How to mock imports in python?

gistfile1.py

/tmp/wow 
15:12  $ cat a.py
import requests

def my_fn():
    return requests.get("not")
/tmp/wow 
15:13  $ cat test.py 
import pytest
import mock
import a

def test_my_fn():
    with mock.patch("a.requests") as MockReq:
        MockReq.get.return_value = "whatever"
        assert a.my_fn() == "whatever"
/tmp/wow 
15:13  $ py.test test.py 
==================================================================== test session starts ====================================================================
platform linux2 -- Python 2.7.14, pytest-2.9.2, py-1.4.33, pluggy-0.3.1
metadata: {'Python': '2.7.14', 'Platform': 'Linux-4.14.11-200.fc26.x86_64-x86_64-with-fedora-26-Twenty_Six', 'Packages': {'py': '1.4.33', 'pytest': '2.9.2', 'pluggy': '0.3.1'}, 'Plugins': {'cov': '2.4.0', 'xdist': '1.15.0', 'html': '1.14.2', 'timeout': '1.2.0', 'metadata': '1.5.0', 'flake8': '0.8.1'}}
rootdir: /tmp/wow, inifile: 
plugins: xdist-1.15.0, timeout-1.2.0, metadata-1.5.0, html-1.14.2, flake8-0.8.1, cov-2.4.0
collected 1 items 

test.py .

================================================================= 1 passed in 0.08 seconds ==================================================================