PEP 750: Tag Strings For Writing Domain-Specific Languages

[MegaIng (Cornelius Krupp)]

@erictraut

First, it complicates the mental model for users of tag strings. They have to assume that all of their interpolation expressions may not be evaluated immediately

The goal of this feature is to define DSLs. Users will need to learn the rule of each individual tag function (i.e. DSL) in isolation, and they should not carry over assumptions about when a value gets evaluated from one to the other.

@erictraut

name = "Bob"
my_str = greet"hello {name}"

name = "Ellen"
print(my_str) # What would you expect here?

Hello Bob

@erictraut

del name
print(my_str) # Will this crash?

No, because the greet DSL eagerly evaluates it’s arguments. This should be spelt out in the documentation of greet.

@erictraut

Fix 1 (my recommendation): Make deferred execution explicit by having the user provide a callable in the interpolation expression. This fix addresses all of the problems I mentioned above.

Strong disagree. This limits to the usefulness of this feature so much that I think it’s useless. Maybe the PEP failed to give a good example, but IMO one of the strongest usecases is lazily evaluated (potentially even intentionally changed variables) templates. Explicitly requiring users to write lambda: breaks the normal reading flow and adds an unnecessary burden that already requires careful documentation reading to use correctly.

@erictraut

(Partial) Fix 2: Document clear expectations for tag functions. This mitigates some, but not all, of the problems. I don’t recommend this fix unless there are objections to fix 1.

This fix involves a clear, documented contract for tag functions. They should be expected to evaluate every interpolation expression exactly once and in order. If they fail to honor this contract, users of tag strings may see unexpected behaviors.

Again, strong disagree. Setting this expectation is limiting the usability of this feature way to much. There should be a strong recommendation to clearly documented when and how the expressions will be evaluated, but there shouldn’t be a warning that not following the normal rules of f-strings will lead to bugs.