You can split your big module into modules inside a sub-package and import stuff in the __init__.py file from the submodules to present a nice interface to the library user.

eg instead of:

    namespace.py
        class C1:
        class C2:

do

    namespace/__init__.py
        from .c1 import C1
        from .c2 import C2

    namespace/c1.py
        class C1:

    namespace/c2.py
        class C2: