Python中的Switch/Case实现是否有任何价值? [英] Is there any value to a Switch / Case implementation in Python?

问题描述

最近，我在网上看到了一些讨论，讨论Python中没有什么等效的"switch/case".我意识到有几种方法可以做类似的事情-有些使用lambda，有些使用字典.关于替代方案，还有其他StackOverflow讨论.甚至还有两个PEP(PEP 0275和PEP 3103)讨论(并拒绝)将switch/case集成到语言中.

我想出了一种我认为是切换/案例的优雅方法.

它最终看起来像这样:

from switch_case import switch, case         # note the import style

x = 42
switch(x)                                    # note the switch statement
if case(1):                                  # note the case statement
    print(1)
if case(2):
    print(2)
if case():                                   # note the case with no args
    print("Some number besides 1 or 2")

所以，我的问题是:这是一个值得创造的东西吗?您对改善它有什么建议吗?

我将包含文件放在github 上，并附有大量示例. (我认为整个include文件大约有50行可执行文件，但是我有1500行示例和文档.)我是否对这件事进行了过度设计，浪费了很多时间，还是有人会觉得这值得?

试图解释为什么这与其他方法不同:
1)可能有多个路径(执行两种或更多种情况)， 在字典方法中比较难.
2)可以检查除等于"以外的比较 (例如case(less_than(1000)).
3)比字典方法和if/elif方法可能更具可读性
4)可以跟踪有多少个True案例.
5)可以限制允许的True个案例. (即执行 前2个真实案例...)
6)允许使用默认情况.

这是一个更详细的示例:

from switch_case import switch, case, between

x=12
switch(x, limit=1)                # only execute the FIRST True case
if case(between(10,100)):         # note the "between" case Function
    print ("%d has two digits."%x)
if case(*range(0,100,2)):         # note that this is an if, not an elif!
    print ("%d is even."%x)       # doesn't get executed for 2 digit numbers,
                                  # because limit is 1; previous case was True.
if case():
    print ("Nothing interesting to say about %d"%x)



# Running this program produces this output:

12 has two digits.

这里是一个示例，试图显示switch_case如何比常规if/else更清晰和简洁:

# conventional if/elif/else:
if (status_code == 2 or status_code == 4 or (11 <= status_code < 20) 
          or status_code==32):
    [block of code]
elif status_code == 25 or status_code == 45:
    [block of code]
if status_code <= 100:
    [block can get executed in addition to above blocks]

# switch_case alternative (assumes import already)
switch(status_code)
if case (2, 4, between(11,20), 32):   # significantly shorter!
    [block of code]
elif case(25, 45):
    [block of code]
if case(le(100)):
    [block can get executed in addition to above blocks]

如果语句反复重复相同的开关，则可以节省很多时间.不确定用例的使用频率如何，但是在某些情况下似乎很有意义.

github上的示例文件包含更多示例.

解决方案

from pyswitch import Switch   # pyswitch can be found on PyPI

myswitch = Switch()

@myswitch.case(42)
def case42(value):
    print "I got 42!"

@myswitch.case(range(10))
def caseRange10(value):
    print "I got a number from 0-9, and it was %d!" % value

@myswitch.caseIn('lo')
def caseLo(value):
    print "I got a string with 'lo' in it; it was '%s'" % value

@myswitch.caseRegEx(r'\b([Pp]y\w)\b')
def caseReExPy(matchOb):
    print r"I got a string that matched the regex '\b[Pp]y\w\b', and the match was '%s'" % matchOb.group(1)

@myswitch.default
def caseDefault(value):
    print "Hey, default handler here, with a value of %r." % value

myswitch(5)  # prints: I got a number from 0-9, and it was 5!
myswitch('foobar')  # prints: Hey, default handler here, with a value of foobar.
myswitch('The word is Python')  # prints: I got a string that matched the regex '\b[Pp]y\w\b', and the match was 'Python'

您明白了.为什么?是的，派发表是使用Python的方式.我只是厌倦了一遍又一遍地编写它们，所以我写了一个类和一些装饰器来为我处理.

