Why does pylint give this warning?

[breamoreboy at gmail.com]

The warning is 'C0103:Method name "__len__" doesn't conform to '_?_?[a-z][A-Za-z0-9]{1,30}$' pattern' but it doesn't complain about __repr__ or __str__.  If there is an explanation out in the wild my search fu has missed it :-(

My setup on Ubuntu 17.10 is:-

$ pylint --version
Using config file /home/mark/.pylintrc
pylint 1.8.1, 
astroid 1.6.0
Python 3.6.3 (default, Oct  3 2017, 21:45:48) 
[GCC 7.2.0]

--
Kindest regards.

Mark Lawrence.