Why “une”? isn’t one of the first things we learn about the French grammar that we don’t use an indefinite article in front of an occupation followed by “être”? e.g., Je suis étudiante, not Je suis une étudiante. What is the subtlety in this sentence that makes “une artiste” legit if this is not an error?
Usually, when there is no extra information, it works as you described, no article :
Alice est étudiante.
Bob est chauffeur.
Usually, when there is extra info describing what kind of étudiante Alice is, or what kind of chauffeur Bob is, then you must use an article:
Alice est une étudiante très disciplinée.
Alice est une étudiante qui a beaucoup de potentiel.
Bob est un chauffeur expérimenté.
So, now to your question / this example:
First, the good news: The article is not mandatory here: “Ma femme est artiste.”, would be fine.
Why do both work here? It’s one of those borderline cases.
Specifically, I think it’s the combination of:
The subject being “Ma femme” (i.e a noun group and not a pronoun) + the word “artiste” itself
So, if you replace “Ma femme” by “Elle”:
Elle est une artiste → The article can’t be used here.
Elle est artiste → OK
Similarly, if you replace the word artiste by médecin:
Ma femme est un(e) médecin → I’d say the article can’t be used here either.
Ma femme est médecin → OK
It’s a complex subject and depending on the specifics, people might disagree as to what is correct or “best”. If you have questions on specific examples, let me know.
Thank you for the amazingly detailed response. I have seen in some other cases where subtly different rules apply when it’s a noun vs pronoun. Given your response, I will patiently wait until my French is more advanced to debate what’s correct / not in what context. For now, it’s really helpful to know that it is not always wrong to have an article followed by être and before occupation. Especially the rule about the article being required when there is extra info!