Mocking subprocess with pytest-subprocess#

For apple-notes-to-sqlite I needed to write some tests that simulated executing the osascript command using the Python subprocess module.

I wanted my tests to run on Linux CI machines, where that command would not exist.

After failing to use unittest.mock.patch to solve this, I went looking for alternatives. I found pytest-subprocess.

Here’s the relevant section of the test I wrote:

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from apple_notes_to_sqlite.cli import cli, COUNT_SCRIPT
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FAKE_OUTPUT = b"""
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The stuff I would expect to be returned by log lines in
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my osascript script.
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"""
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def test_apple_notes_to_sqlite_dump(fp):
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    fp.register_subprocess(["osascript", "-e", COUNT_SCRIPT], stdout=b"2")
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    fp.register_subprocess(["osascript", "-e", fp.any()], stdout=FAKE_OUTPUT)
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    runner = CliRunner()
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    with runner.isolated_filesystem():
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        result = runner.invoke(cli, ["--dump"])
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        # ...

fp is the fixture provided by the package (you need to pip install pytest-subprocess for this to work).

COUNT_SCRIPT here is the first of my osascript constants. It looks like this (in cli.py):

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COUNT_SCRIPT = """
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tell application "Notes"
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    set noteCount to count of notes
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end tell
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log noteCount
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"""

That first fixture line says that any time my program calls osascript -e that-count-script the return value sent to standard output should be a binary string 2.

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fp.register_subprocess(["osascript", "-e", COUNT_SCRIPT], stdout=b"2")

The second call to subprocess made by my script is more complicated - it involves a script that is dynamically generated.

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fp.register_subprocess(["osascript", "-e", fp.any()], stdout=FAKE_OUTPUT)

I eventually figured that using fp.any() was easier than specifying the exact script. This is a wildcard value which matches any string. It returns the full FAKE_OUTPUT variable as the simulated standard out.

What’s useful about pytest-subprocess is that it works for both subprocess.check_output() and more complex subprocess.Popen() calls - both of which I was using in this script.