Python variable and filename conflicts

This is my project structure:

a
├── b.py
└── __init__.py

• File b.py is empty.

• File __init__.py is one line:

  b = 'this is a str'
  

Then the following program gives inconsistent result of a.b:

import a
print(a.b)    # str
import a.b
print(a.b)    # module

What is the best way of detecting this kind of name conflict between a variable and a filename?