5.2 switch and case Statements in Python?

5.2 `switch` and `case` Statements in Python?

Unfortunately, Python doesn’t currently suport switch-case statements. If you’ve worked with other languages before, you may have encountered switch-case or case-when statements - for instance:

In Ruby…

number = 4

puts case number 
     when 0...10
     "a"
     when 10...20
     "b"
     when 30...40
     "c"
     default
     "d"
     end

In Go (i.e., Golang)…

package main 

import "fmt"

var number int = 32

func main() {
  switch number {
    case 10:
      fmt.Println("Hello")
    case 20:
      fmt.Println("20")
    case 30:
      fmt.Println("40")
    default:
      fmt.Println("50")
  }
}

In C++…

#include <iostream>

int main() {
  int number = 20;

  switch(number) {
  case 10:
    std::cout << 20 << "\n";
  case 20:
    std::cout << 30 << "\n";
  case 30:
    std::cout << 40 << "\n";
  }
  return 0;
}

However, I do know of one approach that somewhat mimics the above examples via a dictionary like so:

def getChoice(x):
  choices = {
    20 : "thing",
    30 : "test",
    40 : "try"
  }
  return(choices.get(x, "Can't find what you want!"))

print(getChoice(20))

In other words, one could use a function and a dictionary in the manner above. They can then call the .get() method from the dictionary class to return the appropriate value; otherwise, if x is not present inside the dictionary (i.e., the default statements in the above examples) then we return the string "Can't find what you want!".

5.2.1 Why bother with `switch`-`case` or `case`-`when` statements at all when I have `if`, `elif`, and `else`?

This is a great question.

However, I think there are two points worth noting down about switch-case or case-when statements:

It looks neater

Here’s an example from something that I made in Ruby³ a little while ago. Here, inputs is a two-dimensional array (i.e., a Python list) of strings that I iterate over using the .each method; depending on what the first character of each item in inputs is, I then do one of four things (you don’t have to know what - this is just an example):

inputs.each do |i|
  case i[0]
  when '+'
    insert((i[1] + last_sum_result) % MODULO)
  when '-'
    erase((i[1] + last_sum_result) % MODULO)
  when '?'
    puts search((i[1] + last_sum_result) % MODULO) ? "Found" : "Not found"
  when 's'
    res = sum((i[1] + last_sum_result) % MODULO, (i[2] + last_sum_result) % MODULO)
    puts res
    last_sum_result = res % MODULO
  end
end

However, I can also do the same thing using Ruby’s if, elsif (i.e., the Ruby equivalent of Python’s elif), and else statement:

inputs.each do |i|
  if i[0] == '+'
    insert((i[1] + last_sum_result) % MODULO)
  elsif i[0] == '-'
    erase((i[1] + last_sum_result) % MODULO)
  elsif i[0] == '?'
    puts search((i[1] + last_sum_result) % MODULO) ? "Found" : "Not found"
  elsif i[0] == 's'
    res = sum((i[1] + last_sum_result) % MODULO, (i[2] + last_sum_result) % MODULO)
    puts res
    last_sum_result = res % MODULO
  end
end

Which is easier to read? I personally think that the second code block is harder even though both code blocks essentially perform the same operations!

In some cases, switch-case or case-when statements are faster

To my knowledge, a data structure called a jump table is generated when a program is being compiled. While I do not have much knowledge in this area of computer science, I believe that a jump table is akin to a dictionary in Python, albeit it works in constant time (i.e., \(O(1)\) time). Because of this, a switch-case or a case-when statement can be rather fast!

I suppose one could always look at the following link for more information regarding this: https://mortoray.com/2019/06/29/why-switch-is-better-than-if-else/

Not to worry - Ruby’s syntax is similar enough to Python’s that I think you should have little difficulty understanding the examples!↩︎