5.5Immutability

Functional programming favors immutable data — data that is not modified after creation. This prevents subtle bugs where shared test data is accidentally modified.

# BAD: mutable default — all tests share the same list
def create_order(items=[]):  # list is created once and shared!
    items.append("new_item")
    return items

# GOOD: immutable approach
def create_order(items=None):
    items = list(items or [])  # create a new list each time
    items.append("new_item")
    return items

# BEST: use frozen dataclass for test data
from dataclasses import dataclass

@dataclass(frozen=True)
class TestUser:
    email: str
    password: str
    role: str = "viewer"

admin = TestUser("admin@test.com", "pass123", "admin")
# admin.role = "viewer"  # This raises FrozenInstanceError