Python Alternatives to switch statement from other languages

Python Alternatives to switch statement from other languages is useful in many cases, especially during programming complex items. Learn more here.

Python Alternatives: Use what the language o ers: the if/else construct

Well, if you want a switch/case construct, the most straightforward way to go is to use the good old if/else construct:

def switch(value):
if value == 1:
return "one"
if value == 2:
return "two"
if value == 42:
return "the answer to the question about life, the universe and everything"
raise Exception("No case found!")

it might look redundant, and not always pretty, but that’s by far the most efficient way to go, and it does the job:

switch(1)
one
switch(2)
two
switch(3)

Exception: No case found!
switch(42)
the answer to the question about life the universe and everything

Use a dict of functions

Another straightforward way to go is to create a dictionary of functions:

switch = {
1: lambda: 'one',
2: lambda: 'two',
42: lambda: 'the answer of life the universe and everything',
}

then you add a default function:

def default_case():
raise Exception('No case found!')

and you use the dictionary’s get method to get the function given the value to check and run it. If value does not exists in dictionary, then default_case is run.

switch.get(1, default_case)()
one
switch.get(2, default_case)()
two
switch.get(3, default_case)()
…
Exception: No case found!
switch.get(42, default_case)()
the answer of life the universe and everything

you can also make some syntactic sugar so the switch looks nicer:

def run_switch(value):
return switch.get(value, default_case)()
run_switch(1)
one

Python Alternatives: Use class introspection

You can use a class to mimic the switch/case structure. The following is using introspection of a class (using the getattr() function that resolves a string into a bound method on an instance) to resolve the “case” part.

Then that introspecting method is aliased to the call method to overload the () operator.

class SwitchBase:
def switch(self, case):
m = getattr(self, 'case_{}'.format(case), None)
if not m:
return self.default
return m
call = switch

Then to make it look nicer, we subclass the SwitchBase class (but it could be done in one class), and there we define all the case as methods:

class CustomSwitcher:
def case_1(self):
return 'one'
def case_2(self):
return 'two'
def case_42(self):
return 'the answer of life, the universe and everything!'
def default(self):
raise Exception('Not a case!')

so then we can finally use it:

switch = CustomSwitcher()
print(switch(1))
one
print(switch(2))
two
print(switch(3))

Exception: Not a case!
print(switch(42))
the answer of life, the universe and everything!

GoalKicker.com – Python® Notes for Professionals 577

Section 140.4: Using a context manager

Another way, which is very readable and elegant, but far less efficient than an if/else structure, is to build a class such as follows, that will read and store the value to compare with, expose itself within the context as a callable that will return true if it matches the stored value:

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, cond, *mconds):
return self._val in (cond,)+mconds

then defining the cases is almost a match to the real switch/case construct (exposed within a function below, to make it easier to show off):

def run_switch(value):
with Switch(value) as case:
if case(1):
return 'one'
if case(2):
return 'two'
if case(3):
return 'the answer to the question about life, the universe and everything'

default

raise Exception('Not a case!')

So the execution would be:

run_switch(1)
one
run_switch(2)
two
run_switch(3)

Exception: Not a case!
run_switch(42)
the answer to the question about life, the universe and everything

Nota Bene:

This solution is being offered as the switch module available on pypi.

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