Python's lack of a 'switch' statement has garnered much discussion and even a PEP. The most popular substitute uses dictionaries to map cases to functions, which requires lots of defs or lambdas. While the approach shown here may be O(n) for cases, it aims to duplicate C's original 'switch' functionality and structure with reasonable accuracy.
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# 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.
While the use of 'for ... in' may not be semantically accurate here (it will only ever execute the suite once), the fact that we are trying multiple cases (even if they are all within the same iteration) at least gives it the illusion of making some sense.
For better or worse, changing the switched variable within the suite will have no effect on the proceeding cases. This is a diversion from C functionality.
As noted above, case() (with no arguments) is used in place of 'default'. Another option would be to just omit the last conditional or use 'if True:'. To make that part look even more like the classic idiom, you could: --> change "yield self.match" in __iter__ to "yield self.match, True" --> change "for case in" to "for case, default in" --> change "if case():" to "if default:"
...but, in my opinion, this slight adjustment is just a trade-off of one area of readability for another.
As far as I can tell, people generally use switch for its conciseness and readability, and not for its better lookup time, if that is indeed true.
Alan Haffner's dictionary-based recipe: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/181064
Runsun Pan's dictionary-based recipe: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/269708
PEP 275 - Switching on Multiple Values: http://www.python.org/peps/pep-0275.html