18.1 Data Hiding in Modules

As we've seen, Python modules export all names assigned at the top level of their file. There is no notion of declaring which names should and shouldn't be visible outside the module. In fact, there's no way to prevent a client from changing names inside a module if they want to.

In Python, data hiding in modules is a convention, not a syntactical constraint. If you want to break a module by trashing its names, you can, but we have yet to meet a programmer who would want to. Some purists object to this liberal attitude towards data hiding and claim that it means Python can't implement encapsulation. However, encapsulation in Python is more about packaging than about restricting.

18.1.1 Minimizing from* damage: _X and __all__

As a special case, prefixing names with a single underscore (e.g., _X) prevents them from being copied out when a client imports with a from* statement. This really is intended only to minimize namespace pollution; since from* copies out all names, you may get more than you bargained for (including names that overwrite names in the importer). But underscores aren't "private" declarations: you can still see and change such names with other import forms such as the import statement.

A module 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:

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

When this feature is used, the from* statement will only copy out those names listed in the __all__ list. In effect, this is the converse of the _X convention: __all__ contains names to be copied, but _X identifies names to not 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.

The __all__ list also only has meaning to the from* statement form, and is not a privacy declaration. Module writers can use either trick, to implement modules that are well-behaved when used with from*. See the discussion of __all__ lists in package __init__.py files in Chapter 17; there, they declare submodules to be loaded for a from*.