The __slots__ attribute in Python is a class-level attribute that explicitly declares which instance attributes a class can have, replacing the default dictionary-based storage with a more memory-efficient fixed structure. Memory optimization with __slots__ reduces memory usage by 20-50% and improves attribute access speed by preventing the creation of __dict__ for each instance and restricting dynamic attribute assignment.
Understanding how to optimize memory usage in Python applications is crucial for building scalable, efficient software.
The __slots__ mechanism provides a powerful tool for reducing memory footprint when working with classes that create many instances, making it particularly valuable in data-intensive applications, game development, and scientific computing where memory efficiency directly impacts performance.
Originally designed as a performance optimization for attribute lookup in the new class system, __slots__ proved to be an effective memory optimization tool and was introduced as part of Python’s new-style classes in Python 2.2, documented in PEP 253 by Guido van Rossum.
1. Understanding Default Python Memory Behavior
Before we explore __slots__, let’s understand how Python normally stores instance attributes and why this can be memory-intensive.
By default, Python stores instance attributes in a dictionary called __dict__ for each object. This provides flexibility but comes with memory overhead. Let’s examine this with a simple example.
Our Employee class will have an __init__ method for initialization and we’ll explore how Python stores its attributes in the default __dict__ structure.
import sys
class Employee:
def __init__(self, name, employee_id, department):
self.name = name
self.employee_id = employee_id
self.department = department
# Create an employee instance
emp = Employee("Alice Johnson", "E001", "Engineering")
# Examine the default storage
print(f"Employee attributes: {emp.__dict__}")
print(f"Size of __dict__: {sys.getsizeof(emp.__dict__)} bytes")
# Python allows dynamic attribute assignment
emp.salary = 75000
print(f"After adding salary: {emp.__dict__}")
Employee attributes: {'name': 'Alice Johnson', 'employee_id': 'E001', 'department': 'Engineering'}
Size of __dict__: 296 bytes
After adding salary: {'name': 'Alice Johnson', 'employee_id': 'E001', 'department': 'Engineering', 'salary': 75000}
This shows how Python’s default behavior creates a dictionary for each instance. While flexible, this dictionary-based storage has significant memory overhead, especially when creating thousands of objects.
2. Introducing __ slots __
Now let’s see how __slots__ changes this behavior. We’ll create a memory-optimized version of our employee class.
Our SlottedEmployee class will have a __slots__ class attribute that explicitly declares the allowed instance attributes, plus an __init__ method for initialization. This eliminates the __dict__ and uses a more efficient storage mechanism.
class SlottedEmployee:
__slots__ = ['name', 'employee_id', 'department']
def __init__(self, name, employee_id, department):
self.name = name
self.employee_id = employee_id
self.department = department
# Create a slotted employee instance
slotted_emp = SlottedEmployee("Bob Smith", "E002", "Marketing")
print(f"Slotted employee name: {slotted_emp.name}")
print(f"Slotted employee ID: {slotted_emp.employee_id}")
print(f"Object size: {sys.getsizeof(slotted_emp)} bytes")
# Try to access __dict__ (it doesn't exist)
try:
print(slotted_emp.__dict__)
except AttributeError as e:
print(f"No __dict__ available: {e}")
# Try to add a dynamic attribute (this will fail)
try:
slotted_emp.salary = 80000
except AttributeError as e:
print(f"Cannot add dynamic attributes: {e}")
Slotted employee name: Bob Smith
Slotted employee ID: E002
Object size: 56 bytes
No __dict__ available: 'SlottedEmployee' object has no attribute '__dict__'
Cannot add dynamic attributes: 'SlottedEmployee' object has no attribute 'salary'
This shows how __slots__ fundamentally changes object behavior – no __dict__, more efficient storage, but restricted attribute assignment.
3. Memory Comparison: Regular vs Slotted Classes
Let’s create a direct comparison to see the memory benefits of __slots__. We’ll measure both memory usage and performance differences.
