Python's formatted strings, or "f-strings", came relatively late to the language, but have become a popular feature. F-strings allow a compact representation for the common task of interpolating program data into strings, often in order to output them in some fashion. Some restrictions were placed on f-strings to simplify the implementation of them, but those restrictions are not really needed anymore and, in fact, are complicating the CPython parser. That has led to a Python Enhancement Proposal (PEP) to formalize the syntax of f-strings for the benefit of Python users while simplifying the maintenance of the interpreter itself.
Source: Formalizing f-strings, an article by Jake Edge.