How to test a function with input call?

Question

I have a console program written in Python. It asks the user questions using the command:

some_input = input(\'Answer the question:\', ...)

Answer 1

You can replace sys.stdin with some custom Text IO, like input from a file or an in-memory StringIO buffer:

import sys

class Test:
    def test_function(self):
        sys.stdin = open("preprogrammed_inputs.txt")
        module.call_function()

    def setup_method(self):
        self.orig_stdin = sys.stdin

    def teardown_method(self):
        sys.stdin = self.orig_stdin

this is more robust than only patching input(), as that won't be sufficient if the module uses any other methods of consuming text from stdin.

This can also be done quite elegantly with a custom context manager

import sys
from contextlib import contextmanager

@contextmanager
def replace_stdin(target):
    orig = sys.stdin
    sys.stdin = target
    yield
    sys.stdin = orig

And then just use it like this for example:

with replace_stdin(StringIO("some preprogrammed input")):
    module.call_function()