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
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
其他回答
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
还可以使用列表存储案例,并通过select调用相应的函数-
cases = ['zero()', 'one()', 'two()', 'three()']
def zero():
print "method for 0 called..."
def one():
print "method for 1 called..."
def two():
print "method for 2 called..."
def three():
print "method for 3 called..."
i = int(raw_input("Enter choice between 0-3 "))
if(i<=len(cases)):
exec(cases[i])
else:
print "wrong choice"
也在螺丝台上进行了解释。
Python>=3.10
哇,Python 3.10+现在有一个match/case语法,类似于switch/case等等!
PEP 634——结构模式匹配
匹配/案例的选定特征
1-匹配值:
匹配值类似于另一种语言中的简单开关/大小写:
match something:
case 1 | 2 | 3:
# Match 1-3.
case _:
# Anything else.
#
# Match will throw an error if this is omitted
# and it doesn't match any of the other patterns.
2-匹配结构模式:
match something:
case str() | bytes():
# Match a string like object.
case [str(), int()]:
# Match a `str` and an `int` sequence
# (`list` or a `tuple` but not a `set` or an iterator).
case [_, _]:
# Match a sequence of 2 variables.
# To prevent a common mistake, sequence patterns don’t match strings.
case {"bandwidth": 100, "latency": 300}:
# Match this dict. Extra keys are ignored.
3-捕获变量
解析对象;将其保存为变量:
match something:
case [name, count]
# Match a sequence of any two objects and parse them into the two variables.
case [x, y, *rest]:
# Match a sequence of two or more objects,
# binding object #3 and on into the rest variable.
case bytes() | str() as text:
# Match any string like object and save it to the text variable.
捕获变量在解析数据(如JSON或HTML)时非常有用,这些数据可能以多种不同的模式之一出现。
捕获变量是一项功能。但这也意味着您只需要使用虚线常量(例如:COLOR.RED)。否则,常量将被视为捕获变量并被覆盖。
更多示例用法:
match something:
case 0 | 1 | 2:
# Matches 0, 1 or 2 (value).
print("Small number")
case [] | [_]:
# Matches an empty or single value sequence (structure).
# Matches lists and tuples but not sets.
print("A short sequence")
case str() | bytes():
# Something of `str` or `bytes` type (data type).
print("Something string-like")
case _:
# Anything not matched by the above.
print("Something else")
Python<=3.9
我最喜欢的switch/case Python配方是:
choices = {'a': 1, 'b': 2}
result = choices.get(key, 'default')
简单场景的简短和简单。
比较11行以上的C代码:
// C Language version of a simple 'switch/case'.
switch( key )
{
case 'a' :
result = 1;
break;
case 'b' :
result = 2;
break;
default :
result = -1;
}
您甚至可以通过使用元组来分配多个变量:
choices = {'a': (1, 2, 3), 'b': (4, 5, 6)}
(result1, result2, result3) = choices.get(key, ('default1', 'default2', 'default3'))
您可以使用分派的dict:
#!/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()
输出:
This is case 1
This is case 3
This is case 2
This is case 1
简单,未经测试;每个条件都是独立计算的:没有贯穿,但所有情况都会计算(尽管要打开的表达式只计算一次),除非有break语句。例如
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. ')
指纹是1。是1或2。是什么。(该死!为什么在内联代码块中不能有尾随空格?)若表达式的计算结果为1,则为2。如果表达式的计算结果为2或Was某物。if表达式的计算结果为其他值。