Why doesn't Python hash lists using ID?

Question

When using a dictionary in Python, the following is impossible:

d = {}
d[[1,2,3]] = 4

since 'list' is an unhashable type. However, the id function in Python returns an integer for an object that is guaranteed to be unique for the object's lifetime.

Why doesn't Python use id to hash a dictionary? Are there drawbacks?

Answer 1

The reason is right here (Why must dictionary keys be immutable)

Some unacceptable solutions that have been proposed:

• Hash lists by their address (object ID). This doesn’t work because if you construct a new list with the same value it won’t be found; e.g.:

  mydict = {[1, 2]: '12'}

  print mydict[[1, 2]]

would raise a KeyError exception because the id of the [1, 2] used in the second line differs from that in the first line. In other words, dictionary keys should be compared using ==, not using is.