Recently, I saw some discussions online about how there is no good "switch / case" equivalent in Python. I realize that there are several ways to do something similar - some with lambda, some with dictionaries. There have been other StackOverflow discussions about the alternatives. There were even two PEPs (PEP 0275 and PEP 3103) discussing (and rejecting) the integration of switch / case into the language.

I came up with what I think is an elegant way to do switch / case.

It ends up looking like this:

from switch_case import switch, case         # note the import style

x = 42
switch(x)                                    # note the switch statement
if case(1):                                  # note the case statement
    print(1)
if case(2):
    print(2)
if case():                                   # note the case with no args
    print("Some number besides 1 or 2")

So, my questions are: Is this a worthwhile creation? Do you have any suggestions for making it better?

I put the include file on github, along with extensive examples. (I think the entire include file is about 50 executable lines, but I have 1500 lines of examples and documentation.) Did I over-engineer this thing, and waste a bunch of time, or will someone find this worthwhile?

Edit:

Trying to explain why this is different from other approaches:
1) Multiple paths are possible (executing two or more cases), which is harder in the dictionary method.
2) can do checking for comparisons other than "equals" (such as case(less_than(1000)).
3) More readable than the dictionary method, and possibly if/elif method
4) can track how many True cases there were.
5) can limit how many True cases are permitted. (i.e. execute the first 2 True cases of...)
6) allows for a default case.

Here's a more elaborate example:

from switch_case import switch, case, between

x=12
switch(x, limit=1)                # only execute the FIRST True case
if case(between(10,100)):         # note the "between" case Function
    print ("%d has two digits."%x)
if case(*range(0,100,2)):         # note that this is an if, not an elif!
    print ("%d is even."%x)       # doesn't get executed for 2 digit numbers,
                                  # because limit is 1; previous case was True.
if case():
    print ("Nothing interesting to say about %d"%x)



# Running this program produces this output:

12 has two digits.

Here's an example attempting to show how switch_case can be more clear and concise than conventional if/else:

# conventional if/elif/else:
if (status_code == 2 or status_code == 4 or (11 <= status_code < 20) 
          or status_code==32):
    [block of code]
elif status_code == 25 or status_code == 45:
    [block of code]
if status_code <= 100:
    [block can get executed in addition to above blocks]

# switch_case alternative (assumes import already)
switch(status_code)
if case (2, 4, between(11,20), 32):   # significantly shorter!
    [block of code]
elif case(25, 45):
    [block of code]
if case(le(100)):
    [block can get executed in addition to above blocks]

The big savings is in long if statements where the same switch is repeated over and over. Not sure how frequent of a use-case that is, but there seems to be certain cases where this makes sense.

The example file on github has even more examples.

解决方案

from pyswitch import Switch   # pyswitch can be found on PyPI

myswitch = Switch()

@myswitch.case(42)
def case42(value):
    print "I got 42!"

@myswitch.case(range(10))
def caseRange10(value):
    print "I got a number from 0-9, and it was %d!" % value

@myswitch.caseIn('lo')
def caseLo(value):
    print "I got a string with 'lo' in it; it was '%s'" % value

@myswitch.caseRegEx(r'\b([Pp]y\w)\b')
def caseReExPy(matchOb):
    print r"I got a string that matched the regex '\b[Pp]y\w\b', and the match was '%s'" % matchOb.group(1)

@myswitch.default
def caseDefault(value):
    print "Hey, default handler here, with a value of %r." % value

myswitch(5)  # prints: I got a number from 0-9, and it was 5!
myswitch('foobar')  # prints: Hey, default handler here, with a value of foobar.
myswitch('The word is Python')  # prints: I got a string that matched the regex '\b[Pp]y\w\b', and the match was 'Python'

You get the idea. Why? Yep, dispatch tables are the way to go in Python. I just got tired of writing them over and over, so I wrote a class and some decorators to handle it for me.

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