[Python-Dev] Subtle difference between f-strings and str.format()

[Guido van Rossum]

Hm, without thinking too much about it I'd say it's okay to change the
evaluation order. Can these optimizations be disabled with something like
-O0?

On Wed, Mar 28, 2018 at 8:27 AM, Serhiy Storchaka <storchaka at 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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-- 
--Guido van Rossum (python.org/~guido)
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