Not going to suggest going through and adding type hints to all possible functions, but I think they would be especially useful on the i/o functions since you are traversing libraries and types - this would help the already good interoperability in mixed contexts where people are converting to and from different types, help people know what to expect with static analysis tools. I recognize that you might not have the correct packages imported/installed since they are optionals, but there are a few strategies that might work there -- splitting up the i/o module into a package with submodules where you try and import the necessary packages at the module level and then use string type hints:

something like...

IMPORTED = False
try:
    import networkx as nx
    from networkx.classes.digraph import DiGraph
    IMPORTED = True
except ImportError as e:
    # probably some logging idk
    raise e
    

def to_networkx(m, edge_attribute:str='weight') -> 'DiGraph':
    ...

that's pretty clumsy but hopefully illustrates the idea.

part of: pyOpenSci/software-submission#81