我想用Python编写一个函数,根据输入索引的值返回不同的固定值。
在其他语言中,我会使用switch或case语句,但Python似乎没有switch语句。在此场景中,推荐的Python解决方案是什么?
我想用Python编写一个函数,根据输入索引的值返回不同的固定值。
在其他语言中,我会使用switch或case语句,但Python似乎没有switch语句。在此场景中,推荐的Python解决方案是什么?
当前回答
我发现Python文档中的以下答案非常有用:
你可以通过一系列if…elif。。。埃利夫。。。其他的关于switch语句语法已经有了一些建议,但对于是否以及如何进行范围测试还没有达成共识。有关完整详细信息和当前状态,请参见PEP 275。
对于需要从大量可能性中进行选择的情况,可以创建一个字典,将大小写值映射到要调用的函数。例如:
def function_1(...):
...
functions = {'a': function_1,
'b': function_2,
'c': self.method_1, ...}
func = functions[value]
func()
对于在对象上调用方法,可以通过使用内置的getattr()来检索具有特定名称的方法来进一步简化:
def visit_a(self, ...):
...
...
def dispatch(self, value):
method_name = 'visit_' + str(value)
method = getattr(self, method_name)
method()
建议为方法名称使用前缀,例如本例中的visit_。如果没有这样的前缀,如果值来自不受信任的源,攻击者将能够调用对象上的任何方法。
其他回答
def f(x):
return 1 if x == 'a' else\
2 if x in 'bcd' else\
0 #default
简短易读,具有默认值,支持条件和返回值中的表达式。
然而,它的效率不如字典解决方案。例如,Python必须在返回默认值之前扫描所有条件。
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
如果您想要默认值,可以使用dictionary get(key[,default])函数:
def f(x):
return {
'a': 1,
'b': 2
}.get(x, 9) # 9 will be returned default if x is not found
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)
class Switch:
def __init__(self, value):
self.value = value
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
return False # Allows a traceback to occur
def __call__(self, *values):
return self.value in values
from datetime import datetime
with Switch(datetime.today().weekday()) as case:
if case(0):
# Basic usage of switch
print("I hate mondays so much.")
# Note there is no break needed here
elif case(1,2):
# This switch also supports multiple conditions (in one line)
print("When is the weekend going to be here?")
elif case(3,4):
print("The weekend is near.")
else:
# Default would occur here
print("Let's go have fun!") # Didn't use case for example purposes