Handle pytest 5.4 deprecation to use Node.from_parent

[jacebrowning]

Fixes #26
Depends on #25

---

In order to release this, I believe pytest-describe will need to officially drop support for pytest<5.4, which will in turn require dropping support for python<3.5.

---

To use this patch in your Poetry project, add the following to pyproject.toml:

[tool.poetry.dev-dependencies]

pytest-describe = { git = "https://github.com/jacebrowning/pytest-describe", branch = "pytest-5.4-support" } 

[Cito]

Thank you! Dropping support for Python 2 and Pytest < 5.4 would be even harsher than what I did in #25 though.

[jacebrowning]

I wasn't totally sure when Node.from_parent() was introduced, but its introduction sets the minimum pytest version unless we want to get clever by checking for the existence of the method.

[Cito]

Maybe "use from_parent() except AttributeError: old way" will work?

[jacebrowning]

There would also need to be changes to DescribeBlock.__init__(). Given that this warning only shows up with pytest>=5.4, it seems acceptable to me that the fix would require pytest>=5.4.

[Cito]

Ok, looked into this a bit more. How about this:

Remove the __init__ method alltogether, and create the following from_parent method instead:

    @classmethod
    def from_parent(cls, parent, obj):
        name = obj.__name__
        try:
            from_parent_super = super(DescribeBlock, cls).from_parent
        except AttributeError:  # PyTest < 5.4
            self = cls(name, parent)
        else:
            self = from_parent_super(parent, name=name)
        self._name = getattr(obj, '_mangled_name', name)
        self.funcobj = obj
        return self

Then, in pytest_pycollect_makeitem just return DescribeBlock.from_parent(collector, obj).

This works for me with all supported versions in #25.

Eventually, we should of course desupport the legacy Python and Pytest versions. But for now, I think it would be nice to have a version that is a bit more backward compatible.

[jacebrowning]

Thanks for looking into the API a bit more!

Maybe we should let @ropez decide? If they have limited bandwidth, perhaps trying to support anything other than the latest version of pytest puts too much on their plate? Dropping support for legacy Python also simplifies the build matrix and allows us to drop the legacy super() syntax.

[Cito]

Even then, in order to test libraries that still need to support a broad range of Python versions, it would be nice to publish one last official release that supports that broad version range, and only then remove the Python 2 and older Pytest support, simplify the code for Python 3, and maybe release that as version 1.0 or so. We must not forget to adapt "install_requires" and the classifiers in setup.py when doing that.

[Cito]

This has now been solved in with the alternative fix in 453aa90 that also provides backward compatibility, so we can close this now.