Home Replacements for switch statement in Python?
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Replacements for switch statement in Python?

Michael Schneider
1#
Michael Schneider Published in 2008-09-13 00:36:30Z

I want to write a function in Python that returns different fixed values based on the value of an input index.

In other languages I would use a switch or case statement, but Python does not appear to have a switch statement. What are the recommended Python solutions in this scenario?

Greg Hewgill
2#
Greg Hewgill Reply to 2008-09-13 00:38:24Z

You could use a dictionary:

def f(x):
    return {
        'a': 1,
        'b': 2,
    }[x]
binarysubstrate
3#
binarysubstrate Reply to 2014-02-16 05:06:42Z

I've always liked doing it this way

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

From here

Solomon Ucko
4#
Solomon Ucko Reply to 2016-01-20 16:21:52Z

If you are really just returning a predetermined, fixed value, you could create a dictionary with all possible input indexes as the keys, along with their corresponding values. Also, you might not really want a function to do this - unless you're computing the return value somehow.

Oh, and if you feel like doing something switch-like, see here.

Matthew Schinckel
5#
Matthew Schinckel Reply to 2015-06-10 14:07:33Z

In addition to the dictionary methods (which I really like, BTW), you can also use if-elif-else to obtain the switch/case/default functionality:

if x == 'a':
    # Do the thing
elif x == 'b':
    # Do the other thing
if x in 'bc':
    # Fall-through by not using elif, but now the default case includes case 'a'!
elif x in 'xyz':
    # Do yet another thing
else:
    # Do the default

This of course is not identical to switch/case - you cannot have fall-through as easily as leaving off the break; statement, but you can have a more complicated test. Its formatting is nicer than a series of nested ifs, even though functionally that's what it is closer to.

miya
6#
miya Reply to 2008-09-13 17:42:03Z

There's a pattern that I learned from Twisted Python code.

class SMTP:
    def lookupMethod(self, command):
        return getattr(self, 'do_' + command.upper(), None)
    def do_HELO(self, rest):
        return 'Howdy ' + rest
    def do_QUIT(self, rest):
        return 'Bye'

SMTP().lookupMethod('HELO')('foo.bar.com') # => 'Howdy foo.bar.com'
SMTP().lookupMethod('QUIT')('') # => 'Bye'

You can use it any time you need to dispatch on a token and execute extended piece of code. In a state machine you would have state_ methods, and dispatch on self.state. This switch can be cleanly extended by inheriting from base class and defining your own do_ methods. Often times you won't even have do_ methods in the base class.

Edit: how exactly is that used

In case of SMTP you will receive HELO from the wire. The relevant code (from twisted/mail/smtp.py, modified for our case) looks like this

class SMTP:
    # ...

    def do_UNKNOWN(self, rest):
        raise NotImplementedError, 'received unknown command'

    def state_COMMAND(self, line):
        line = line.strip()
        parts = line.split(None, 1)
        if parts:
            method = self.lookupMethod(parts[0]) or self.do_UNKNOWN
            if len(parts) == 2:
                return method(parts[1])
            else:
                return method('')
        else:
            raise SyntaxError, 'bad syntax'

SMTP().state_COMMAND('   HELO   foo.bar.com  ') # => Howdy foo.bar.com

You'll receive ' HELO foo.bar.com ' (or you might get 'QUIT' or 'RCPT TO: foo'). This is tokenized into parts as ['HELO', 'foo.bar.com']. The actual method lookup name is taken from parts[0].

(The original method is also called state_COMMAND, because it uses the same pattern to implement a state machine, i.e. getattr(self, 'state_' + self.mode))

Jeremy Cantrell
7#
Jeremy Cantrell Reply to 2008-09-19 15:37:15Z

expanding on the "dict as switch" idea. if you want to use a default value for your switch:

def f(x):
    try:
        return {
            'a': 1,
            'b': 2,
        }[x]
    except KeyError:
        return 'default'
Nilpo
8#
Nilpo Reply to 2017-07-06 22:51:30Z

If you'd like defaults you could use the dictionary get(key[, default]) method:

def f(x):
    return {
        'a': 1,
        'b': 2
    }.get(x, 9)    # 9 is default if x not found
M. Utku ALTINKAYA
9#
M. Utku ALTINKAYA Reply to 2008-11-27 04:24:36Z
def f(x):    
  return {'a': 1,'b': 2,}.get(x) or "Default"
bluish
10#
bluish Reply to 2009-06-05 14:20:52Z

I have made a (relatively) flexible and re-usable solution for this. It can be found at GitHub as this gist. If the result of the switch function is callable, it is automatically called.

