调用子类函数减少代码复制答案

【问题标题】：Calling child class function to reduce code replication调用子类函数减少代码复制
【发布时间】：2019-10-18 18:59:29
【问题描述】：

我希望通过在父类中使用公共代码来减少子类中的代码复制，但仍需要根据子类进行一些更改的特定处理。我知道从父母那里打电话给孩子是不好的。如何完成代码缩减？（或者我应该尝试一下吗？）

这是一个例子：

class Address:

    def __init__(self, street, city, postal_code):

        self._street = street
        self._city = city
        self._postal_code = self.valid_postal_code(postal_code)

    def valid_postal_code(self, postal_code):
        """ Returns a validated postal code """
        #
        # A big bunch of code common to all postal codes
        #

        if child == Usa:
            return Usa.valid_postal_code(postal_code)
        else:
            return Canada.valid_postal_code(postal_code)    

class Usa(Address):

    def valid_postal_code(self, postal_code):
        """ Returns a validated US zip code """
        # Must be 5 digits or 5 digits plus dash 4 digits
        if len(postal_code) != 5 and len(postal_code) != 10:
            raise Exception("Bad postal code")

        return postal_code


class Canada(Address):

    def valid_postal_code(self, postal_code):
        """ Returns a validates Canadian postal code """
        # Must be A#A #A#
        if len(postal_code) == 6:
            postal_code = postal_code[0:3] + " " + postal_code[3:3]
        if len(postal_code) != 7:
            raise Exception("Bad postal code")

        return postal_code.upper()

【问题讨论】：

美国和加拿大的地址实际上没有任何共同之处，只是它们有邮政编码。基本上，Address.valid_postal_code 只会有一个通用的占位符定义（如pass 或return True 或return False），并且每个子类都必须用适当的定义覆盖它。
让# A big bunch of code common to all postal codes 成为通用部分，但子类中的不同验证逻辑应该保留
每个子节点都应该通过super().valid_postal_code() 调用父节点，而不是父节点调用子节点实现，因为父节点不一定知道（或关心）哪些子节点存在。

标签： python class reduction

【解决方案1】：

每个覆盖方法都应该调用父方法。

class Address:

    def __init__(self, street, city, postal_code):

        self._street = street
        self._city = city
        self._postal_code = self.valid_postal_code(postal_code)

    def valid_postal_code(self, postal_code):
        """ Returns a validated postal code """
        #
        # A big bunch of code common to all postal codes
        #
        ...


class Usa(Address):

    def valid_postal_code(self, postal_code):
        """ Returns a validated US zip code """

        # Must be 5 digits or 5 digits plus dash 4 digits
        if not super().valid_postal_code(postal_code) or len(postal_code) != 5 and len(postal_code) != 10:
            raise Exception("Bad postal code")

        return postal_code


class Canada(Address):

    def valid_postal_code(self, postal_code):
        """ Returns a validates Canadian postal code """
        # Must be A#A #A#
        if len(postal_code) == 6:
            postal_code = postal_code[0:3] + " " + postal_code[3:3]
        if not super().valid_postal_code() or len(postal_code) != 7:
            raise Exception("Bad postal code")

        return postal_code.upper()

【讨论】：

【解决方案2】：

有两种可能：

让父代码包含公共部分，子代码包含细节
- 父级必须返回中间结果

或

有两个父方法，其中一个是存根并被子方法覆盖
- 父级将中间结果传递到存根中

第二种方法看起来像：

class Address:

    def __init__(...):

    def valid_postal_code(self, postal_code):
        """ Returns a validated postal code """
        #
        # A big bunch of code common to all postal codes
        #
        return self._validate_postal_code(postal_code)

    def _validate_postal_code(self, postal_code):
        raise NotImplementedError


class Usa(Address):

    def _validate_postal_code(self, postal_code):
        """ Returns a validated US zip code """
        # Must be 5 digits or 5 digits plus dash 4 digits
        if len(postal_code) != 5 and len(postal_code) != 10:
            raise Exception("Bad postal code")
        return postal_code


class Canada(Address):

    def _validate_postal_code(self, postal_code):
        """ Returns a validates Canadian postal code """
        # Must be A#A #A#
        if len(postal_code) == 6:
            postal_code = postal_code[0:3] + " " + postal_code[3:3]
        if len(postal_code) != 7:
            raise Exception("Bad postal code")
        return postal_code.upper()

【讨论】：