First, even in Java you don't know the return type / invariants ...

     int n = func_that_returns_positive_even_number()

What you need is to document the thing:

    def func_that_returns_positive_even_number():
          """Returns positive even number."""

This comment will be available when typing "help(func_that_returns_positive_even_number)" in the Python console btw.

Or if you're paranoid and that function can totally break your code:

         n = func_that_returns_positive_even_number()
         assert isinstance(n, int) and n % 2 == 0 and n >= 0

Or to make extra sure this will hold in the future (i.e. protecting from code-changes done by other people) ...

     import unittest

     class TestMyFunc(unittest.TestCase):
           def test_is_positive_and_even(self):
                n = func_that_returns_positive_even_number()
                self.assertTrue( n % 2 == 0 and n >= 0 )