Python: 'Private' module in a package

While there are not explicit private keywords there is a convention to have put private functions start with a single underscore but a double leading underscore will make it so others cannot easily call the function from outside the module. See the following from PEP 8

- _single_leading_underscore: weak "internal use" indicator.  E.g. "from M
  import *" does not import objects whose name starts with an underscore.

- single_trailing_underscore_: used by convention to avoid conflicts with
  Python keyword, e.g.

  Tkinter.Toplevel(master, class_='ClassName')

- __double_leading_underscore: when naming a class attribute, invokes name
  mangling (inside class FooBar, __boo becomes _FooBar__boo; see below).

- __double_leading_and_trailing_underscore__: "magic" objects or
  attributes that live in user-controlled namespaces.  E.g. __init__,
  __import__ or __file__.  Never invent such names; only use them
  as documented.

To make an entire module private, don't include it __init__.py file.

One thing to be aware of in this scenario is indirect imports. If in mypack you

from mypack._mod_b import foo
foo()

Then a user can

from mypack import foo
foo()

and be none the wiser. I recommend importing as

from mypack import _mod_b
_mod_b.foo()

then a user will immediately see a red flag when they try to

from mypack import _mod_b

As for actual directory structure, you could even extend Jeremy's answer into a _package_of_this_kind package, where anything in that can have any 'access modifiers' on it you like - users will know there be dragons

The solution I've settled on is to create a sub-package 'private' and place all the modules I wish to hide in there. This way they stay stowed away, leaving mypack's module list cleaner and easier to parse.

To me, this doesn't look unpythonic either.

I prefix private modules with an underscore to communicate the intent to the user. In your case, this would be mypack._mod_b

This is in the same spirit (but not completely analogous to) the PEP8 recommendation to name C-extension modules with a leading underscore when it’s wrapped by a Python module; i.e., _socket and socket.