Consider using Lib/test/list_tests.py to test collections.deque

[larryhastings]

Feature or enhancement

Bug description:

collections.deque is a list-like object that supports fast insertion and removal of elements at either the beginning or the end. Although it doesn't state this goal explicitly, I believe it's intended to be a drop-in replacement for list.

Lib/test/list_tests.py is a file containing unit tests for list-like objects. It's designed to be used with any arbitrary list-like object. However, currently collections.deque doesn't test against it.

I assume the collections.deque object is working fine, and testing it against Lib/test/list_tests.py wouldn't uncover anything. Still, I think more tests are always a good thing. So I propose to incorporate list_tests.py into the deque test suite. It would only be a small amount of work, and who knows, the tests might discover something someday.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[kianelbo]

I did:

class TestList(test_list.ListTest):
    type2test = deque

but contrary to your expectation lots of tests failed.

[brianschubert]

deque doesn't implement a number of list-like behaviors:

>>> d = deque([1, 2, 3])

>>> d[1:]
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    d[1:]
    ~^^^^
TypeError: sequence index must be integer, not 'slice'

>>> d.sort()
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    d.sort()
    ^^^^^^
AttributeError: 'collections.deque' object has no attribute 'sort'

so tests that require those would need to be bypassed.

I see that deque is already tested against the Lib/test/seq_test.py tests. I wonder if that already covers most of the list-like behaviors that deque does implement?

cpython/Lib/test/test_deque.py

Line 906 in 2602d8a

class TestSequence(seq_tests.CommonTest):

[mpkocher]

Although it doesn't state this goal explicitly, I believe it's intended to be a drop-in replacement for list

There's some overlap, but a deque has fundamentally different "O(n)" characteristic than a list for several different access patterns. A deque and list are intended to be used in different use cases.

https://docs.python.org/3/library/collections.html#collections.deque

Also, we should try to clarify terminology if possible. list implements the MutableSequence interface. It's unclear what "list-like" means.

https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes

[serhiy-storchaka]

list_tests.py was designed to share tests between list and UserList.

To test less "list-like" objects, there is seq_tests.py. It is also used to test tuple and deque.

See 1dde95d.

[picnixz]

It's unclear what "list-like" means.

I used "list-like" in the recent commit I pushed. List-like really means "same functionalities as a list" for me or if you want, can be used as a drop-in replacement whenever a list is expected. List-like is essentially a virtual subclass of list.