Membership Operators (`in` / `not in`)

Membership operators check whether a value exists inside an iterable.

An iterable is a collection that can hold multiple values, such as a string, list, tuple, set, or dictionary.

What you'll learn

• Use in and not in with strings, lists, dictionaries, and sets
• Understand that strings search for substrings
• Learn that dictionaries check keys by default
• Avoid the common nested-list misconception

Membership operators

Operator	Meaning
in	Returns True if the value exists
not in	Returns True if the value does not exist

value in iterable
value not in iterable

Important: The container must be an iterable.

Iterable	Non-iterable
str	int
list	float
tuple	bool
set	complex
dict	—

Strings: search for substrings

Strings look for a continuous sequence of characters.

s = "PYTHON"

print("P" in s)
print("PYT" in s)
print("HON" in s)
print("PTO" in s)

True
True
True
False

"PTO" fails because those letters are not consecutive in "PYTHON".

Strings with slicing

Membership also works on slices.

s = "PYTHON"

print("PTO" in s[::2])
print("NOH" in s[::-1])

True
True

Why?

• s[::2] → "PTO"
• s[::-1] → "NOHTYP"

Lists

Lists search for complete elements, not partial text.

lst = [10, "Rossum", 34.56, 2 + 3j]

print(34.56 in lst)
print("Ross" in lst)
print("Ross" in lst[1])

True
False
True

"Ross" is not a list element, but it is a substring of "Rossum".

Dictionaries

By default, membership checks keys, not values.

d = {
    10: "Apple",
    20: "Mango",
    30: "Kiwi",
}

print(10 in d)
print("Apple" in d)

True
False

To search values, use .values().

print("Apple" in d.values())

True

Sets

Sets search for individual elements.

vowels = {"a", "e", "i", "o", "u"}

print("a" in vowels)
print("x" in vowels)

True
False

A set does not treat another set as one of its elements unless that set is actually stored inside it.

The nested list trap

Many beginners expect this to be True:

[10, 20, 30] in [10, 20, 30, 40, 50]

Actual result: False

Why? Python searches for one element. The outer list contains 10, 20, 30, 40, and 50 — not the list [10, 20, 30] as a single element.

Summary table

Container	in checks
String	Consecutive substring
List	Whole element
Tuple	Whole element
Set	Whole element
Dictionary	Keys
dict.values()	Values
dict.items()	Key–value pairs

Practice exercises

Exercise 1 — Predict the result:

"HON" in "PYTHON"

Answer: True

Exercise 2 — Predict:

"Apple" in {10: "Apple"}

Why is the answer not True? (Keys are tested — 10 is a key, "Apple" is a value.)

Exercise 3 — Explain why this returns False:

[1, 2] in [1, 2, 3]

The inner [1, 2] is not a single element of the outer list.

Quick knowledge check

1. What does in return when the value is missing?
   False

2. Does in on a dictionary check values?
   No. It checks keys by default.

3. Is "PTO" in "PYTHON" True?
   No. The letters are not consecutive in that order.

Summary

• in and not in work with iterables.
• Strings search for consecutive substrings.
• Lists, tuples, and sets search for whole elements.
• Dictionaries check keys unless you use .values() or .items().
• A smaller list or set is not automatically inside a larger one.

Next lesson: Identity Operators (is / is not) — value equality (==) vs object identity (is).