Best practice for nested Python module imports

Question

Suppose I have a Python module "main.py":

import math           # from the standard Python library
import my_own_module

...

foo = math.cos(bar)

And I also need to import the standard math module in "my_own_module.py":

import math

...

baz = math.sin(qux)

In this case I think import math in "main.py" is redundant and can be omitted.

What's best practice in this case:

1. Omit import math from "main.py" becuase it's redundant? Or,
2. Keep import math in "main.py" to clarify that the code in that module requires it?

Answer 1

The reference to math.cos in main.py means that import math is required in main.py, regardless of whether my_own_module.py imports it or not. It is not redundant, and it cannot be omitted (and if you try to omit it, you'll get an error).

Answer 2

import math

does something else than simply including the full text of one file into the other.

It introduces a new namespace with the name math, and this math name will be known in your current namespace.

If you omit the

import math

from your main.py file, your command

foo = math.cos(bar)

becomes illegal, as the math symbol will be not (recognized) in the main.py namespace.