PEP 842 proposes __export__ for controlled module visibility in Python

This PEP proposes the introduction of an `__export__` variable within Python modules. Modules can define this variable, typically a list or tuple of strings, to explicitly declare which members (variables, classes, functions) are intended for external access. Any member not listed in `__export__` will raise an `ImportError` when accessed from outside the module, effectively limiting visibility and preventing accidental use of internal components.

The motivation behind `__export__` is to address the ambiguity of private names (often denoted by underscores) and the difficulties in managing imported module visibility. By requiring explicit export, developers can clearly distinguish between public and internal APIs, reducing the risk of breaking changes for users and decreasing the mental overhead associated with prefixing names.

When `__export__` is defined, all module attribute accesses will be filtered against it, raising `ImportError` for unexported names. Additionally, the module's `__dir__()` function will be modified to exclude unexported names, unless a custom `__dir__` is defined. Dunder names (`__name__`, `__dict__`, etc.) remain accessible regardless of `__export__`.

If a module defines `__export__` but not `__all__`, the `__all__` variable will be implicitly set to the contents of `__export__`. This ensures that wildcard imports (`from module import *`) will only import the explicitly exported members, further enforcing module API boundaries.

The behavior of `__export__` is not overridden by a module's `__getattr__()` function, as `__getattr__` is only invoked for names not already defined. Furthermore, lazy imports not listed in `__export__` will not be resolved when accessed outside the module, maintaining their lazy state.