Immutable vs Mutable Objects in Python 2026 – Best Practices for Writing Functions
Understanding the difference between immutable and mutable objects is fundamental to writing correct, predictable, and efficient Python functions. In 2026, this knowledge directly impacts code safety, performance, and debugging experience.
TL;DR — Key Takeaways 2026
- Immutable: int, float, str, tuple, frozenset, bytes — cannot be changed after creation
- Mutable: list, dict, set, custom classes — can be modified in place
- Immutable objects are safe to pass to functions
- Mutable objects can cause unexpected side effects if modified inside functions
- Prefer immutable data structures when possible for safer code
1. Immutable vs Mutable Behavior
# Immutable - behaves like "pass by value"
def increment(n: int) -> int:
n += 1 # Creates a new integer object
return n
x = 10
increment(x)
print(x) # Still 10
# Mutable - can be modified in place
def add_item(items: list) -> None:
items.append("new") # Modifies the original list!
my_list = ["a", "b"]
add_item(my_list)
print(my_list) # ['a', 'b', 'new'] ← Changed!
2. Best Practices in 2026
# Safe approach with mutable objects
def process_items(items: list) -> list:
# Work on a copy to avoid side effects
items_copy = items.copy()
items_copy.append("processed")
return items_copy
# Better: Use immutable data when possible
from typing import Tuple
def process_tuple(items: Tuple) -> Tuple:
return items + ("processed",)
3. When to Choose Immutable vs Mutable
- Use immutable (tuple, frozenset, NamedTuple) for data that should not change
- Use mutable (list, dict, set) when you need to modify the collection
- Pass copies (
.copy()ordeepcopy()) when you must modify mutable objects - Document side effects clearly in function docstrings
Conclusion
Understanding immutable vs mutable objects is critical for writing safe and predictable functions. In 2026, the best practice is to prefer immutable data structures whenever possible and to be explicit about mutation when using mutable objects.
Next steps:
- Review your functions and make mutation behavior explicit
- Related articles: Writing Functions in Python 2026 • Pass by Assignment in Python 2026