The "magic methods" wrapped in in double-underscores are special because other parts of the language's syntax rely on them always meaning the same thing. (Some

The "magic methods" wrapped in in double-underscores are special because other parts of the language's syntax rely on them always meaning the same thing. (Some room for programmer misbehavior here.) Example: If __len__ is defined on an object x, and __nonzero__ (Py2) or __bool__ (Py3) is not defined, then "not x" will test if x.__len__() == 0. Similarly, __contains__ interacts with "foo in x", and __eq__ and __hash__ are expected to play well together for hashable objects (e.g. unique members of a set). The double underscores are a useful code smell that indicates you're changing how an object interacts with Python's syntax.