Eq () and hash () in Python: Difference between revisions
Jump to navigation
Jump to search
| Line 18: | Line 18: | ||
2. Store (hash_value, key, value) in the bucket at the location hash_value % len(buckets) | 2. Store (hash_value, key, value) in the bucket at the location hash_value % len(buckets) | ||
3. If the bucket array needs resizing, | 3. If the bucket array needs resizing, **re-use the previously computed hash value** to re-insert all previously stored values. This is why is important that the key is immutable: if the key is mutable and it changes while the key/value pair is stored in the dictionary, lookup and resizing will not work. | ||
Revision as of 16:17, 11 September 2022
Internal
Overview
__eq__()
__hash__()
Dictionaries and the __hash__() Function
To store a key into a dictionary, Python performs the following sequence:
1. Call __hash__() on the key and compute the hash of the key. If the key is not hashable, raise a TypeError.
2. Store (hash_value, key, value) in the bucket at the location hash_value % len(buckets)
3. If the bucket array needs resizing, **re-use the previously computed hash value** to re-insert all previously stored values. This is why is important that the key is immutable: if the key is mutable and it changes while the key/value pair is stored in the dictionary, lookup and resizing will not work.