Test various functions with "empty" trees / tree sequences

[hyanwong]

Functions like Tree.draw_text() and TreeSequence.draw_text() fail at the moment with ValueError: negative dimensions are not allowed if there are no edges, e.g.

tskit.TableCollection(1).tree_sequence().first().draw_text()

And there's no trivial way to create empty trees/tree sequences: you have to go via a TableCollection as in the example above. This makes me think that we haven't been testing many of our functions against these instances. It might be useful to feed such empty instances into all the functions / methods that deal with Trees or TreeSequences. I wonder if this can be done automatically somehow?

[jeromekelleher]

A lot of the methods are being tested on an empty/null tree sequence, but it's not systematic. I guess we should add this to the list of example topologies when we're making the set of standard fixtures for pytest.

I think we should test drawing functions in a separate PR (the pattern there is clear - add a test for this case).

[hyanwong]

Yep - I just noticed that there was an empty TS in test_highlevel.py::get_example_tree_sequences which I moved to tsutil.py in #931 . So perhaps it's just the drawing code. Having a standard set of examples would be useful, which is why I moved the functions in that PR, but we probably need something more systematic, as you say.

[benjeffery]

Closing for inactivity and labelling "future", please re-open if you plan to work on this.