import pytest
import re

def test_string_rep():
    cases = ['', ' ', 'a', 'a s !']
    expected = ['', '%20', 'a', 'a%20s%20!']
    for c, e in zip(cases, expected):
        assert string_rep(c) == e

pytest.main()

def string_rep(input):
    p = re.compile(' ')
    return p.sub('%20', input)