访问者模式和依赖注入模式 19/30 | Python 主题月

写在前面

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这个月是 Python 活动月，我决定尝试用 Python 来刷这 30 天的每日一题和随机一题。然后如果周末有精力，我想捣鼓捣鼓这个python-patterns

设计模式对我来说更多的是学习而不是我的个人经验总结，所以我很可能理解偏，如果有大佬见到了请及时指出，我之所以选择在掘金来写一些个人的东西是因为这里的文章质量更高，我不希望后来者看到了这些文章被误导。

访问者模式

class Node:
    pass


class A(Node):
    pass


class B(Node):
    pass


class C(A, B):
    pass


class Visitor:
    def visit(self, node, *args, **kwargs):
        meth = None
        for cls in node.__class__.__mro__:
            meth_name = "visit_" + cls.__name__
            meth = getattr(self, meth_name, None)
            if meth:
                break

        if not meth:
            meth = self.generic_visit
        return meth(node, *args, **kwargs)

    def generic_visit(self, node, *args, **kwargs):
        print("generic_visit " + node.__class__.__name__)

    def visit_B(self, node, *args, **kwargs):
        print("visit_B " + node.__class__.__name__)


def main():
    """
    >>> a, b, c = A(), B(), C()
    >>> visitor = Visitor()

    >>> visitor.visit(a)
    generic_visit A

    >>> visitor.visit(b)
    visit_B B

    >>> visitor.visit(c)
    visit_B C
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod()

依赖注入模式

import datetime
from typing import Callable


class ConstructorInjection:
    def __init__(self, time_provider: Callable) -> None:
        self.time_provider = time_provider

    def get_current_time_as_html_fragment(self) -> str:
        current_time = self.time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


class ParameterInjection:
    def __init__(self) -> None:
        pass

    def get_current_time_as_html_fragment(self, time_provider: Callable) -> str:
        current_time = time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


class SetterInjection:
    """Setter Injection"""

    def __init__(self):
        pass

    def set_time_provider(self, time_provider: Callable):
        self.time_provider = time_provider

    def get_current_time_as_html_fragment(self):
        current_time = self.time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


def production_code_time_provider() -> str:
    """
    Production code version of the time provider (just a wrapper for formatting
    datetime for this example).
    """
    current_time = datetime.datetime.now()
    current_time_formatted = f"{current_time.hour}:{current_time.minute}"
    return current_time_formatted


def midnight_time_provider() -> str:
    """Hard-coded stub"""
    return "24:01"


def main():
    """
    >>> time_with_ci1 = ConstructorInjection(midnight_time_provider)
    >>> time_with_ci1.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">24:01</span>'

    >>> time_with_ci2 = ConstructorInjection(production_code_time_provider)
    >>> time_with_ci2.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">...</span>'

    >>> time_with_pi = ParameterInjection()
    >>> time_with_pi.get_current_time_as_html_fragment(midnight_time_provider)
    '<span class="tinyBoldText">24:01</span>'

    >>> time_with_si = SetterInjection()

    >>> time_with_si.get_current_time_as_html_fragment()
    Traceback (most recent call last):
    ...
    AttributeError: 'SetterInjection' object has no attribute 'time_provider'

    >>> time_with_si.set_time_provider(midnight_time_provider)
    >>> time_with_si.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">24:01</span>'
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod(optionflags=doctest.ELLIPSIS)

小结

参考文献

• 无