Computer Science Why?

Why Can't a List Be a Dictionary Key?

Why can't you use a Python list as a dictionary key?

Lesson notes

Key points

Because a dictionary stores each key in the bucket chosen by the key's hash at insertion time.
Because a list is mutable, appending to it after insertion would change its hash value.
Because a changed hash sends future lookups to a different bucket, the stored entry would become unreachable.

Definitions

Hashable: An object whose hash never changes in its lifetime and that can be compared for equality.

Takeaway: So keys must be hashable: use immutable types like tuples or strings.

Quick check: What property must a dictionary key's hash have over time?

It must grow
It must stay constant
It must be prime

A changing hash would send lookups to the wrong bucket.

Hash picks bucketMutabilityLost entry

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