Python type hinting: when to use MutableSequence vs List

Question

I have been reading about typing.Sequence and typing.MutableSequence. There isn't a ton of "light reading" out there on either of the two, everything goes straight into details.

From this answer to Can you specify variance in a Python type annotation?

Sequence is the read-only version of List

So that leads me to wonder, what is the difference between MutableSequence, and just a plain List?

More Details

The best source I could find was the The standard type hierarchy section of the Python Data model.

From reading the section Mutable sequences, it seems like MutableSequence might be a "parent" of List?

In other words, one can use them interchangeably, just MutableSequence is a bit less restrictive?

Sequence is not a read-only version of List. The answer you read was wrong about that. — user2357112
– user2357112, Commented Jun 6, 2020 at 23:23
@user2357112supportsMonica from reading Lists (docs.python.org/3.6/library/stdtypes.html#lists), it says "Lists are mutable sequences". So am I correct in my understanding that MutableSequence is a superset of List? — Intrastellar Explorer
– Intrastellar Explorer, Commented Jun 6, 2020 at 23:36
@IntrastellarExplorer: Depending on how you interpret superset, you may be viewing this backwards; List is a superset of MutableSequence's guaranteed features. MutableSequence is the minimum requirement, but a list can do stuff that not all MutableSequences can do, e.g. list can hold any object, while other mutable sequences like array.array are limited to numeric types in specific ranges. — ShadowRanger
– ShadowRanger, Commented Jun 6, 2020 at 23:47
Yes @ShadowRanger good point. I have changed my wording in the question from "superset" to "parent", for clarity. — Intrastellar Explorer
– Intrastellar Explorer, Commented Jun 6, 2020 at 23:55

user2357112 · Accepted Answer · 2020-06-06 23:29:14Z

8

MutableSequence represents arbitrary mutable sequences. For example, an instance of array.array satisfies MutableSequence.

List is specifically just lists. If an object is not a list, it doesn't satisfy List.

Use MutableSequence when you want to express "mutable sequence". Use List when you want to express "list".

answered Jun 6, 2020 at 23:29

user2357112

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