Modifying an array requires no allocation; persistent vectors always do.

Linear types are another approach where you're essentially passing around exclusive access to a single mutable structure, so all effects are local. See: Rust/Clean.

Python is an awfully weird language to desire this feature in. You don't even statically know what type profile is, so checking the class you think it will be doesn't actually answer the question. Also effects would necessarily be part of the public api of a given class (preventing someone from later changing verified_email to a "property"), which is quite unpythonic.