GeeF
11#
GeeF Reply to 2010-09-30 08:31:55Z

Let's say you don't want to just return a value, but want to use methods that change something on an object. Using the approach stated here would be:

result = {
  'a': obj.increment(x),
  'b': obj.decrement(x)
}.get(value, obj.default(x))

What happens here is that python evaluates all methods in the dictionary. So even if your value is 'a', the object will get incremented and decremented by x.

Solution:

func, args = {
  'a' : (obj.increment, (x,)),
  'b' : (obj.decrement, (x,)),
}.get(value, (obj.default, (x,)))

result = func(*args)

So you get a list containing a function and its arguments. This way, only the function pointer and the argument list get returned, not evaluated. 'result' then evaluates the returned function call.

bluish
12#
bluish Reply to 2011-11-08 10:25:17Z

If you're searching extra-statement, as "switch", I built a python module that extends Python. It's called ESPY as "Enhanced Structure for Python" and it's available for both Python 2.x and Python 3.x.

For example, in this case, a switch statement could be performed by the following code:

macro switch(arg1):
    while True:
        cont=False
        val=%arg1%
        socket case(arg2):
            if val==%arg2% or cont:
                cont=True
                socket
        socket else:
            socket
        break

that can be used like this:

a=3
switch(a):
    case(0):
        print("Zero")
    case(1):
        print("Smaller than 2"):
        break
    else:
        print ("greater than 1")

so espy translate it in Python as:

a=3
while True:
    cont=False
    if a==0 or cont:
        cont=True
        print ("Zero")
    if a==1 or cont:
        cont=True
        print ("Smaller than 2")
        break
    print ("greater than 1")
    break
KurzedMetal
13#
KurzedMetal Reply to 2013-05-31 14:24:52Z
class switch(object):
    value = None
    def __new__(class_, value):
        class_.value = value
        return True

def case(*args):
    return any((arg == switch.value for arg in args))

Usage:

while switch(n):
    if case(0):
        print "You typed zero."
        break
    if case(1, 4, 9):
        print "n is a perfect square."
        break
    if case(2):
        print "n is an even number."
    if case(2, 3, 5, 7):
        print "n is a prime number."
        break
    if case(6, 8):
        print "n is an even number."
        break
    print "Only single-digit numbers are allowed."
    break

Tests:

n = 2
#Result:
#n is an even number.
#n is a prime number.
n = 11
#Result:
#Only single-digit numbers are allowed.
John Doe
14#
John Doe Reply to 2011-07-07 06:12:47Z

My favorite one is a really nice recipe. You'll really like it. It's the closest one I've seen to actual switch case statements, especially in features.

Here's an example:

# 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!"
htmlfarmer
15#
htmlfarmer Reply to 2012-10-17 21:52:45Z

If you have a complicated case block you can consider using a function dictionary lookup table...

If you haven't done this before its a good idea to step into your debugger and view exactly how the dictionary looks up each function.

NOTE: Do not use "()" inside the case/dictionary lookup or it will call each of your functions as the dictionary / case block is created. Remember this because you only want to call each function once using a hash style lookup.

def first_case():
    print "first"

def second_case():
    print "second"

def third_case():
    print "third"

mycase = {
'first': first_case, #do not use ()
'second': second_case, #do not use ()
'third': third_case #do not use ()
}
myfunc = mycase['first']
myfunc()
Tarrasch
16#
Tarrasch Reply to 2012-09-28 15:00:11Z

Greg's solutions will not work for unhashable entries. For example when indexing lists.

# doesn't work
def give_me_array(key)
  return {
      [1, 0]: "hello"
    }[key]

Luckily though tuples are hashable.

# works
def give_me_array(key)
  return {
      (1, 0): "hello"
    }[tuple(key)]

Similarly there probably are immutable (thus probably hashable) versions of dictionaries or sets too.

emu
17#
emu Reply to 2012-11-05 20:05:12Z
def f(x):
     return 1 if x == 'a' else\
            2 if x in 'bcd' else\
            0 #default

Short and easy to read, has a default value and supports expressions in both conditions and return values.

However, it is less efficient than the solution with a dictionary. For example, Python has to scan through all the conditions before returning the default value.

