优美胜于丑陋 import this

博客地址：Specific-Dispatch

前言

表驱动法是一种编辑模式（ Scheme ）——从表里面查找信息而不使用逻辑语句（if 和 case）。事实上，凡是能通过逻辑语句来选择的事物，都可以通过查表来选择。

对简单的情况而言，使用逻辑语句更为容易和直白。但随着逻辑链的越来越复杂，查表法也就愈发显得更具吸引力。

Python 的switch case

由于 Python 中没有switch case关键词，所以对于每一种情况的逻辑语句只能用if,elif,else来实现，显得很不 Pythonic.

def handle_case(case):
    if case == 1:
        print('case 1')
    elif case == 2:
        print('case 2')
    else:
        print('default case')

而受到PEP-443: Single-dispatch generic functions的启发，很容易就能实现如下装饰器：

from functools import update_wrapper
from types import MappingProxyType
from typing import Hashable, Callable, Union


def specificdispatch(key: Union[int, str] = 0) -> Callable:
    """specific-dispatch generic function decorator.

    Transforms a function into a generic function, which can have different
    behaviours depending upon the value of its key of arguments or key of keyword arguments.
    The decorated function acts as the default implementation, and additional
    implementations can be registered using the register() attribute of the
    generic function.
    """

    def decorate(func: Callable) -> Callable:
        registry = {}

        def dispatch(key: Hashable) -> Callable:
            """
            Runs the dispatch algorithm to return the best available implementation
            for the given *key* registered on *generic_func*.
            """
            try:
                impl = registry[key]
            except KeyError:
                impl = registry[object]
            return impl

        def register(key: Hashable, func: Callable=None) -> Callable:
            """
            Registers a new implementation for the given *key* on a *generic_func*.
            """
            if func is None:
                return lambda f: register(key, f)

            registry[key] = func
            return func

        def wrapper_index(*args, **kw):
            return dispatch(args[key])(*args, **kw)

        def wrapper_keyword(*args, **kw):
            return dispatch(kw[key])(*args, **kw)

        registry[object] = func
        if isinstance(key, int):
            wrapper = wrapper_index
        elif isinstance(key, str):
            wrapper = wrapper_keyword
        else:
            raise KeyError('The key must be int or str')
        wrapper.register = register
        wrapper.dispatch = dispatch
        wrapper.registry = MappingProxyType(registry)
        update_wrapper(wrapper, func)

        return wrapper

    return decorate

而之前的代码就能很优美的重构成这样：

@specificdispatch(key=0)
def handle_case(case):
    print('default case')

@handle_case.register(1)
def _(case):
    print('case 1')

@handle_case.register(2)
def _(case):
    print('case 2')

handle_case(1) # case 1
handle_case(0) # default case

而对于这样的架构，即易于扩展也利于维护。

更多实例

class Test:
    @specificdispatch(key=1)
    def test_dispatch(self, message, *args, **kw):
        print(f'default: {message} args:{args} kw:{kw}')

    @test_dispatch.register('test')
    def _(self, message, *args, **kw):
        print(f'test: {message} args:{args} kw:{kw}')

test = Test()
# default: default args:(1,) kw:{'test': True}
test.test_dispatch('default', 1, test=True)
# test: test args:(1,) kw:{'test': True}
test.test_dispatch('test', 1, test=True)

@specificdispatch(key='case')
def handle_case(case):
    print('default case')

@handle_case.register(1)
def _(case):
    print('case 1')

@handle_case.register(2)
def _(case):
    print('case 2')

handle_case(case=1)  # case 1
handle_case(case=0)  # default case