Comment by Marazan

I'm sorry, I clearly haven't explained myself well as otherwise you would not have wasted a huge amount of text tying yourself in knots based clearly on a mistaken apprehension of what I was saying.

For clarity I reproduce the original function you gave and then I present what the change I am suggesting is

  def cool_function(x):
    for i in range(x):
      print(i)

My change

  def cool_function(x, output_stream=sys.stdout):
    for i in range(x):
      print(i, file=output_stream)

Does it now become clear what I am suggesting? My new function can be used as a 1-for-1 replacement for the old function, no code of the system needs changed as the default value provided to the new variable ensures semantically identical operation without changing any further code. Yet it is now unit testable

But now we can write a unit test like

  def test_output():
    output = io.StringIO()  
    cool_function(1, output)   
    contents = output.getvalue()
    assert contents=="1\n"

So I've made the code unit testable, kept semantics completely identical and not had to worrty about any weird IO concerns that you have. No monkey patching, no weird file IO, no bizarelly re-implemnting list(range(x)).