[转帖] Python 中 switch 的解决办法

K*K posted @ Thu, 08 Jan 2009 19:25:46 +0800 in 未归类 with tags python switch , 5805 readers

其他语言中，switch语句大概是这样的

switch (var)
{
case value1: do_some_stuff1();
case value2: do_some_stuff2();
…
case valueN: do_some_stuffN();
default: do_default_stuff();
}

而python本身没有switch语句，解决方法有以下3种:

A.使用dictionary

values = {
value1: do_some_stuff1,
value2: do_some_stuff2,
…
valueN: do_some_stuffN,
}
values.get(var, do_default_stuff)()

具体请参考: Python switch statement 这个方法的缺点是:我不知道do_some_stuff是不是允许多个语句，如何实现，也许是可以的，我不知道-.-

B.使用lambda

在PHP中，

switch ($value) {
case 'a':
$result = $x * 5;
break;
case 'b':
$result = $x + 7;
break;
case 'c':
$result = $x - 2;
break;
}

在python中:

result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}[value](x)

具体参考: Switch statements in Python

C.Brian Beck提供了一个类 switch 来实现其他语言中switch的功能

# This class provides the functionality we want. You only need to look at
# this if you want to know how this works. It only needs to be defined
# once, no need to muck around with its internals.

class switch(object):
def __init__(self, value):
self.value = value
self.fall = False

def __iter__(self):
“”"Return the match method once, then stop”"”
yield self.match
raise StopIteration

def match(self, *args):
“”"Indicate whether or not to enter a case suite”"”
if self.fall or not args:
return True
elif self.value in args: # changed for v1.5, see below
self.fall = True
return True
else:
return False

# The following example is pretty much the exact use-case of a dictionary,
# but is included for its simplicity. Note that you can include statements
# in each suite.

v = ‘ten’
for case in switch(v):
if case(‘one’):
print 1
break
if case(‘two’):
print 2
break
if case(‘ten’):
print 10
break
if case(‘eleven’):
print 11
break
if case(): # default, could also just omit condition or ‘if True’
print “something else!”
# No need to break here, it’ll stop anyway

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