CODEWITHSUNDEEP
pythonpython-unittest
Ray Salemi
Ray Salemi

Reputation: 5913

How do I stop AssertionError messages from subprocess.check_call() in unittest

I have a test that is calling a script using subprocess.check_call. I have assertions inside the script checking the validity of the arguments and I want to check the error cases. So I've done this:

try:
    self.assertRaises(subprocess.CalledProcessError, subprocess.check_call, ['myscript.py', 'myBadArgument'])
except AssertionError:
    pass

The test actually passes since the correct assertion gets raised, but I want to keep the assertion messages and associated stack dump from getting printed to the screen.

How do I do this? Thanks!

Upvotes: 1

Views: 784

Answers (2)

Ray Salemi
Ray Salemi

Reputation: 5913

The problem here was that I was overusing the assert() statement. I was using assert to check for things such as whether the user gave me files that existed, or values that made sense in the script.

This was a misunderstanding on my part. I now see that the correct approach is to check the data and raise the appropriate exceptions in my library elements, then catch those exceptions in my user-facing scripts and provide a call-stack free message.

Once I cleaned this up my test passed with the correct exceptions and with no unneeded messages.

Upvotes: 1

user2357112
user2357112

Reputation: 281476

To suppress the subprocess's error messages, redirect its stderr:

from subprocess import DEVNULL

with self.assertRaises(subprocess.CalledProcessError):
    subprocess.run(['myscript.py', 'myBadArgument'], stdout=DEVNULL, stderr=DEVNULL)

Also, don't catch AssertionError. That doesn't do anything to suppress the subprocess's error messages; it only causes your test to incorrectly pass even if the subprocess doesn't raise an error.

Upvotes: 3

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