我发现一种常见的交换机结构：

switch ...parameter...
case p1: v1; break;
case p2: v2; break;
default: v3;

可以用Python表示如下：

(lambda x: v1 if p1(x) else v2 if p2(x) else v3)

或以更清晰的方式格式化：

(lambda x:
     v1 if p1(x) else
     v2 if p2(x) else
     v3)

Python版本不是一个语句，而是一个表达式，其计算结果为一个值。

switch语句只是if/elif/else的语法糖。任何控制语句所做的都是基于某个条件（即决策路径）来授权作业。为了将其包装到模块中并能够基于其唯一id调用作业，可以使用继承和Python中的任何方法都是虚拟的这一事实来提供派生类特定的作业实现，作为特定的“case”处理程序：

#!/usr/bin/python

import sys

class Case(object):
    """
        Base class which specifies the interface for the "case" handler.
        The all required arbitrary arguments inside "execute" method will be
        provided through the derived class
        specific constructor

        @note in Python, all class methods are virtual
    """
    def __init__(self, id):
        self.id = id

    def pair(self):
        """
            Pairs the given id of the "case" with
            the instance on which "execute" will be called
        """
        return (self.id, self)

    def execute(self): # Base class virtual method that needs to be overridden
        pass

class Case1(Case):
    def __init__(self, id, msg):
        self.id = id
        self.msg = msg
    def execute(self): # Override the base class method
        print("<Case1> id={}, message: \"{}\"".format(str(self.id), self.msg))

class Case2(Case):
    def __init__(self, id, n):
        self.id = id
        self.n = n
    def execute(self): # Override the base class method
        print("<Case2> id={}, n={}.".format(str(self.id), str(self.n)))
        print("\n".join(map(str, range(self.n))))


class Switch(object):
    """
        The class which delegates the jobs
        based on the given job id
    """
    def __init__(self, cases):
        self.cases = cases # dictionary: time complexity for the access operation is 1
    def resolve(self, id):

        try:
            cases[id].execute()
        except KeyError as e:
            print("Given id: {} is wrong!".format(str(id)))



if __name__ == '__main__':

    # Cases
    cases=dict([Case1(0, "switch").pair(), Case2(1, 5).pair()])

    switch = Switch(cases)

    # id will be dynamically specified
    switch.resolve(0)
    switch.resolve(1)
    switch.resolve(2)

除了字典方法（我很喜欢，BTW），您还可以使用if elif else来获得switch/case/default功能：

if x == 'a':
    # Do the thing
elif x == 'b':
    # Do the other thing
if x in 'bc':
    # Fall-through by not using elif, but now the default case includes case 'a'!
elif x in 'xyz':
    # Do yet another thing
else:
    # Do the default

当然，这与switch/case不同——你不可能像放弃break语句那样轻易地通过，但你可以进行更复杂的测试。它的格式比一系列嵌套的if更好，尽管在功能上更接近。

Python 3.10（2021）引入了match-case语句，该语句提供了Pythons“switch”的一流实现。例如：

def f(x):
    match x:
        case 'a':
            return 1
        case 'b':
            return 2
        case _:
            return 0   # 0 is the default case if x is not found

match-case语句比这个简单的示例强大得多。

以下原始答案写于2008年，当时还未提供匹配案例：

你可以用字典：

def f(x):
    return {
        'a': 1,
        'b': 2,
    }[x]

虽然已经有了足够的答案，但我想指出一个更简单、更强大的解决方案：

class Switch:
    def __init__(self, switches):
        self.switches = switches
        self.between = len(switches[0]) == 3

    def __call__(self, x):
        for line in self.switches:
            if self.between:
                if line[0] <= x < line[1]:
                    return line[2]
            else:
                if line[0] == x:
                    return line[1]
        return None


if __name__ == '__main__':
    between_table = [
        (1, 4, 'between 1 and 4'),
        (4, 8, 'between 4 and 8')
    ]

    switch_between = Switch(between_table)

    print('Switch Between:')
    for i in range(0, 10):
        if switch_between(i):
            print('{} is {}'.format(i, switch_between(i)))
        else:
            print('No match for {}'.format(i))


    equals_table = [
        (1, 'One'),
        (2, 'Two'),
        (4, 'Four'),
        (5, 'Five'),
        (7, 'Seven'),
        (8, 'Eight')
    ]
    print('Switch Equals:')
    switch_equals = Switch(equals_table)
    for i in range(0, 10):
        if switch_equals(i):
            print('{} is {}'.format(i, switch_equals(i)))
        else:
            print('No match for {}'.format(i))

输出：

Switch Between:
No match for 0
1 is between 1 and 4
2 is between 1 and 4
3 is between 1 and 4
4 is between 4 and 8
5 is between 4 and 8
6 is between 4 and 8
7 is between 4 and 8
No match for 8
No match for 9

Switch Equals:
No match for 0
1 is One
2 is Two
No match for 3
4 is Four
5 is Five
No match for 6
7 is Seven
8 is Eight
No match for 9

扩展Greg Hewgill的答案-我们可以使用装饰器封装字典解决方案：

def case(callable):
    """switch-case decorator"""
    class case_class(object):
        def __init__(self, *args, **kwargs):
            self.args = args
            self.kwargs = kwargs

        def do_call(self):
            return callable(*self.args, **self.kwargs)

return case_class

def switch(key, cases, default=None):
    """switch-statement"""
    ret = None
    try:
        ret = case[key].do_call()
    except KeyError:
        if default:
            ret = default.do_call()
    finally:
        return ret

然后可以将其与@case decorator一起使用

@case
def case_1(arg1):
    print 'case_1: ', arg1

@case
def case_2(arg1, arg2):
    print 'case_2'
    return arg1, arg2

@case
def default_case(arg1, arg2, arg3):
    print 'default_case: ', arg1, arg2, arg3

ret = switch(somearg, {
    1: case_1('somestring'),
    2: case_2(13, 42)
}, default_case(123, 'astring', 3.14))

print ret

好消息是，这已经在NeoPySwitch模块中完成。只需使用pip进行安装：

pip install NeoPySwitch