Alden
18#
Alden Reply to 2013-02-04 17:05:55Z

Just mapping some a key to some code is not really and issue as most people have shown using the dict. The real trick is trying to emulate the whole drop through and break thing. I don't think I've ever written a case statement where I used that "feature". Here's a go at drop through.

def case(list): reduce(lambda b, f: (b | f[0], {False:(lambda:None),True:f[1]}[b | f[0]]())[0], list, False)

case([
    (False, lambda:print(5)),
    (True, lambda:print(4))
])

I was really imagining it as a single statement. I hope you'll pardon the silly formatting.

reduce(
    initializer=False,
    function=(lambda b, f:
        ( b | f[0]
        , { False: (lambda:None)
          , True : f[1]
          }[b | f[0]]()
        )[0]
    ),
    iterable=[
        (False, lambda:print(5)),
        (True, lambda:print(4))
    ]
)

I hope that's valid python. It should give you drop through. of course the boolean checks could be expressions and if you wanted them to be evaluated lazily you could wrap them all in a lambda. I wouldn't be to hard to make it accept after executing some of the items in the list either. Just make the tuple (bool, bool, function) where the second bool indicates whether or not to break or drop through.

Harry247
19#
Harry247 Reply to 2013-06-06 14:01:46Z

also use the List for store the cases ,and call corresponding function by select-

cases = ['zero()','one()','two()','three()']

def zero():
  print "method for 0 called..."
def one():
  print "method for 1 called..."
def two():
  print "method for 2 called..."
def three():
  print "method for 3 called..." 

i = int(raw_input("Enter choice between 0-3 "))

if(i<=len(cases)):
 exec(cases[i])
else:
 print "wrong choice"

also explained at screwdesk

user49809
20#
user49809 Reply to 2013-07-25 18:23:33Z

Defining:

def switch1(value, options):
  if value in options:
    options[value]()

allows you to use a fairly straightforward syntax, with the cases bundled into a map:

def sample1(x):
  local = 'betty'
  switch1(x, {
    'a': lambda: print("hello"),
    'b': lambda: (
      print("goodbye," + local),
      print("!")),
    })

I kept trying to redefine switch in a way that would let me get rid of the "lambda:", but gave up. Tweaking the definition:

def switch(value, *maps):
  options = {}
  for m in maps:
    options.update(m)
  if value in options:
    options[value]()
  elif None in options:
    options[None]()

Allowed me to map multiple cases to the same code, and to supply a default option:

def sample(x):
  switch(x, {
    _: lambda: print("other") 
    for _ in 'cdef'
    }, {
    'a': lambda: print("hello"),
    'b': lambda: (
      print("goodbye,"),
      print("!")),
    None: lambda: print("I dunno")
    })

Each replicated case has to be in its own dictionary; switch() consolidates the dictionaries before looking up the value. It's still uglier than I'd like, but it has the basic efficiency of using a hashed lookup on the expression, rather than a loop through all the keys.

JD Graham
21#
JD Graham Reply to 2013-10-12 18:43:57Z

I didn't find the simple answer I was looking for anywhere on Google search. But I figured it out anyway. It's really quite simple. Decided to post it, and maybe prevent a few less scratches on someone else's head. The key is simply "in" and tuples. Here is the switch statement behavior with fall-through, including RANDOM fall-through.

l = ['Dog', 'Cat', 'Bird', 'Bigfoot',
     'Dragonfly', 'Snake', 'Bat', 'Loch Ness Monster']

for x in l:
    if x in ('Dog', 'Cat'):
        x += " has four legs"
    elif x in ('Bat', 'Bird', 'Dragonfly'):
        x += " has wings."
    elif x in ('Snake',):
        x += " has a forked tongue."
    else:
        x += " is a big mystery by default."
    print(x)

print()

for x in range(10):
    if x in (0, 1):
        x = "Values 0 and 1 caught here."
    elif x in (2,):
        x = "Value 2 caught here."
    elif x in (3, 7, 8):
        x = "Values 3, 7, 8 caught here."
    elif x in (4, 6):
        x = "Values 4 and 6 caught here"
    else:
        x = "Values 5 and 9 caught in default."
    print(x)

Provides:

Dog has four legs
Cat has four legs
Bird has wings.
Bigfoot is a big mystery by default.
Dragonfly has wings.
Snake has a forked tongue.
Bat has wings.
Loch Ness Monster is a big mystery by default.

Values 0 and 1 caught here.
Values 0 and 1 caught here.
Value 2 caught here.
Values 3, 7, 8 caught here.
Values 4 and 6 caught here
Values 5 and 9 caught in default.
Values 4 and 6 caught here
Values 3, 7, 8 caught here.
Values 3, 7, 8 caught here.
Values 5 and 9 caught in default.
Community
22#
Community Reply to 2017-05-23 11:47:36Z

I liked Mark Bies's answer

Since the x variable must used twice, I modified the lambda functions to parameterless.

