Q19: `@staticmethod` vs `@classmethod` vs Instance Methods: - Python & FastAPI

In Simple Words:

Instance method: Takes self; accesses and mutates instance state.
@classmethod: Takes cls; accesses class state; ideal for alternative factory constructors.
@staticmethod: Takes neither self nor cls; acts as an isolated pure function inside a class namespace.

python

class Date:
    def __init__(self, year, month, day):
        self.year, self.month, self.day = year, month, day

    @classmethod
    def from_string(cls, date_str): # Factory constructor
        y, m, d = map(int, date_str.split("-"))
        return cls(y, m, d)

    @staticmethod
    def is_valid_year(year):
        return 1900 <= year <= 2100