Testing CLI Scripts in Python & Sphinx doctest

[ cli python sphinx stdio-mgr testing ]

I have a couple of Python projects that implement CLIs: sphobjinv and h5cube . For both of these, I have automated tests/documentation in place checking/illustrating the CLI commands (e.g., for sphobjinv v1.0 ). The doctest module in the Python standard library and the doctest extension within Sphinx work really well for testing REPL code snippets, but these are intrinsically “API-like” interactions and I haven’t been able to find anything particularly satisfying for doctesting CLI invocations. So, I put something together myself. To illustrate here, I’ll focus on Sphinx doctest–if you’re interested in how I set up things on the testing side, see here.

Because doctest expects inputs to be interpreted at the REPL, there’s little choice but to construct a CLI runner as a Python function. The version of cli_run implemented as of this writing is:

def cli_run(argstr, inp=''):
    '''Run as if argstr was passed to shell.

    Can't handle quoted arguments.
    import sys

    import sphobjinv.cmdline as cli
    from stdio_mgr import stdio_mgr

    old_argv = sys.argv
    sys.argv = argstr.strip().split()

    with stdio_mgr(inp) as (i_, o_, e_):
        except SystemExit:
            sys.argv = old_argv

        output = o_.getvalue() + e_.getvalue()


Most of this isn’t particularly novel or fancy. It just imports the module containing my main() and swaps out the actual sys.argv temporarily with the test invocation to be run. Once sys.argv is swapped out, the ArgumentParser within main() sees the swapped arguments list and acts on it appropriately, calling into my CLI code. The try/except to pass the SystemExit is needed to avoid fouling of the doctest execution from internal sys.exit() calls. Usage in docs is as simple as:

>>> cli_run('command arg1 arg2 arg3'):
<expected output here>

The big advantage over anything else I’ve seen comes from using stdio_mgr , which I just put together recently. In addition to allowing mocking of both stdout and stderr (via o_ and e_), it provides a means for mocking stdin (via i_) as well. Thus, if you want to test a portion of your CLI that involves user input at the console, stdio-mgr provides a clean, concise way to do this. As a specific example, a ‘Y/N’ confirmation for overwriting a file is what I’ll be using it on shortly in the sphobjinv docs –the doctest source will probably end up looking something like:

>>> cli_run('sphobjinv convert plain objects_attrs.inv', inp='y\n')
File exists. Overwrite (Y/N)? y
Conversion completed.
'.../objects_attrs.inv' converted to '.../objects_attrs.txt' (plain).

The mocked user input inp (which MUST be newline terminated!!) is read from an input call just as if it’d been typed at the console, in response to the File Exists. Overwrite (Y/N)? prompt.

Written on June 16, 2018