Python: why can I put mutable object in a dict or set?

by

Python doesn’t test for mutable objects, it tests for hashable objects.

Custom class instances are by default hashable. That’s fine because the default __eq__ implementation for such classes only tests for instance identity and the hash is based of the same information.

In other words, it doesn’t matter that you alter the state of your instance attributes, because the identity of an instance is immutable anyway.

As soon as you implement a __hash__ and __eq__ method that take instance state into account you might be in trouble and should stop mutating that state. Only then would a custom class instance no longer be suitable for storing in a dictionary or set.

More Related Contents:

  1. Hashable, immutable
  2. What would a “frozen dict” be?
  3. hash function in Python 3.3 returns different results between sessions
  4. Why do -1 and -2 both hash to -2 in CPython? [duplicate]
  5. Python dictionary : TypeError: unhashable type: ‘list’
  6. Hashing a file in Python
  7. Types that define `__eq__` are unhashable?
  8. Why are python strings and tuples are made immutable?
  9. How to replace Django’s primary key with a different integer that is unique for that table
  10. Python string with space and without space at the end and immutability
  11. What does hash do in python?
  12. Immutable numpy array?
  13. TypeError: unhashable type: ‘dict’, when dict used as a key for another dict [duplicate]
  14. How to sum all the values in a dictionary?
  15. How to implement a good __hash__ function in python [duplicate]
  16. Why are Python strings immutable? Best practices for using them
  17. What is time complexity of a list to set conversion?
  18. How to hash a string into 8 digits?
  19. Built in Python hash() function
  20. Reversible hash function?
  21. Why are integers immutable in Python?
  22. Does Python have an immutable list?
  23. Python, subclassing immutable types
  24. Python consistent hash replacement
  25. Pandas: Modify a particular level of Multiindex
  26. Unexpected Behavior of Extend with a list in Python [duplicate]
  27. How to hash a large object (dataset) in Python?
  28. python tuple is immutable – so why can I add elements to it
  29. Mutable and Immutable Strings in python
  30. Why must dictionary keys be immutable?

Leave a Comment