Yielding for testability in python

I was writing a small cli tool and wanted to try out typer. I’m a big fan of fast api, so I thought a cli library from the same author was worth a look. This blog post will cover a pattern I ended up using to help with testing.

The app and the first attempt at tests

I won’t go into too much detail about my actual problem, but I ended up with something like the following:

import typer

def my_command(some_input: str):
    typer.echo(f"Starting up")
    # Do the first thing
    typer.echo(f"Result of first thing was X")
    # Do the second  thing
    typer.echo(f"Result of second thing was X")
    # Do some clean up
    typer.echo(f"All done")

if __name__ == "__main__":

The value in the code I was writing was the output of typer.echo. So I wanted to test it. Initially in my tests I was monkey patching typer.echo and asserting that it matched what I wanted. This was okay, but I don’t like monkey patching with mocks as it often becomes very tangled and complicated.

A solution? Yielding.

Since I already had a function representing my command invocation I thought it would be really nice if I could just return the output. Then I could call the function and assert that it had output what I wanted. However, given that quite significant delays could happen in between each of my echo statements I didn’t really want to wait until the end to get all my output. So instead yield seemed like a good candidate. I updated my code to look something like this:

class Echo:
    message: str

def my_command(some_input: str):
    yield Echo(f"Starting up")
    # Do the first thing
    yield Echo(f"Result of first thing was X")
    # Do the second  thing
    yield Echo(f"Result of second thing was X")
    # Do some clean up
    yield Echo(f"All done")

Now my function was a generator. The code looked almost identical which was nice. I did need to add a layer on top to run this generator but this was not too complicated:

import typer

def run_cli(generator_func):
    for output_line in generator_func():

and now my test cases could look like this:

def test_something():
    actual = list(my_command("input"))
    expected = [
        Echo("first message"), 
        Echo("second message"), 
    assert actual == expected

I’m counting this as a success. My tests became a lot simpler which was my initial goal. In addition, the my_command function lost its dependency on the typer library and became agnostic to how the input is displayed to the user (though this wasn’t really my initial goal).

Thoughts? Comments? Send me a tweet!