An unhashable object cannot serve as a dictionary key or set element. Using one in either position raises TypeError because dictionaries and sets rely on stable hash values for retrieval. section 3.5 section 3.5.2
Listing 3.6 Failed creation of dict and set objects
failed_dict = {[0, 2]: "even"}
# ERROR: TypeError: unhashable type: 'list'
failed_set = {{"a": 0}}
# ERROR: TypeError: unhashable type: 'dict'
These failed constructions show the restriction directly: lists cannot be dictionary keys, and dictionaries cannot be set elements. section 3.5
Hashability depends on stable hashing. A mutable object could change its contents after insertion, creating a conflict between its old lookup position and its new value. Dictionaries and sets therefore accept common immutable types such as int, float, str, tuple, bool, and None, but reject dict, list, and set. section 3.5.2
Checking hashability
from collections.abc import Hashable items = [{"a": 1}, [1], {1}, 1, "test", (1, 2), None] for item in items: print(type(item).__name__, isinstance(item, Hashable)) # dict False # list False # set False # int True # str True # tuple True # NoneType True Use isinstance(value, Hashable) when code needs to validate a potential dictionary key or set element before insertion. section 3.5.2
When choosing between containers:
- Use lists and tuples when flexible element storage is the main need. section 3.5
- Use dictionaries for key-value retrieval and sets for membership-oriented collections, using only hashable keys or elements. section 3.5
- Remember that immutability commonly accompanies hashability, but the practical test is whether the object is accepted as hashable. section 3.5.2
