学ぶ classmethod and staticmethod as Descriptors

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classmethod and staticmethod are built-in descriptors. They use __get__ to change what gets passed as the first argument when a method is called. Understanding them as descriptors explains exactly how they work – and why they behave differently from regular methods.

Regular Methods Are Non-Data Descriptors

Before looking at classmethod and staticmethod, it helps to know that regular functions stored as class attributes are themselves non-data descriptors. Their __get__ binds the instance as the first argument:


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# A regular function's __get__ binds the instance
class Report:
    def generate(self):
        return f"Report from {self}"

print(type(Report.__dict__["generate"]))  # <class 'function'>
print(hasattr(Report.__dict__["generate"], "__get__"))  # True

report = Report()
bound_method = Report.__dict__["generate"].__get__(report, Report)
print(bound_method())  # Same as report.generate()

`classmethod` — Binding the Class

classmethod wraps a function so that __get__ binds the class as the first argument instead of the instance. This happens whether the method is called on an instance or on the class directly:


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# classmethod binds the class, not the instance
class Transaction:
    _currency = "USD"

    def __init__(self, transaction_id, amount):
        self.transaction_id = transaction_id
        self.amount = amount

    @classmethod
    def get_currency(cls):
        return cls._currency

    @classmethod
    def from_string(cls, raw_string):
        # Parsing "TX-001:4500.0" into a Transaction
        transaction_id, amount = raw_string.split(":")
        return cls(transaction_id, float(amount))

# Calling on the class and on an instance – both receive the class
print(Transaction.get_currency())           # "USD"

transaction = Transaction.from_string("TX-001:4500.0")
print(transaction.transaction_id)           # "TX-001"
print(transaction.amount)                   # 4500.0

How `classmethod.get` Works


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# Inspecting classmethod as a descriptor
class Config:
    debug = False

    @classmethod
    def is_debug(cls):
        return cls.debug

print(type(Config.__dict__["is_debug"]))           # <class 'classmethod'>
print(hasattr(Config.__dict__["is_debug"], "__get__"))  # True

# __get__ returns a bound method with the class pre-filled
bound = Config.__dict__["is_debug"].__get__(None, Config)
print(bound())  # False – same as Config.is_debug()

`staticmethod` — No Binding at All

staticmethod wraps a function so that __get__ returns the original function unchanged – no instance, no class is bound. It behaves like a plain function that happens to live in a class namespace:


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# staticmethod – no binding, just a plain function in class scope
class CurrencyConverter:
    _rate = 1.08  # EUR to USD

    @staticmethod
    def format_amount(amount, symbol):
        return f"{symbol}{amount:,.2f}"

    @classmethod
    def convert_to_usd(cls, eur_amount):
        usd_amount = eur_amount * cls._rate
        return cls.format_amount(usd_amount, "$")

print(CurrencyConverter.format_amount(4500.0, "€"))  # €4,500.00
print(CurrencyConverter.convert_to_usd(4500.0))      # $4,860.00