Leading underscore before the name of Python module

Question

I have searched around for this but I did not find any answer. I'm looking at a Python code written by someone else and there is a module say "GUI_Module". This module contains a class which contains GUI related methods. The module is then imported in the __main__ Python file which makes use of those methods.

Instead of being imported like this with its actual name:

import GUI_Module

it is imported like this with a leading underscore:

import _GUI_Module

Now I know about most meanings of underscores in Python but I have not found anything that explains what a single underscore before the name of a module that we are importing is supposed to do.

Answer 1

In the implementation of the standard library in CPython, the _ prefix seems to indicate that a module is "private", in the sense that you shouldn't import it directly. Usually, this is because its functionality is imported and exposed by some "public", documented module.

Such a private module may be an accelerator module - a fast C implementation that exists alongside a slower pure-Python implementation of the same module (and which the pure-Python version tries to import everything from, so that the faster version is always automatically used if available).

Not all modules implemented in C are accelerator modules, though. In some cases, the _-prefixed module written in C is the only implementation available. For instance, CPython doesn't contain a pure-Python implementation of the ctypes or sqlite modules; these modules respectively depend on importing stuff from the _ctypes or _sqlite3 modules (which are implemented in C) and hence can't be used by a Python interpreter that doesn't support Python's C API.

And not all _-prefixed modules are even implemented in C! For example, the _pydecimal module is a pure-Python implementation of the decimal module. The implementation of decimal in CPython tries to import _decimal (implemented in C) if it's available, and if not, falls back to importing _pydecimal.

The only thing that seems to be universally true of these _-prefixed modules is that they're all implementation details of some documented, "public" module that doesn't have a _-prefix, and that you're supposed to use that module instead of directly importing stuff from the _-prefixed one.