used for
> f-strings. Either introduce a new AST node type, or add a boolean flag to
> JoinedStr.
>
> 2. Change the semantic of f-strings. Make it closer to the semantic of
> str.format(): evaluate all subexpressions first than format them. This can be
> implemented in two ways:
>
> 2a) Add additional instructions for stack manipulations. This will slow down
> f-strings.
>
> 2b) Introduce a new complex opcode that will replace FORMAT_VALUE and
> BUILD_STRING. This will speed up f-strings.
>
> 3. Transform str.format() into an f-string with changing semantic, and ignore
> this change. This is not new. The optimizer already changes semantic.
> Non-optimized "if a and True:" would call bool(a) twice, but optimized code
> calls it only once.
>
> [1] https://bugs.python.org/issue28307
> [2] https://bugs.python.org/issue28308
>
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