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When module modname has an attribute named __all__, the attribute’s value is the list of the attribute names that are bound by this type of from statement.

From

Cover of Python in a Nutshell, 3rd Edition

Note

If you want to restrict those attributes imported by other modules using "from xxxxx import xxxxx", you put a list of attributes you want to expose in all. These attributes are only ones that will be imported into another module.