Python中switch语句的替换?
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Python中switch语句的替换?,python,switch-statement,Python,Switch Statement,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。在这种情况下,推荐的Python解决方案是什么?下面的原始答案是在2008年编写的。从那时起,Python3.10(2021)引入了一条语句,该语句为Python提供了“开关”的一流实现。例如: def f(x): match x: case 'a': return 1 case
在其他语言中,我会使用
switch
或case
语句,但Python似乎没有switch
语句。在这种情况下,推荐的Python解决方案是什么?下面的原始答案是在2008年编写的。从那时起,Python3.10(2021)引入了一条语句,该语句为Python提供了“开关”的一流实现。例如:
def f(x):
match x:
case 'a':
return 1
case 'b':
return 2
match
-case
语句比这个简单的示例强大得多
你可以使用字典:
def f(x):
return {
'a': 1,
'b': 2,
}[x]
下面的原始答案写于2008年。从那时起,Python3.10(2021)引入了一条语句,该语句为Python提供了“开关”的一流实现。例如:
def f(x):
match x:
case 'a':
return 1
case 'b':
return 2
match
-case
语句比这个简单的示例强大得多
你可以使用字典:
def f(x):
return {
'a': 1,
'b': 2,
}[x]
我一直喜欢这样做
result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}[value](x)
我一直喜欢这样做
result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}[value](x)
除了字典方法(顺便说一句,我非常喜欢),您还可以使用
if
-elif
-else
来获得开关
/案例
/默认功能:
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-您不可能像退出break
语句那样容易失败,但您可以进行更复杂的测试。它的格式比一系列嵌套的if
s更好,尽管功能上它更接近于此。除了字典方法(顺便说一句,我非常喜欢)之外,您还可以使用if
-elif
-else
来获得开关
/案例
/默认功能:
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-您不可能像退出break
语句那样容易失败,但您可以进行更复杂的测试。它的格式比一系列嵌套的if
s更好,尽管在功能上更接近于此。我从Twisted Python代码中学到了一种模式
class SMTP:
def lookupMethod(self, command):
return getattr(self, 'do_' + command.upper(), None)
def do_HELO(self, rest):
return 'Howdy ' + rest
def do_QUIT(self, rest):
return 'Bye'
SMTP().lookupMethod('HELO')('foo.bar.com') # => 'Howdy foo.bar.com'
SMTP().lookupMethod('QUIT')('') # => 'Bye'
您可以在需要分派令牌并执行扩展代码段的任何时候使用它。在状态机中,您将拥有状态
方法,并在self.state
上进行调度。通过从基类继承并定义自己的do
方法,可以干净地扩展此开关。通常情况下,基类中甚至没有do
方法
编辑:这到底是怎么用的
如果是SMTP,您将从电报中收到HELO
。相关代码(来自twisted/mail/smtp.py
,针对我们的案例进行了修改)如下所示
class SMTP:
# ...
def do_UNKNOWN(self, rest):
raise NotImplementedError, 'received unknown command'
def state_COMMAND(self, line):
line = line.strip()
parts = line.split(None, 1)
if parts:
method = self.lookupMethod(parts[0]) or self.do_UNKNOWN
if len(parts) == 2:
return method(parts[1])
else:
return method('')
else:
raise SyntaxError, 'bad syntax'
SMTP().state_COMMAND(' HELO foo.bar.com ') # => Howdy foo.bar.com
您将收到'HELO foo.bar.com'
(或者您可能会收到'QUIT'
或'RCPT TO:foo'
)。这被标记为部分
为['HELO',foo.bar.com']
。实际的方法查找名称取自部分[0]
(最初的方法也称为state\u COMMAND
,因为它使用相同的模式来实现状态机,即getattr(self,'state\u'+self.mode)
)我从TwistedPython代码中学到了一种模式
class SMTP:
def lookupMethod(self, command):
return getattr(self, 'do_' + command.upper(), None)
def do_HELO(self, rest):
return 'Howdy ' + rest
def do_QUIT(self, rest):
return 'Bye'
SMTP().lookupMethod('HELO')('foo.bar.com') # => 'Howdy foo.bar.com'
SMTP().lookupMethod('QUIT')('') # => 'Bye'
您可以在需要分派令牌并执行扩展代码段的任何时候使用它。在状态机中,您将拥有状态
方法,并在self.state
上进行调度。通过从基类继承并定义自己的do
方法,可以干净地扩展此开关。通常情况下,基类中甚至没有do
方法
编辑:这到底是怎么用的
如果是SMTP,您将从电报中收到HELO
。相关代码(来自twisted/mail/smtp.py
,针对我们的案例进行了修改)如下所示
class SMTP:
