不同于C语言和SHELL，python中没有switch case语句，关于为什么没有，官方的解释是这样的
使用Python模拟实现的方法：
def switch_if(fun, x, y):
if fun == 'add':
return x + y
elif fun == 'sub':
return x - y
elif fun == 'mul':
return x * y
elif fun == 'div':
return x / y
else:
return None
def switch_dict(fun, x, y):
return {
'add': lambda: x + y,
'sub': lambda: x - y,
'mul': lambda: x * y,
'div': lambda: x / y,
}.get(fun,None)()
print("switch_if('add',1,2):",switch_if('add',1,2))
print("switch_if('sub',1,2):",switch_if('sub',1,2))
print("switch_if('mul',1,2):",switch_if('mul',1,2))
print("switch_if('div',1,2):",switch_if('div',1,2))
print("switch_dict('add',1,2):",switch_dict('add',1,2))
print("switch_dict('sub',1,2):",switch_dict('sub',1,2))
print("switch_dict('mul',1,2):",switch_dict('mul',1,2))
print("switch_dict('div',1,2):",switch_dict('div',1,2))
switch_if('add',1,2): 3
switch_if('sub',1,2): -1
switch_if('mul',1,2): 2
switch_if('div',1,2): 0.5
switch_dict('add',1,2): 3
switch_dict('sub',1,2): -1
switch_dict('mul',1,2): 2
switch_dict('div',1,2): 0.5
