slots and tuple
- Python course
- Expert
- Lesson 40/47
- 5 min
Learn memory savings with __slots__ and tuple.
What is __slots__?
__slots__ prevents instances of a class from creating a __dict__. Instead only the specified fields get space; this saves memory.
Normally each instance carries a __dict__ dict; this allows dynamic attribute addition but costs memory. Defining __slots__ = ['x', 'y'] makes instances carry only these fields; no __dict__. When thousands of instances are created the saving matters. But dynamic attributes cannot be added; this is a trade-off.
Measuring memory savings
You check whether an instance has __dict__ with hasattr(obj, '__dict__'). In a class with __slots__ this returns False.
In a normal class it returns True. This is the simplest way to confirm slots is working. To measure the saving you use sys.getsizeof; but __dict__ itself is a separate object and getsizeof does not count it. To see the full saving you must add the __dict__ size too.
Practice Compare __dict__ with slots
Write a WithSlots class with __slots__ = ['x', 'y']. Write a WithoutSlots class (no slots). Create an instance of each; set x=1, y=2 on both. Print hasattr(a, '__dict__') and hasattr(b, '__dict__') on separate lines.
Starter code
# write two classes: one with slots
# create instances and check __dict__Sample solution
class WithSlots:
__slots__ = ["x", "y"]
class WithoutSlots:
pass
a = WithSlots()
a.x = 1
a.y = 2
b = WithoutSlots()
b.x = 1
b.y = 2
print(hasattr(a, "__dict__"))
print(hasattr(b, "__dict__"))Expected output
False True
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match slots concepts
| Term | Meaning |
|---|---|
__slots__ |
Field list |
__dict__ |
Dynamic dict |
memory |
Saves memory |
trade-off |
Less flexibility |
2 What does __slots__ prevent?
class Point:
__slots__ = ['x', 'y']- Creating __dict__ Correct answer
- Inheritance
- Method definitions
- Calling __init__
Explanation: __slots__ prevents instances from carrying __dict__; it saves memory.
3 Complete the __slots__ definition
class Point:
____ = ['x', 'y']class Point:
__slots__ = ['x', 'y']Explanation: __slots__ = ['x', 'y'] specifies the fields.