A grid maps fixed coordinate pairs to stored values.
What You Need to Understand
A dictionary key must be hashable so its hash value can be used in lookup. Strings, integers and immutable tuples of hashable contents work; lists and mutable dictionaries do not.
When to Use It
A grid maps fixed coordinate pairs to stored values.
Step-by-Step Python Example
cells = {(0, 0): "empty", (0, 1): "wall"}
print(cells[(0, 1)])
print(hash((0, 1)))How the Example Works
A tuple of integers can act as one coordinate key. Its hash can be calculated because its members are hashable.
Example Output
wall
-1950498447580522560
teamA Second Example
Compare this smaller variation with the main example, then experiment with your own values.
groups = {frozenset({"a", "b"}): "team"}
print(groups[frozenset({"b", "a"})])Important Behavior and Edge Cases
Hashability is not identical to immutability of every nested object, and equal keys must have equal hashes. (1, 2) works, but ([1], 2) cannot be hashed.
Try It Yourself
Try using an immutable frozenset as a dictionary key.
Common Beginner Mistakes
- Hashability is not identical to immutability of every nested object, and equal keys must have equal hashes.
(1, 2)works, but([1], 2)cannot be hashed. - Treating missing dictionary keys, replacement precedence, and shallow copies as if they had identical behavior in every context.
Key Takeaways
- A dictionary key must be hashable so its hash value can be used in lookup.
- A tuple of integers can act as one coordinate key.
- Hashability is not identical to immutability of every nested object, and equal keys must have equal hashes.