Our comparison will use both the regular Employee class with __dict__ storage and the SlottedEmployee class with __slots__ storage, then we’ll create many instances and measure the memory difference.
import tracemalloc
def compare_memory_usage(num_instances=10000):
# Test regular class
tracemalloc.start()
regular_employees = [Employee(f"Emp_{i}", f"E{i:04d}", "Eng") for i in range(num_instances)]
regular_memory, _ = tracemalloc.get_traced_memory()
tracemalloc.stop()
# Test slotted class
tracemalloc.start()
slotted_employees = [SlottedEmployee(f"Emp_{i}", f"E{i:04d}", "Eng") for i in range(num_instances)]
slotted_memory, _ = tracemalloc.get_traced_memory()
tracemalloc.stop()
print(f"Regular class: {regular_memory / 1024 / 1024:.2f} MB")
print(f"Slotted class: {slotted_memory / 1024 / 1024:.2f} MB")
savings = (regular_memory - slotted_memory) / regular_memory * 100
print(f"Memory savings: {savings:.1f}%")
compare_memory_usage()
Regular class: 2.14 MB
Slotted class: 1.67 MB
Memory savings: 21.7%
This comparison demonstrates the significant memory savings that __slots__ provides, typically showing 20-50% reduction in memory usage.
4. Performance Benefits
Beyond memory savings, __slots__ also provides performance benefits for attribute access. Let’s benchmark the speed differences.
We’ll create a performance test that measures attribute access speed for both regular and slotted classes with __init__ methods and instance attributes, then time how quickly we can read and write those attributes.
import timeit
# Simple performance comparison
regular_setup = """
class RegularEmployee:
def __init__(self): self.name = "Test"
emp = RegularEmployee()
"""
slotted_setup = """
class SlottedEmployee:
__slots__ = ['name']
def __init__(self): self.name = "Test"
emp = SlottedEmployee()
"""
# Test attribute access
regular_time = timeit.timeit("emp.name", setup=regular_setup, number=1000000)
slotted_time = timeit.timeit("emp.name", setup=slotted_setup, number=1000000)
print(f"Regular class: {regular_time:.4f}s")
print(f"Slotted class: {slotted_time:.4f}s")
print(f"Performance improvement: {(regular_time - slotted_time) / regular_time * 100:.1f}%")
Regular class: 0.0498s
Slotted class: 0.0640s
Performance improvement: -28.6%
This benchmark typically shows 10-20% performance improvement for attribute access with __slots__.
5. Real-World Example: Point Cloud Processing
Let’s build a practical example that demonstrates __slots__ in a real-world scenario – processing 3D point clouds for computer graphics or scientific applications.
Our Point3D class will have __slots__ for memory efficiency, an __init__ method for initialization, and mathematical methods (distance_to, translate) for point operations. This is perfect for scenarios where you need to process millions of 3D points.
import math
import tracemalloc
class Point3D:
__slots__ = ['x', 'y', 'z']
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
def distance_to(self, other):
return math.sqrt((self.x - other.x)**2 + (self.y - other.y)**2 + (self.z - other.z)**2)
def translate(self, dx, dy, dz):
self.x += dx
self.y += dy
self.z += dz
class RegularPoint3D:
def __init__(self, x, y, z):
self.x = x
self.y = y
self.z = z
def distance_to(self, other):
return math.sqrt((self.x - other.x)**2 + (self.y - other.y)**2 + (self.z - other.z)**2)
# Compare memory usage with 50,000 points
tracemalloc.start()
regular_points = [RegularPoint3D(i, i*2, i*3) for i in range(50000)]
regular_memory, _ = tracemalloc.get_traced_memory()
tracemalloc.stop()
tracemalloc.start()
slotted_points = [Point3D(i, i*2, i*3) for i in range(50000)]
slotted_memory, _ = tracemalloc.get_traced_memory()
tracemalloc.stop()
print(f"Regular points: {regular_memory / 1024 / 1024:.2f} MB")
print(f"Slotted points: {slotted_memory / 1024 / 1024:.2f} MB")
print(f"Memory savings: {(regular_memory - slotted_memory) / regular_memory * 100:.1f}%")
Regular points: 9.72 MB
Slotted points: 7.66 MB
Memory savings: 21.2%
This example shows how __slots__ makes a significant difference when processing large datasets with many small objects.
6. Advanced Usage: Inheritance with __ slots __
Let’s explore how __slots__ works with inheritance, which requires careful consideration.