# ...
def do_UNKNOWN(self, rest):
raise NotImplementedError, 'received unknown command'
def state_COMMAND(self, line):
line = line.strip()
parts = line.split(None, 1)
if parts:
method = self.lookupMethod(parts[0]) or self.do_UNKNOWN
if len(parts) == 2:
return method(parts[1])
else:
return method('')
else:
raise SyntaxError, 'bad syntax'
SMTP().state_COMMAND(' HELO foo.bar.com ') # => Howdy foo.bar.com
您将收到'HELO foo.bar.com'
(或者您可能会收到'QUIT'
或'RCPT TO:foo'
)。这被标记为部分
为['HELO',foo.bar.com']
。实际的方法查找名称取自部分[0]
(原始方法也称为state\u COMMAND
,因为它使用相同的模式来实现状态机,即getattr(self,'state\u'+self.mode)
)扩展了“dict as switch”的思想。如果要为交换机使用默认值:
def f(x):
try:
return {
'a': 1,
'b': 2,
}[x]
except KeyError:
return 'default'
扩展“dict as switch”的概念。如果要为交换机使用默认值:
def f(x):
try:
return {
'a': 1,
'b': 2,
}[x]
except KeyError:
return 'default'
如果希望使用默认值,可以使用dictionary方法:
如果希望使用默认值,可以使用dictionary方法:
我使用的解决方案:
这里发布的两个解决方案的组合,相对容易阅读并支持默认值
result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}.get(whatToUse, lambda x: x - 22)(value)
在哪里
在dict中查找“lambda x:x-2”
,并将其与x=23一起使用
.get('xxx', lambda x: x - 22)(44)
在dict中找不到它,使用默认的“lambda x:x-22”
和x=44
我使用的解决方案:
这里发布的两个解决方案的组合,相对容易阅读并支持默认值
result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}.get(whatToUse, lambda x: x - 22)(value)
在哪里
在dict中查找“lambda x:x-2”
,并将其与x=23一起使用
.get('xxx', lambda x: x - 22)(44)
在dict中找不到它,并使用默认的lambda x:x-22”
和x=44
假设您不想只返回一个值,而是想使用更改对象上某些内容的方法。使用此处所述的方法将是:
result = {
'a': obj.increment(x),
'b': obj.decrement(x)
}.get(value, obj.default(x))
在这里,Python计算字典中的所有方法
因此,即使您的值为“a”,对象也将以x的形式递增,递减
解决方案:
func, args = {
'a' : (obj.increment, (x,)),
'b' : (obj.decrement, (x,)),
}.get(value, (obj.default, (x,)))
result = func(*args)
这样就得到了一个包含函数及其参数的列表。只有这边
# Prints 'bar' then 'baz'.
with Switch(2) as case:
while case(0):
print('foo')
while case(1, 2, 3):
print('bar')
while case(4, 5):
print('baz')
break
while case.default:
print('default')
break
def first_case():
print "first"
def second_case():
print "second"
def third_case():
print "third"
mycase = {
'first': first_case, #do not use ()
'second': second_case, #do not use ()
'third': third_case #do not use ()
}
myfunc = mycase['first']
myfunc()
def f(x):
return 1 if x == 'a' else\
2 if x in 'bcd' else\
0 #default
def switch1(value, options):
if value in options:
options[value]()
def sample1(x):
local = 'betty'
switch1(x, {
'a': lambda: print("hello"),
'b': lambda: (
print("goodbye," + local),
print("!")),
})
def switch(value, *maps):
options = {}
for m in maps:
options.update(m)
if value in options:
options[value]()
elif None in options:
options[None]()
def sample(x):
switch(x, {
_: lambda: print("other")
for _ in 'cdef'
}, {
'a': lambda: print("hello"),
'b': lambda: (
print("goodbye,"),
print("!")),
None: lambda: print("I dunno")
})
l = ['Dog', 'Cat', 'Bird', 'Bigfoot',
'Dragonfly', 'Snake', 'Bat', 'Loch Ness Monster']
for x in l:
if x in ('Dog', 'Cat'):
x += " has four legs"
elif x in ('Bat', 'Bird', 'Dragonfly'):
x += " has wings."
elif x in ('Snake',):
x += " has a forked tongue."
else:
x += " is a big mystery by default."
print(x)
print()
for x in range(10):
if x in (0, 1):
x = "Values 0 and 1 caught here."
elif x in (2,):
x = "Value 2 caught here."
elif x in (3, 7, 8):
x = "Values 3, 7, 8 caught here."
elif x in (4, 6):
x = "Values 4 and 6 caught here"
else:
x = "Values 5 and 9 caught in default."
print(x)
Dog has four legs
Cat has four legs
Bird has wings.
Bigfoot is a big mystery by default.
Dragonfly has wings.
Snake has a forked tongue.
Bat has wings.
Loch Ness Monster is a big mystery by default.
Values 0 and 1 caught here.
Values 0 and 1 caught here.
Value 2 caught here.
Values 3, 7, 8 caught here.
