Is there an official or common knowledge standard minimal interface for a "list-like" object?

Question

I keep seeing functions and documentation like this and this (to name a few) which operate on or refer to list-like objects.

I'm quite aware of what exactly an actual list is (dir(list)), and can deduce what (often varying) methods from a list are necessary in most references to a "list-like object", however the number of times I see it referenced has left me with the following question:

Is there an official or common knowledge standard minimal interface for a "list-like" object? Is it as simple as actualizing__getitem__, or is it agreed that additional things like __len__ and __setitem__are required as well?

This may seem like semantics, but I can't help but think that if there does not exist a standard minimal interface requirement, various ideas of "list-likeness" could cause some issues/improper handling. Perhaps this is just a slight downside to Python's duck typing?

Answer 1

See the collections.abc module. Of the abstract base classes listed there, list in Python implements Iterable, Container, Sized, Sequence and MutableSequence. Now, of these, Iterable, Sequence and MutableSequence could be casually called list-like.

However, I would understand the term list-like to mean that it is a MutableSequence - has at least the methods __getitem__, __setitem__, __delitem__ and __len__, expecting also it to have the mixin methods mentioned in the documentation, such as append.

If there is no need for __setitem__ and __delitem__ it should be called a sequence instead - the assumption is that if something accepts a sequence, it does not need to be mutable, thus str, bytes, tuple etc also work there.

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Your two links highlight the vagueness of the term:

The plotly API requires that the list-like objects will be serialized to a JSON array by the internal PlotlyJSONEncoder that delegates most of the encoding to the Python JSONEncoder. However, the latter encodes only tuple and list (and subclasses) to a JSON array; thus the list-like here means a list, a tuple or subclasses thereof. A custom sequence object that is not a subclass of either will result in TypeError: [...] is not JSON serializable.

The unzip recipe you linked to requires an object that behaves like a Sequence, (mutability is not required), thus a tuple or str, or any custom object implementing Sequence will do there.

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TL;DR list-like is a vague term. It is preferable to use the terms iterable, sequence and mutable sequence instead, now that these are defined in collections.abc.

Answer 2

The technical term for a "list-like object" is sequence. At the very least it supports ordering (i.e. two objects with the same elements but different order are not equal), indexing (foo[bar] such that bar is an integer less than the length of the sequence), and containment checking (in), and has a given length. It should support iteration, but if not then Python will simulate it using indexing.