python - 在方法开头组合检查

Question

我有几种类似于以下的方法:

@property
def tpr_num_days(self):
    """
    The duration, in days, of the TPR interval.
    """
    if not self.prices:
        raise RuntimeError("Must initialize self.prices first.")

    # do something

@property
def revenue_during_tpr_is_greatest(self):
    """
    Tells us if the Revenue that was generated during the TPR was both greater
    than the revenue that was generated in the same-duration interval before the TPR and after the TPR.
    """
    if not self.prices:
        raise RuntimeError("Must initialize self.prices first.")

    # do something

def build_pricing_data(self):
    """
    This will build a list of pricing objects.

    Note that we are currently excluding "returns" because the price of the returned
    item may be different than the current price of the product, which could open up
    a can of worms trying to track down when that item was purchased.
    """
    cursor.execute('''SELECT
                            date,
                            max(if(offer_code='sdbuy', price, null)) sd_buy_price,
                            max(if(offer_code='hdbuy', price, null)) hd_buy_price,
                            max(if(offer_code='sdrent', price, null)) sd_rent_price,
                            max(if(offer_code='hdrent', price, null)) hd_rent_price,
                            sum(revenue) revenue
                      FROM price_date WHERE apple_id=%s and territory=%s and revenue > 0
                      GROUP BY date
                      ORDER BY date
                      ''', (self.apple_id, self.territory))

    # let's make sure it's re-initialized in case it's called multiple times.
    self.prices = []     
    for row in cursor:
        pricing_obj = {"date": item[0], "SDBUY": item[1], "HDBUY": item[2], "SDRENT": item[3], "HDRENT": item[4], "REVENUE": item[5]}
        self.prices.append(pricing_obj)

除了在许多方法的开头使用这个 if 语句，还有什么更好的方法来封装它？

Answer 1

您可以使用函数装饰器:

import functools

def requires_price(func):
    @functools.wraps(func)
    def wrapper(obj, *args, **kwargs):
        if not obj.prices:
            raise RuntimeError("Must initialize self.prices first.")

        return func(obj, *args, **kwargs)

    return wrapper

使用方式如下:

@property
@requires_price
def tpr_num_days(self):
    ... # do something

或者您可以更进一步，将装饰器实现为 descriptor ，这可以让您省略 @property:

class price_property:
    def __init__(self, func):
        self.func = func

    def __get__(self, instance, owner):
        if instance is None:
            return self

        if not instance.prices:
            raise RuntimeError("Must initialize self.prices first.")

        return self.func(instance)

使用方式如下:

@price_property
def tpr_num_days(self):
    ... # do something