Python in and not in Membership Operators
# Basic membership check using 'in' and 'not in'
my_list = [3, 4, 5, 8, 3]
print(5 in my_list) # True
print(9 not in my_list) # Truex in my_iterable and x not in my_iterable Membership operators are used to test whether a value or variable is found within a sequence or iterable (such as lists, tuples, strings, or dictionaries).
The in operator returns True if the specified element is present in the sequence, and False otherwise. Conversely, the not in operator returns True if the element is not present. Both in and not in are reserved keywords in Python. For object identity comparisons, see is and not is, or check iterator states with Iterator and any().
| Operator | Description | Example |
|---|---|---|
in | Returns True if value is found in the sequence | 5 in [3, 4, 5] → True |
not in | Returns True if value is NOT found in the sequence | 9 not in [3, 4, 5] → True |
1. Syntax & Formal Parameters 🔝
result = x in my_iterable
result = x not in my_iterableParameters: x is the target element to search for; my_iterable is any iterable data structure (list, tuple, string, set, dictionary keys, or generator).
2. Using Lists with Membership Operators 🔝
Checking presence of elements inside a list:
my_list = [3, 4, 5, 8, 3]
if 5 in my_list:
print("Yes it is Present")
else:
print("NO it is NOT Present")
# Using not in
if 9 not in my_list:
print("Yes it is NOT Present")Expected output
Yes it is Present
Yes it is NOT Present3. Checking Substrings in Strings 🔝
Membership operators work seamlessly with strings to check for substrings or individual characters:
url = 'plus2net.com'
if '2' in url:
print("yes 2 is available inside")
else:
print("No 2 is not available inside")
assert ('net' in url) == TrueExpected output
yes 2 is available inside4. Using Dictionaries (Dictionary Keys) 🔝
When used with a dictionary, in checks against the keys, not the values:
my_dict = {0: 'A', 1: 'B'}
print(1 in my_dict) # True (key 1 exists)
print('A' in my_dict) # False ('A' is a value, not a key)
my_dict2 = {'A': 'X', 'B': 'Y', 'C': 'Z'}
print('C' in my_dict2) # True
print('Y' not in my_dict2) # True (Y is a value)5. Using Tuples (Tuple Iterables) 🔝
my_tuple = (6, 1, 12, 7)
print(7 in my_tuple) # True
print(8 not in my_tuple) # True
assert (1 in my_tuple) == TrueFrequently Asked Questions (FAQ) 🔝
Does `in` check dictionary keys or dictionary values?
By default, the in operator checks only against dictionary keys. To check if a value exists, use value in my_dict.values().
What is the time complexity of the `in` operator in Python?
For lists, tuples, and strings, the time complexity is O(n) because Python performs a linear search. For sets and dictionaries (when checking keys), the time complexity is O(1) on average due to hash table lookups.
Can `not in` be combined with `if` statements and loops?
Yes! not in is frequently used in conditional branches (if x not in my_list:) and filter comprehensions to ensure uniqueness or exclude unwanted items.
Further Reading & Related Tutorials 🔝
- All Python Operators — Overview of arithmetic, logical, comparison, and membership operators in Python.
- is and not is Identity Operators — Learn object identity checks vs. equality and membership.
- Python List Methods — Managing sequences, appending, and searching elements.
- Python Dictionaries — Working with key-value mappings and hash lookups.
- Python Tuples — Immutable sequence iterables and packing/unpacking.
- Python Iterators — Traversing custom iterable objects efficiently.
- Python any() Function — Evaluating truthiness across iterable elements.
Practice in Google Colab 🔝
Test membership checks across lists, strings, dictionaries, and tuples in our interactive Google Colab companion notebook:
Interactive Practice: Colab & GitHub
Launch the notebook directly in Google Colab to run live code cells or inspect the source repository on GitHub:
View on GitHub
Subhendu Mohapatra
Author & Instructor at plus2net
I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.
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