_foo
foo_
__foo
_Foo__bar
__foo__
foo__bar
All of that is valid Python, and some of those forms mean different things depending on where they are used.
Otherwise, a leading underscore indicates a private method but isn't enforced. A double leading underscore is also a private method but is "enforced" by giving it an unpredictable name. Double underscore (on both sides) means the function is digging in to python's API, like if you want to give a class some behaviour with + or = or [].
It's not trivial, and not particularly intuitive, but it's not necessarily terribly confusing.
edit: I googled a bit and PEP8 explicitly says "Thus class_ is better than clss". and "single_trailing_underscore_: used by convention to avoid conflicts with Python keyword, e.g..."
The fourth form is the mangling used for __x names internally (__x field in class Foo is actually _Foo__x
I don't know where GP saw sixth form, but considering all other forms are from real-world usage, someone probably uses it too.
But I'd still argue the average Python codebase tends to be pretty legible and simple to read.