BUG: Empty string node results in unclear exception

[rossbar]

This was originally reported to NetworkX in networkx/networkx#4834. [Edit: was pointing to #4855]

A minimal reproducing example:

>>> import pygraphviz as pgv
>>> A = pgv.AGraph()
>>> A.add_node("")  # An empty string
Traceback (most recent call last)
   ...
TypeError: decoding to str: need a bytes-like object, NoneType found

As pointed out in the original issue, this may be expected (assuming graphviz doesn't allow empty nodes) but at the very least a more informative exception should be raised.

I think the problem resides around here:

pygraphviz/pygraphviz/agraph.py

Lines 1854 to 1855 in 8048d8c

	if nh is not None:
	n = super().__new__(self, gv.agnameof(nh), graph.encoding)

as gv.agnameof(nh) returns None when nh derives from an empty string. After verifying that graphviz indeed does not allow empty strings as nodes, this should be simple to fix by adding an extra condition or a try/except here.

[dschult]

Is the PR number referenced in the comment correct? That’s a pr about sphinx.

I do see that agnameof Is defined twice in pygraphviz.i . Once just as calling the function and then a few lines later being defined directly using Python code. The comments suggest this is to handle anonymous edges. I’m not sure of the impact of that change. But it seems that it would be better to use different names for the two functions.

GraphViz itself allows empty strings as labels for nodes. At least the GraphViz playground works just fine with input of a dot file that uses “” as the node’s unique identifier.

[herrjemand]

Can confirm it is still painfully exists