Minimizing from * Damage: _X and __all__

As a special case, you can prefix names with a single underscore (e.g., _X) to prevent them from being copied out when a client imports a module’s names with a from * statement. This really is intended only to minimize namespace pollution; because from * copies out all names, the importer may get more than it’s bargained for (including names that overwrite names in the importer). Underscores aren’t “private” declarations: you can still see and change such names with other import forms, such as the import statement.

Alternatively, you can achieve a hiding effect similar to the _X naming convention by assigning a list of variable name strings to the variable __all__ at the top level of the module. For example:

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__all__ = ["Error", "encode", "decode"]     # Export these only

When this feature is used, the from * statement will copy out only those names listed in the __all__ list. In effect, this is the converse of the _X convention: __all__ identifies names to be copied, while _X identifies names not to be copied. Python looks for an __all__ list in the module first; if one is not defined, from * copies all names without a single leading underscore.

Like the _X convention, the __all__ list has meaning only to the from * statement form and does not amount to a privacy declaration. Module writers can use either trick to implement modules that are well behaved when used with from *. (See also the discussion of __all__ lists in package __init__.py files in Chapter 23; there, these lists declare submodules to be loaded for a from *.)