Membership Operator

The in operator checks if a value exists in a collection.

With Lists

let nums = [10, 20, 30, 40, 50]

if 20 in nums:
    print("Found 20!")

if 99 in nums:
    print("Won't print - 99 not in list")

With Sets

let colors = #{"red", "green", "blue"}

if "red" in colors:
    print("Red is in the set")

With Strings (Substring Check)

let message = "Hello, World!"

if "World" in message:
    print("Found 'World' in message")

if "Desi" in message:
    print("Won't print")

With Tuples

let coords = (1, 2, 3, 4, 5)

if 3 in coords:
    print("Found 3 in tuple")

Custom Classes

Classes can implement __contains__ to support the in operator:

class MySet:
    pub data: list[int]

    pub def __new__(self, values: list[int]):
        self.data = values

    pub def __contains__(self, item: int) -> bool:
        # Custom logic to check membership
        for x in self.data:
            if x == item:
                return true
        return false

let s = MySet([10, 20, 30, 40, 50])

if 20 in s:
    print("20 is in MySet")  # This prints

if 99 in s:
    print("Won't print")

Tip

The __contains__ method decides how to compare elements. Use value equality (like ==) for intuitive behavior.

Negation

Negate the whole membership test with not:

let nums = [1, 2, 3]

if not (5 in nums):
    print("5 is not in the list")

There is no not in operator

Python's x not in y is not Desi syntax. Write not (x in y) — the parentheses are required, because not binds tighter than in.