Eq () and hash () in Python: Difference between revisions

From NovaOrdis Knowledge Base
Jump to navigation Jump to search
Line 12: Line 12:
===Dictionaries and the <tt>__hash__()</tt> Function===
===Dictionaries and the <tt>__hash__()</tt> Function===


To store a key into a dictionary, Python performs the following sequence:
To store a key into a [[Python_Language_Dictionary#Dictionaries_and_the_hash_.28.29_Function|dictionary]], Python performs the following sequence:


1. Call <code>__hash__()</code> on the key and compute the hash of the key. If the key is not hashable, raise a <code>TypeError</code>.
1. Call <code>__hash__()</code> on the key and compute the hash of the key. If the key is not hashable, raise a <code>TypeError</code>.

Revision as of 16:16, 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.