What Makes an Object Hashable in Python?

Understanding Hashability in Python

In Python programming, we often encounter the concept of hashable objects. Understanding what it means for an object to be hashable is crucial for working effectively with data structures like dictionaries and sets.

Defining Hashability

According to Python's glossary, an object is considered hashable if it satisfies the following conditions:

• It has an immutable hash value that remains constant throughout its lifetime. To provide this functionality, the object must implement the __hash__() method.
• It can be compared to other objects using the __eq__() or __cmp__() methods.

Significance of Hashability

Hashability plays a vital role in the implementation of data structures in Python:

• Dictionary Keys: Dictionaries use hash values to efficiently search for keys. Hashable objects serve as valid dictionary keys because their hash values remain constant, making it easy to locate their associated values.
• Set Members: Sets also rely on hash values to store unique elements. Hashable objects can be used as set members since their uniqueness can be guaranteed through their consistent hash values.

Hashability in Built-in Types

All immutable built-in objects in Python are hashable, including:

• Numbers (integers, floats, complex numbers)
• Strings
• Tuples
• Frozen sets

In contrast, mutable containers like lists and dictionaries are not hashable because their hash values can change as their contents change.

Custom Class Hashability

Objects instantiated from user-defined classes are hashable by default. However, they behave differently than built-in types:

• They all compare unequal by default.
• Their hash values are calculated based on their object IDs using the id() function.

The above is the detailed content of What Makes an Object Hashable in Python?. For more information, please follow other related articles on the PHP Chinese website!