Updated as of 2020-04-12 1:41pm PDT

Problem

There is at least one place in NetworkX that uses collections.OrderedDict as a substitute for an insertion-ordered set. This is an undesirable workaround: it makes code harder than necessary to read.

Consider this example:

visited[child] = None

What do you think it does? I wouldn't fault anyone for (on a first pass) thinking that it marks child as not visited. Well, in reality, adds an item to an collections.OrderedDict (used as an insertion-ordered set).

In my view, the above code doesn't just "read badly" it is also misleading. In context of a particular algorithm, simple_paths.py, it makes it harder to follow.

Solution

There are options. I understand that Python's collections library does not provide an ordered set container. I recommend either:

• using an existing ordered set library, or
• add a library-wide wrapper on top of OrderedDict.

For example, please consider IndexedSet from the boltons package:

IndexedSet is a collections.MutableSet that maintains insertion order and uniqueness of inserted elements. It’s a hybrid type, mostly like an OrderedSet, but also list-like, in that it supports indexing and slicing.