The most Pythonic way to check if an element exists is to use the in operator. Sets provide O(1) average time complexity for membership checks due to their underlying hash table implementation.

The in operator returns True if the set contains an element and False otherwise.

Syntax

element in set

Basic membership check

Let’s declare a Set named “stocks” and check for two different elements, and see if the set contains those elements.

stocks = {"Kaynes", "Dixon", "PGEL"}

print("Kaynes" in stocks)
# Output: True

print("Syrma" in stocks)
# Output: False

Case sensitivity

The in operator checks for case-sensitivity as well. So, lowercase and uppercase make a huge difference.

langs = {"Python", "Php", "JavaScript"}

print("php" in langs)
# Output: False

print("Php" in langs)
# Output: True

Mixed types

What if the set contains different types of elements, and we check for a specific type? If it exists, returns True; otherwise, False.

data = {21, "Krunal", (19, 10)}

print(21 in data)
# Output: True

print("Krunal" in data)
# Output: True

print((19, 21) in data)
# Output: False

print((19, 10) in data)
# Output: True

print([1, 2] in data)
# TypeError: unhashable type: 'list'

Sets can only contain hashable types. In the last line, we are checking for a list, so it threw a TypeError: unhashable type: ‘list’ because the list is an unhashable type.

Empty Set

If the input Set is empty and you try to check any element against it, it always returns False.

data = {}

print(21 in data)

# Output: False

None as an element

Since None is hashable, it qualifies to be an element in a set. What if we want to check None as an element? Well, if the Set contains None, the “in” operator returns True for the existence of None; otherwise, it returns False.

null_set = {19, None, "khushi"}

print(None in null_set)  

# Output: True

Alternate approaches

The following are alternative approaches that you can use, but they are less reliable than the “in” operator.

1. Using the “not in” operator
2. Using operator.countOf() method

Approach 1: Using the not in operator

The not in operator is the negation of the in operator. So, not in operator returns False if the set contains an element. If the set does not contain an element, it returns True.

main_set = {19, 21, "Kaynes", -10, True}

print(48 not in main_set)
# Output: True

print("Kaynes" not in main_set)
# Output: False

Since, the Kaynes is included in the main_set, it returns False. 48 is not included in the main_set, so it returns True.

Approach 2: Using an operator.countOf()

The operator.countOf() method checks if specific elements are present in the set. If the element is present in the set, the .countOf() method returns True, otherwise False.

import operator as op

main_set = {19, 21, "Kaynes", -10, True}

print(op.countOf(main_set, 21) > 0)
# Output: True

print(op.countOf(main_set, 46) > 0)
# Output: False

That’s all!