I have to run with results[value](value)

In [2]: result = {
    ...:   'a': lambda x: 'A',
    ...:   'b': lambda x: 'B',
    ...:   'c': lambda x: 'C'
    ...: }
    ...: result['a']('a')
    ...: 
Out[2]: 'A'

In [3]: result = {
    ...:   'a': lambda : 'A',
    ...:   'b': lambda : 'B',
    ...:   'c': lambda : 'C',
    ...:   None: lambda : 'Nothing else matters'

    ...: }
    ...: result['a']()
    ...: 
Out[3]: 'A'

Edit: I noticed that I can use None type with with dictionaries. So this would emulate switch ; case else

leo
23#
leo Reply to 2015-05-29 17:25:57Z

I found that a common switch structure:

switch ...parameter...
case p1: v1; break;
case p2: v2; break;
default: v3;

can be expressed in Python as follows:

(lambda x: v1 if p1(x) else v2 if p2(x) else v3)

or formatted in a clearer way:

(lambda x:
     v1 if p1(x) else
     v2 if p2(x) else
     v3)

Instead of being a statement, the python version is an expression, which evaluates to a value.

Tadhg McDonald-Jensen
24#
Tadhg McDonald-Jensen Reply to 2017-12-15 20:25:07Z
class Switch:
    def __init__(self, value): self._val = value
    def __enter__(self): return self
    def __exit__(self, type, value, traceback): return False # Allows traceback to occur
    def __call__(self, *mconds): return self._val in mconds

from datetime import datetime
with Switch(datetime.today().weekday()) as case:
    if case(0):
        # Basic usage of switch
        print("I hate mondays so much.")
        # Note there is no break needed here
    elif case(1,2):
        # This switch also supports multiple conditions (in one line)
        print("When is the weekend going to be here?")
    elif case(3,4): print("The weekend is near.")
    else:
        # Default would occur here
        print("Let's go have fun!") # Didn't use case for example purposes
ChaimG
25#
ChaimG Reply to 2015-08-16 21:58:26Z

My favorite Python recipe for switch/case is:

choices = {'a': 1, 'b': 2}
result = choices.get(key, 'default')

Short and simple for simple scenarios.

Compare to 11+ lines of C code:

// C Language version of a simple 'switch/case'.
switch( key ) 
{
    case 'a' :
        result = 1;
        break;
    case 'b' :
        result = 2;
        break;
    default :
        result = -1;
}

You can even assign multiple variables by using tuples:

choices = {'a': (1, 2, 3), 'b': (4, 5, 6)}
(result1, result2, result3) = choices.get(key, ('default1', 'default2', 'default3'))
pbfy0
26#
pbfy0 Reply to 2015-10-24 01:17:46Z
# simple case alternative

some_value = 5.0

# this while loop block simulates a case block

# case
while True:

    # case 1
    if some_value > 5:
        print ('Greater than five')
        break

    # case 2
    if some_value == 5:
        print ('Equal to five')
        break

    # else case 3
    print ( 'Must be less than 5')
    break
runsun
27#
runsun Reply to 2016-01-06 18:05:24Z

For the sake of completeness, here are some of my attempts back in stone-age:

http://code.activestate.com/recipes/269708-some-python-style-switches/?in=user-1521341

I especially enjoy the use of "3. Select values with 'range comparisons'"

J_Zar
28#
J_Zar Reply to 2016-04-20 14:54:43Z

I think the best way is to use the python language idioms to keep your code testable. As showed in previous answers, I use dictionaries to take advantage of python structures and language and keep the "case" code isolated in different methods. Below there is a class, but you can use directly a module, globals and functions. The class has methods that can be tested with isolation. Dependending to your needs, you can play with static methods and attributes too.

class ChoiceManager:

    def __init__(self):
        self.__choice_table = \
        {
            "CHOICE1" : self.my_func1,
            "CHOICE2" : self.my_func2,
        }

    def my_func1(self, data):
        pass

    def my_func2(self, data):
        pass

    def process(self, case, data):
        return self.__choice_table[case](data)

ChoiceManager().process("CHOICE1", my_data)

It is possible to take advantage of this method using also classes as keys of "__choice_table". In this way you can avoid isinstance abuse and keep all clean and testable.

Supposing you have to process a lot of messages or packets from the net or your MQ. Every packet has its own structure and its management code (in a generic way). With the above code it is possible to do something like this:

class PacketManager:

    def __init__(self):
        self.__choice_table = \
        {
            ControlMessage : self.my_func1,
            DiagnosticMessage : self.my_func2,
        }

    def my_func1(self, data):
        # process the control message here
        pass

    def my_func2(self, data):
        # process the diagnostic message here
        pass

    def process(self, pkt):
        return self.__choice_table[pkt.__class__](pkt)

pkt = GetMyPacketFromNet()
PacketManager().process(pkt)


# isolated test or isolated usage example
def test_control_packet():
    p = ControlMessage()
    PacketManager().my_func1(p)

So complexity is not spread in the code flow but it is rendered in code structure.

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