Preserve default total timeout when other timeout is changed in ClientSession.__init__()

[wombatonfire]

There is a default total timeout

DEFAULT_TIMEOUT: Final[ClientTimeout] = ClientTimeout(total=5 * 60)

used by the ClientSession when no explicit session-level timeout is provided:

if timeout is sentinel or timeout is None:
    self._timeout = DEFAULT_TIMEOUT

This is, definitely, a good thing, you don't want to wait forever in the worst-case scenario.

The problem is, it's very easy to unintentionally reset this default:

timeout = ClientTimeout(connect=5.0)
session = ClientSession(timeout=timeout)

Because of the way how ClientSession is initialized

if timeout is sentinel or timeout is None:
    self._timeout = DEFAULT_TIMEOUT
else:
    self._timeout = timeout

when you change some other - more specific - timeout and do not explicitly provide the value for the total timeout, the default for the total is not preserved and is reset to None.

It looks to me like an undesirable side-effect and something you, probably, do not expect. Documentation also does not cover this behavior.

I think the better approach would be to merge the default total timeout value, when it's not being overridden, with whatever more specific timeout is passed on ClientSession.__init__(). Something that was done in earlier versions for the now deprecated read_timeout and conn_timeout:

self._timeout = attr.evolve(self._timeout, connect=conn_timeout)

If this makes sense, I'm happy to provide the patch.

[Dreamsorcerer]

I think changing the default for total in ClientTimeout seems like a reasonable approach. Probably will need to be in v4 only though, as this could cause backwards-compatibility issues. Feel free to create a PR for that.

[wombatonfire]

@Dreamsorcerer thanks for the feedback! PR #7275 has been created.