Making a "switch" function

crystalattice · #1 08-May-2006, 02:07

For those who don't know, Python does not have a switch() function, unlike many other languages. Instead you have to use if/elif statements, like so:

Python Code:

if x == 1:
    print "one"
elif x ==2:
    print "two"
else:
    print "three"

However, I just stumbled upon a site (http://www.voidspace.org.uk/python/articles/OOP.shtml) that gives a nifty way of making a switch() style function using dictionaries. It's slightly more complicated than using if/elif statements, but it may make more sense if you have a lot of options. Which way is better is up to you to decide.

Python Code:

def function1():
    print 'You chose one.'
def  function2():
    print 'You chose two.'
def  function3():
    print 'You chose three.'
#
# switch is our dictionary of functions
switch = {
    'one': function1,
    'two': function2,
    'three': function3,
    }
#
# choice can eithe be 'one', 'two', or 'three'
choice = raw_input('Enter one, two, or three :')
#
# call one of the functions
try:
    result = switch[choice]
except KeyError:
    print 'I didn\'t understand your choice.'
else:
    result()

LuciWiz · #2 08-May-2006, 02:36

I had somewhat similar problems in C++ with string elements : you can't use a switch statement on strings! When configuring an application with XML files, making programmatic decisions based on string comparison can make sense.
The solution was to use maps. I had to add each string element to the map, in a similar way you use the dictionary. Tying the string to an action is more complicated however, and even without such a requirement, the Python code looks much nicer!

Thanks,
Lucian

crystalattice · #3 09-May-2006, 14:48

I remember in my C++ class having a problem that had me trying to use strings with a switch(). I didn't know about maps at the time so I kludged a similar way to do it by using integers as the switch() argument, then tying the number to a string.

It probably wasn't as "elegant" as a map, but I agree that the Python way is easier.

Ene Uran · #4 18-Oct-2006, 15:11

Quote:

Originally Posted by crystalattice

For those who don't know, Python does not have a switch() function, unlike many other languages. Instead you have to use if/elif statements, like so:

Python Code:

if x == 1:
    print "one"
elif x ==2:
    print "two"
else:
    print "three"

However, I just stumbled upon a site (http://www.voidspace.org.uk/python/articles/OOP.shtml) that gives a nifty way of making a switch() style function using dictionaries. It's slightly more complicated than using if/elif statements, but it may make more sense if you have a lot of options. Which way is better is up to you to decide.

Python Code:

def function1():
    print 'You chose one.'
def  function2():
    print 'You chose two.'
def  function3():
    print 'You chose three.'
#
# switch is our dictionary of functions
switch = {
    'one': function1,
    'two': function2,
    'three': function3,
    }
#
# choice can eithe be 'one', 'two', or 'three'
choice = raw_input('Enter one, two, or three :')
#
# call one of the functions
try:
    result = switch[choice]
except KeyError:
    print 'I didn\'t understand your choice.'
else:
    result()

Even in C++ the switch/case construct has slower performance than the "if/else if" construct. I would stick with if/elif in Python, particularly if your functions carry arguments, than the switch dictionary becomes rather tough.

crystalattice · #5 19-Oct-2006, 13:03

The if/elif construct is actually really easy to use and it makes sense. I've never tried the above switch dictionary because it's a lot more work to get the same thing.

I wasn't aware of the speed difference in C++ though. How bad can the performance hit be, assuming worst case scenarios?

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Making a "switch" function

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