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

[Chris Angelico]

On Fri, Mar 30, 2018 at 1:08 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Thu, Mar 29, 2018 at 11:28 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>> On Wed, Mar 28, 2018 at 06:27:19PM +0300, Serhiy Storchaka wrote:
>>
>>> The optimizer already changes
>>> semantic. Non-optimized "if a and True:" would call bool(a) twice, but
>>> optimized code calls it only once.
>>
>> I don't understand this. Why would bool(a) be called twice, and when did
>> this change? Surely calling it twice would be a bug.
>>
>> I just tried the oldest Python 3 I have on this computer, 3.2, and bool
>> is only called once.
>
> Technically not bool() itself, but the equivalent. Here's some similar code:

Wow, I'm good. Premature send much? Nice going, Chris. Let's try that
again. Here's some similar code:

>>> def f(a):
...     if a and x:
...         print("Yep")
...
>>> class Bool:
...     def __bool__(self):
...         print("True?")
...         return True
...
>>> x = 1
>>> f(Bool())
True?
Yep

This is, however, boolifying a, then boolifying x separately. To bool
a twice, you'd need to write this instead:

>>> def f(a):
...     if a or False:
...         print("Yep")
...

In its optimized form, this still only boolifies a once. But we can
defeat the optimization:

>>> def f(a):
...     cond = a or False
...     if cond:
...         print("Yep")
...
>>> f(Bool())
True?
True?
Yep

The "or False" part implies a booleanness check on its left operand,
and the 'if' statement performs a boolean truthiness check on its
result. That means two calls to __bool__ in the unoptimized form. But
it gets optimized, on the assumption that __bool__ is a pure function.
The version assigning to a temporary variable does one check before
assigning, and then another check in the 'if'; the same thing without
the temporary skips the second check, and just goes ahead and enters
the body of the 'if'.

Technically that's a semantic change. But I doubt it'll hurt anyone.

ChrisA