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

Terry Reedy tjreedy at udel.edu
Fri Mar 30 10:33:22 EDT 2018

On 3/30/2018 6:29 AM, Serhiy Storchaka wrote:

29.03.18 18:06, Terry Reedy пише:

On 3/28/2018 11:27 AM, 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.

Perhaps Ref 3.3.1 object.bool entry, after " should return False or True.", should say something like "Should not have side-effects, as redundant bool calls may be optimized away (bool(bool(ob)) should have the same result as bool(ob))." Do you meant that it should be idempotent operation? Because bool(bool(ob)) always have the same result as bool(ob)) if bool(ob) returns True or False.

That is what the parenthetical comment says, but it is not right in the context and should be deleted.

For the "if a and True:" example, 'redundant bool calls may be optimized away.' might be better written as 'duplicate implied bool calls may be avoided.'

What I am trying to say is that we define the intended behavior of special methods, and we should define what an implementation may actually expect. The current optimizer expects bool to have no side effects, at least none that it need respect.

Having said what bool should do, we can also say what it should not do to avoid possible surprises -- at least in production code, as opposed to 'testing' code like the examples in this thread.

-- Terry Jan Reedy

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