python中Switch/Case实现的示例代码

学习Python过程中，发现没有switch-case,过去写C习惯用Switch/Case语句，官方文档说通过if-elif实现。所以不妨自己来实现Switch/Case功能。

使用if…elif…elif…else 实现switch/case

可以使用if…elif…elif..else序列来代替switch/case语句，这是大家最容易想到的办法。但是随着分支的增多和修改的频繁，这种代替方式并不很好调试和维护。

方法一

通过字典实现

def foo(var):
  return {
      'a': 1，
      'b': 2,
      'c': 3,
  }.get(var,'error')  #'error'为默认返回值，可自设置

方法二

通过匿名函数实现

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

通过定义类实现

参考Brian Beck通过类来实现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

发现其实实现Switch Case需要被判断的变量是可哈希的和可比较的，这与Python倡导的灵活性有冲突。在实现上，优化不好做，可能到最后最差的情况汇编出来跟If Else组是一样的。所以Python没有支持。

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