TIL you can hash None in Python

Recently I saw some Python code which looked something like:

d = {
  None: "value",
  "another_key": "another_value",

It had never really occurred to me you could use None (null) as a key to a dictionary before. What this also meant is that None must be hashable as only hashable objects can be keys in a dictionary. These objects include strings, tuples, sets but don’t include lists as they are mutable and not hashable.

Lists as keys?

Using a list as a key won’t provide a consistent hash therefore you wouldn’t be able to find the value associated with that list. Again this is because lists are mutable so their hash would change.

In [1]: a = ["a", 1, 2, 3]

In [2]: hash(a)
TypeError                     Traceback (most recent call last)
Cell In [2], line 1
----> 1 hash(a)

TypeError: unhashable type: 'list'

In fact for this very reason you cannot even hash them using the builtin hash function in python.


If we do try to use the built-in hash function on None you will notice we do get a hash (int) as expected.

In [1]: hash(None)
Out[1]: 8783956518372

So yeah you can use None as a key in a dictionary.

JSON Caveat

One thing to note is when transforming a dictionary into JSON. The None will become a string. So when you convert it back from JSON to a python dictionary it will be something like:

d = {
  "None": "value",
  # ...