Question

In Flask-PyMongo, they use self._Collection__database to represent the database object that this Collection object belongs to:

class Collection(collection.Collection):
    """Custom sub-class of :class:`pymongo.collection.Collection` which
    adds Flask-specific helper methods.
    """

    def __getattr__(self, name):
    attr = super(Collection, self).__getattr__(name)
    if isinstance(attr, collection.Collection):
        db = self._Collection__database
        return Collection(db, attr.name)
    return attr

Why is self._Collection__database not self.__database?

test <a>and <i>

Answer 1

Flask-PyMongo is not arbitrarily choosing that name.
The name is the result of name mangling:

Since there is a valid use-case for class-private members (namely to
avoid name clashes of names with names defined by subclasses), there
is limited support for such a mechanism, called name mangling. Any
identifier of the form __spam (at least two leading underscores, at
most one trailing underscore) is textually replaced with
_classname__spam, where classname is the current class name with leading underscore(s) stripped.

In the parent-class definition, the attribute is defined as self.__database and Python “mangles” the name (to self._ClassName__attributename) so that any subclasses do not overwrite with their own assignment to their own self.__database attribute.