Apprendre __slots__ vs __dict__ | Writing Memory-Efficient Code

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Every Python object stores its instance attributes in a dictionary called __dict__. Dictionaries are flexible but carry significant memory overhead. For classes with a fixed set of attributes – especially when thousands of instances are created – __slots__ replaces __dict__ with a compact fixed-size structure.

The Default: `dict`

By default, every instance of a class gets its own __dict__ that can hold any attribute:


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import sys

# Default class using __dict__ for attribute storage
class Transaction:
    def __init__(self, transaction_id, amount, currency):
        self.transaction_id = transaction_id
        self.amount = amount
        self.currency = currency

transaction = Transaction("TX001", 4500.0, "USD")

print(transaction.__dict__)           # {'transaction_id': 'TX001', 'amount': 4500.0, 'currency': 'USD'}
print(sys.getsizeof(transaction))     # Object size – does not include __dict__ overhead
print(sys.getsizeof(transaction.__dict__))  # Dict overhead – typically 184+ bytes

The dict overhead is constant per instance regardless of how many attributes are stored.

Using `slots`

Declaring __slots__ tells Python to allocate only the listed attributes – no __dict__ is created:


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import sys

# Class using __slots__ for compact attribute storage
class Transaction:
    __slots__ = ("transaction_id", "amount", "currency")

    def __init__(self, transaction_id, amount, currency):
        self.transaction_id = transaction_id
        self.amount = amount
        self.currency = currency

transaction = Transaction("TX001", 4500.0, "USD")

print(sys.getsizeof(transaction))     # Smaller – no __dict__ overhead
print(hasattr(transaction, "__dict__"))  # False

Measuring the Difference

The savings become significant when many instances are created:


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import sys
import tracemalloc

# Comparing memory usage between __dict__ and __slots__ at scale
class RecordWithDict:
    def __init__(self, record_id, value):
        self.record_id = record_id
        self.value = value

class RecordWithSlots:
    __slots__ = ("record_id", "value")

    def __init__(self, record_id, value):
        self.record_id = record_id
        self.value = value

tracemalloc.start()

records_dict = [RecordWithDict(record_id, record_id * 1.5) for record_id in range(100000)]
snapshot_dict = tracemalloc.take_snapshot()

del records_dict
tracemalloc.clear_traces()

records_slots = [RecordWithSlots(record_id, record_id * 1.5) for record_id in range(100000)]
snapshot_slots = tracemalloc.take_snapshot()

dict_total = sum(s.size for s in snapshot_dict.statistics("lineno"))
slots_total = sum(s.size for s in snapshot_slots.statistics("lineno"))

print(f"With __dict__:  {dict_total / 1024:.1f} KB")
print(f"With __slots__: {slots_total / 1024:.1f} KB")

tracemalloc.stop()

Limitations of `slots`

__slots__ comes with trade-offs:

You cannot add attributes not listed in __slots__ at runtime;
Inheritance requires care — subclasses that don't define __slots__ get __dict__ back;
Pickling and some dynamic patterns require additional work.

Use __slots__ when: