I want to use stdin in a pytest test

Question

The PyTest documentation states that stdin is redirected to null as no-one will want to do interactive testing in a batch test context. This is true, but interactive is not the only use of stdin. I want to test code that uses stdin just as it would use any other file. I am happy with stdout and sterr being captured but how to actually have stdin connected to an io.StringIO object say in a PyTest conformant way?

Answer 1

You can monkeypatch it:

def test_method(monkeypatch):
    monkeypatch.setattr('sys.stdin', io.StringIO('my input'))
    # test code

Answer 2

Maybe you could run your script as a subprocess? In Python 3.6:

import subprocess

def test_a_repl_session():
    comlist = ['./executable_script.py']
    script = b'input\nlines\n\n'
    res = subprocess.run(comlist, input=script,
            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    assert res.returncode == 0
    assert res.stdout
    assert res.stderr == b''