Values 4 and 6 caught here
Values 5 and 9 caught in default.
Values 4 and 6 caught here
Values 3, 7, 8 caught here.
Values 3, 7, 8 caught here.
Values 5 and 9 caught in default.
In [2]: result = {
...: 'a': lambda x: 'A',
...: 'b': lambda x: 'B',
...: 'c': lambda x: 'C'
...: }
...: result['a']('a')
...:
Out[2]: 'A'
In [3]: result = {
...: 'a': lambda : 'A',
...: 'b': lambda : 'B',
...: 'c': lambda : 'C',
...: None: lambda : 'Nothing else matters'
...: }
...: result['a']()
...:
Out[3]: 'A'
switch ...parameter...
case p1: v1; break;
case p2: v2; break;
default: v3;
(lambda x: v1 if p1(x) else v2 if p2(x) else v3)
(lambda x:
v1 if p1(x) else
v2 if p2(x) else
v3)
match something:
case 0 | 1 | 2:
# Matches 0, 1 or 2
print("Small number")
case [] | [_]:
# Matches an empty or single value sequence
# Matches lists and tuples but not sets
print("A short sequence")
case str() | bytes():
# Something of `str` or `bytes` type
print("Something string-like")
case _:
# Anything not matched by the above
print("Something else")
choices = {'a': 1, 'b': 2}
result = choices.get(key, 'default')
// C Language version of a simple 'switch/case'.
switch( key )
{
case 'a' :
result = 1;
break;
case 'b' :
result = 2;
break;
default :
result = -1;
}
choices = {'a': (1, 2, 3), 'b': (4, 5, 6)}
(result1, result2, result3) = choices.get(key, ('default1', 'default2', 'default3'))
# simple case alternative
some_value = 5.0
# this while loop block simulates a case block
# case
while True:
# case 1
if some_value > 5:
print ('Greater than five')
break
# case 2
if some_value == 5:
print ('Equal to five')
break
# else case 3
print ( 'Must be less than 5')
break
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
@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
pip install NeoPySwitch
def numbers_to_strings(argument):
switcher = {
0: "zero",
1: "one",
2: "two",
}
return switcher.get(argument, "nothing")
function(argument){
switch(argument) {
case 0:
return "zero";
case 1:
return "one";
case 2:
return "two";
default:
return "nothing";
}
}
if something:
return "first thing"
elif somethingelse:
return "second thing"
elif yetanotherthing:
return "third thing"
else:
return "default thing"
for case in [expression]:
if case == 1:
print(end='Was 1. ')
if case == 2:
print(end='Was 2. ')
break
if case in (1, 2):
print(end='Was 1 or 2. ')
print(end='Was something. ')
result = {
'case1': foo1,
'case2': foo2,
'case3': foo3,
}.get(option)(parameters_optional)
option = number['type']
result = {
'number': value_of_int, # result = value_of_int(number['value'])
'text': value_of_text, # result = value_of_text(number['value'])
'binary': value_of_bin, # result = value_of_bin(number['value'])
}.get(option)(value['value'])
option = number['type']
result = {
'number': func_for_number, # result = func_for_number()
'text': func_for_text, # result = func_for_text()
'binary': func_for_bin, # result = func_for_bin()
}.get(option)()
def dispatch(self, value):
method_name = 'visit_' + str(value)
method = getattr(self, method_name)
method()
if x == 1: print('first')
elif x == 2: print('second')
elif x == 3: print('third')
else: print('did not place')
def decision_time( key, *args, **kwargs):
def action1()
"""This function is a closure - and has access to all the arguments"""
pass
def action2()
"""This function is a closure - and has access to all the arguments"""
pass
def action3()
"""This function is a closure - and has access to all the arguments"""
pass
return {1:action1, 2:action2, 3:action3}.get(key,default)()
def ToUpper(lcChar):
if (lcChar == 'a' or lcChar == 'A'):
return 'A'
elif (lcChar == 'b' or lcChar == 'B'):
return 'B'
...
elif (lcChar == 'z' or lcChar == 'Z'):
return 'Z'
else:
return None # or something
def ConvertToReason(code):
if (code == 200):
return 'Okay'
elif (code == 400):
return 'Bad Request'
elif (code == 404):
return 'Not Found'
else:
return None
def f(x):
dictionary = {'a':1, 'b':2, 'c':3}
return dictionary.get(x,'Not Found')
##Returns the value for the letter x;returns 'Not Found' if x isn't a key in the dictionary
#!/usr/bin/env python
def case1():
print("This is case 1")
def case2():
print("This is case 2")
def case3():
print("This is case 3")
token_dict = {
"case1" : case1,
"case2" : case2,
"case3" : case3,
}
def main():
cases = ("case1", "case3", "case2", "case1")
for case in cases:
token_dict[case]()
if __name__ == '__main__':
main()