Should Sequence support __add__?

[jamesbraza]

The Python docs on Sequence mentions it supports concatenation here. The current Sequence (link) doesn't include __add__.

Please see the below code:

from typing import Sequence

# Can add List
list_1: Sequence[int] = [1, 2]
list_2: Sequence[int] = [3, 4]
assert len(list_1 + list_2) == 4

# Can't add range
range_1 = range(5)
range_2 = range(6)
concatenated_ranges = range_1 + range_2

Running this code throws a TypeError on the last line:

Traceback (most recent call last):
  File "/path/to/foo.py", line 11, in <module>
    concatenated_ranges = range_1 + range_2
TypeError: unsupported operand type(s) for +: 'range' and 'range'

Running mypy==0.931 on this I get the below errors:

path/to/foo.py:6:12: error: Unsupported left operand type for + ("Sequence[int]")  [operator]
path/to/foo.py:11:23: error: Unsupported left operand type for + ("range")  [operator]
Found 2 errors in 1 file (checked 21 source files)

I believe mypy is raising these errors per typeshed Sequence not implementing __add__.

So, there are two possibilities:

• Either: the Python docs should mention __add__ isn't universally implemented for all Sequence
• Or range should implement __add__ and we should add Sequence.__add__ should be added to typeshed

What do you think?

[JelleZijlstra]

range() couldn't support __add__, because range objects can't be discontinuous. I guess it could return a list, but that could quickly eat a lot of memory and it would be inconsistent with other sequences.

Also, even sequences that do support __add__ only support adding their own type; you can't do list + tuple. That makes it not very useful in the typing world: if you see a: Sequence; b: Sequence; a + b, you can't know whether the + is safe because you don't know the concrete types of a and b.

There is tension between the informal notion of "sequence" in the Python documentation, which can mean slightly different things depending on context, and the precise notion of a Sequence in typing, which defines an exact set of supported methods. At the C level, a sequence (according to https://docs.python.org/3/c-api/sequence.html#c.PySequence_Check) just needs to support __getitem__.

The Python docs link you link already say (in footnote 7 to the table) that range doesn't support * or +.

[jamesbraza]

That makes a ton of sense, I realize now it doesn't make sense to change. Thank you for a detailed response @JelleZijlstra!