[python] 類實現Switch case語句

python中Switch/Case實現

學習Python過程中，發現沒有switch-case,過去寫C習慣用Switch/Case語句，官方文檔說通過if-elif實現。所以不妨自己來實現Switch/Case功能。

方法一

通過字典實現

def foo(var):
    return {
            'a': 1，
            'b': 2,
            'c': 3,
    }.get(var,'error')    #'error'為默認返回值，可自設置

方法二

通過匿名函數實現

def foo(var,x):
    return {
            'a': lambda x: x+1,
            'b': lambda x: x+2,
            'c': lambda x: x+3, 
    }[var](x)

方法三

通過定義類實現
參考Brian Beck通過類來實現Swich-case

# 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

# break is used here to look as much like the real thing as possible, but
# elif is generally just as good and more concise.

# Empty suites are considered syntax errors, so intentional fall-throughs
# should contain 'pass'
c = 'z'
for case in switch(c):
    if case('a'): pass # only necessary if the rest of the suite is empty
    if case('b'): pass
    # ...
    if case('y'): pass
    if case('z'):
        print "c is lowercase!"
        break
    if case('A'): pass
    # ...
    if case('Z'):
        print "c is uppercase!"
        break
    if case(): # default
        print "I dunno what c was!"

# As suggested by Pierre Quentel, you can even expand upon the
# functionality of the classic 'case' statement by matching multiple
# cases in a single shot. This greatly benefits operations such as the
# uppercase/lowercase example above:
import string
c = 'A'
for case in switch(c):
    if case(*string.lowercase): # note the * for unpacking as arguments
        print "c is lowercase!"
        break
    if case(*string.uppercase):
        print "c is uppercase!"
        break
    if case('!', '?', '.'): # normal argument passing style also applies
        print "c is a sentence terminator!"
        break
    if case(): # default
        print "I dunno what c was!"

# Since Pierre's suggestion is backward-compatible with the original recipe,
# I have made the necessary modification to allow for the above usage.

---

查看Python官方:PEP 3103-A Switch/Case Statement
發現其實實現Switch Case需要被判斷的變量是可哈希的和可比較的，這與Python倡導的靈活性有沖突。在實現上，優化不好做，可能到最后最差的情況匯編出來跟If Else組是一樣的。所以Python沒有支持。