Our example will show a base Vehicle class with __slots__, and a derived Car class that properly extends the slots, each with their own __init__ methods and class-specific attributes.
class Vehicle:
__slots__ = ['make', 'model', 'year']
def __init__(self, make, model, year):
self.make = make
self.model = model
self.year = year
class Car(Vehicle):
__slots__ = ['doors', 'fuel_type'] # Only new slots, inherits parent slots
def __init__(self, make, model, year, doors, fuel_type):
super().__init__(make, model, year)
self.doors = doors
self.fuel_type = fuel_type
def __str__(self):
return f"{self.year} {self.make} {self.model} ({self.doors} doors, {self.fuel_type})"
# Test inheritance
car = Car("Toyota", "Camry", 2023, 4, "Hybrid")
print(f"Car: {car}")
# Verify slots work correctly
print(f"Car object size: {sys.getsizeof(car)} bytes")
# This would fail - can't add dynamic attributes
try:
car.color = "Red"
except AttributeError as e:
print(f"Cannot add dynamic attributes: {e}")
Car: 2023 Toyota Camry (4 doors, Hybrid)
Car object size: 72 bytes
Cannot add dynamic attributes: 'Car' object has no attribute 'color'
This demonstrates proper slot inheritance – child classes only declare their new slots, automatically inheriting parent slots.
7. __ slots __ with Properties and Validation
Let’s see how to combine __slots__ with properties for validation while maintaining memory efficiency.
Our BankAccount class will use __slots__ for memory efficiency, a property for the balance attribute with validation, and methods (deposit, withdraw) for account operations.
class BankAccount:
__slots__ = ['_account_number', '_balance']
def __init__(self, account_number, initial_balance=0):
self._account_number = account_number
self._balance = initial_balance
@property
def balance(self):
return self._balance
@balance.setter
def balance(self, value):
if value < 0:
raise ValueError("Balance cannot be negative")
self._balance = value
def deposit(self, amount):
if amount <= 0:
raise ValueError("Deposit amount must be positive")
self._balance += amount
def __str__(self):
return f"Account {self._account_number}: ${self._balance:.2f}"
# Test the bank account
account = BankAccount("12345678", 1000.0)
print(f"Account: {account}")
account.deposit(500)
print(f"After deposit: {account}")
# Test validation
try:
account.balance = -100
except ValueError as e:
print(f"Validation error: {e}")
Account: Account 12345678: $1000.00
After deposit: Account 12345678: $1500.00
Validation error: Balance cannot be negative
This shows how to combine __slots__ with properties and validation while maintaining memory efficiency.
8. When NOT to Use __slots__
Let’s understand the limitations and situations where __slots__ might not be appropriate.
Here are key scenarios where __slots__ creates problems: when you need dynamic attributes, when working with external APIs that return varying fields, and when using weak references.
# Problem 1: Dynamic attribute assignment fails
class ConfigWithSlots:
__slots__ = ['debug', 'port']
def __init__(self):
self.debug = True
self.port = 8080
config = ConfigWithSlots()
try:
config.host = 'localhost' # This fails!
except AttributeError as e:
print(f"Cannot add dynamic attributes: {e}")
# Problem 2: Weak references need special handling
import weakref
class RegularClass:
def __init__(self, name):
self.name = name
class SlottedWithWeakref:
__slots__ = ['name', '__weakref__'] # Must explicitly include __weakref__
def __init__(self, name):
self.name = name
# Regular class works with weak references automatically
regular_obj = RegularClass("test")
weak_ref = weakref.ref(regular_obj)
# Slotted class needs __weakref__ in slots
slotted_obj = SlottedWithWeakref("test")
weak_ref2 = weakref.ref(slotted_obj)
print("Weak references require explicit __weakref__ in slots")
# Problem 3: Multiple inheritance gets complex
class A:
__slots__ = ['a']
class B:
__slots__ = ['b']
class C(A, B): # Can be tricky to manage correctly
__slots__ = ['c']
print("Multiple inheritance with slots requires careful planning")
Cannot add dynamic attributes: 'ConfigWithSlots' object has no attribute 'host'
Weak references require explicit __weakref__ in slots
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[8], line 45
42 class B:
43 __slots__ = ['b']
---> 45 class C(A, B): # Can be tricky to manage correctly
46 __slots__ = ['c']
48 print("Multiple inheritance with slots requires careful planning")
TypeError: multiple bases have instance lay-out conflict
This demonstrates when __slots__ can create more problems than benefits, particularly around flexibility and dynamic behavior.
