include a is_subset assert method

[ssbarnea]

I think it is common to want to assert that a nested dictonary is a subset of another one and it would be very useful to add such an assert function to pytest itself.

One good example is at https://stackoverflow.com/a/57675231/99834

[asottile]

pytest doesn't currently house any assertion functions -- this would be better done in some assertion library or plugin

[ssbarnea]

@asottile I was afraid that this may be the case. Now the big question is if there is another existing library or plugin which could host this assertion. I do really want to avoid creating another project for it.

[nicoddemus]

While @asottile is correct, I do wonder if we should perhaps have an official assertion library/plugin, as it solves practical problems. Immediately comes to mind how often I use fnmatch_lines even though it is private.

But before we introduce assertions, I would like to see a possible list of "assertion helpers" that should be included, so we can take a look at the landscape and decide if this should be indeed a separate library (perhaps under pytest-dev directly) or introduced into pytest itself (I tend for the former, as it doesn't depend on the internals and might have a different release cycle than the core).

This has been brought up before: #2376

[nicoddemus]

@ssbarnea I'm closing this now, let's continue this in #2376