Match True in python

Question

In JavaScript, using the switch statement, I can do the following code:

switch(true){
    case 1 === 1:
        console.log(1)
        break

    case 1 > 1:
        console.log(2)
        break

    default:
        console.log(3)
        break
}

And it’s going to return 1, since JavaScript switch is comparing true === (1 === 1)

But the same does not happen when I try it with Python Match statement, like as follows:

match True:
    case 1 = 1:
        print(1)

    case 1 > 1:
        print(2)

    case _:
        print(3)

It returns:

File "<stdin>", line 2
    case 1 = 1:
           ^
SyntaxError: invalid syntax

And another error is returned if I try it this way:

Check1 = 1 == 1
Check2 = 1 > 1

match True:
    case Check1:
        print(1)

    case Check2:
        print(2)

    case _:
        print(3)

It returns:

case Check1:
         ^^^^^^
SyntaxError: name capture 'Check1' makes remaining patterns unreachable

What would be the cleanest/fastest way to do many different checks without using a lot of if’s and elif’s?

Asked By: William Brochensque junior

||

Source

Answer 1

In JavaScript, using the switch statement, I can do the following code

I definitely wouldn’t be using JavaScript as any form of litmus or comparator for python.

If you used 1==1 in your first test case, the below is what both of your test cases are ultimately doing.

match True:
    case True:
        print(1)

    case False: #will never get hit
        print(2)

    case _:     #will never get hit
        print(3)

This is why you get the error for the second version. True will only ever be True, so no other case will ever be hit.

Based on your example, it seems like you are trying to use match/case just to determine the "truthiness" of an expression. Put the expression in the match.

match a==1:
    case True: 
        pass
    case False:
        pass

If you have a lot of expressions, you could do something like the below, although I don’t think this is very good.

a = 2

match (a==1, a>1):
    case (True, False):
        print('equals 1')
    case (False, True):
        print('greater than 1')
    case _:
        print(_)

#OR

match ((a>1) << 1) | (a==1):
    case 1:
        print('equals 1')
    case 2:
        print('greater than 1')
    case _:
        print(_)

cases should be possible results of the match, NOT expressions that attempt to emulate the match. You’re doing it backwards. The below link should tell you pretty much everything that you need to know about match/case, as-well-as provide you with alternatives.

Match/Case Examples and Alternatives

Answered By: OneMadGypsy

Answer 2

Check if you are using Python 3.10, if not then use this instead and
also the match case isn’t meant to be used liked this, you’re better off using switch case if you’re just trying to print something out

The switch case in python is used by creating a function for an ‘if-else-if’ statement and declaring the case above like:

match = int(input("1-3: "))


def switch(case): #Function
    if case == 1:
        print(1)

    elif case > 1:
        print(2)

    else:
        print(3)


switch(match) #Main driver

else is the ‘default’ and you can add as many elif statements.

Answered By: sascha

Answer 3

If you don’t want to use the match statement from Python 3.10, you can create a switch-like statement with a one line function and use it with a one-pass for loop. It would look very similar to the javascript syntax:

def switch(v): yield lambda *c:v in c

for case in switch(True):

    if case(1 == 1):
        print(1)
        break

    if case( 1 > 1):
        print(2)
        break
else:
    print(3)

Note that if you don’t use breaks, the conditions will flow through (which would allow multiple cases to be executed if that’s what you need).

Answered By: Alain T.

Match True in python

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