An unhashable object cannot serve as a dictionary key or set element because its value cannot provide a stable hash. Mutable built-in types such as lists are unhashable: changing the object could change its hash, leaving a stored key impossible to find. section 4.1
A list cannot be a dictionary key
```python key = ["red", "green"] # A list is mutable, so it cannot safely be used as a key. data = {key: "colors"} ``` A dictionary must be able to find a key after insertion. If the list changed, its hash could change too, so Python prevents this use rather than allowing an unreachable key-value pair. section 4.1
Keep these distinctions in mind:
- Hashable and immutable are not identical terms, although they overlap substantially. section 4.1
- Hashing supports fast dictionary-key lookup and key uniqueness; it does not provide value lookup. section 4.1
