65 lines
2.1 KiB
Markdown
65 lines
2.1 KiB
Markdown
---
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title: "How to mock stdout runtime attribute of subprocess.Popen - Python ?"
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date: 2018-02-16
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layout: post
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category: Programming
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image: /img/blog/how-to-mock-stdout-runtime-attribute-of-subprocess-popen-python.png
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description: "Everything is in the title ! Subprocess library is hard to test..."
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---
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[](/img/blog/how-to-mock-stdout-runtime-attribute-of-subprocess-popen-python.png)
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After 4 days of struggling ~~and having browsed half of StackOverflow answers~~, I've decided to publish a blog post accounting for the Python mocking operation described below.
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While populating test cases for the [archey project](https://git.io/archey4), I wanted to mock some `subprocess.Popen` calls, used in contexts like this one :
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{% highlight python %}
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check_output(
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['grep', '-E', '3D|VGA|Display'],
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stdin=Popen(['lspci'],
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stdout=PIPE,
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stderr=DEVNULL).stdout
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).decode().split(': ')[1].rstrip()
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{% endhighlight %}
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So the first thought you might have would be something like :
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{% highlight python %}
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@mock.patch.object(
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'module.submodule.Popen',
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'stdout',
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'Mocked string',
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create=True
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)
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{% endhighlight %}
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... but this does not work, as `Popen` expects a _file-like_ object to read from.
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In order to sum this mess up, the problems here are :
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* `subprocess.check_output` call needs a readable pipe to work
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* `stdout` attribute is **a runtime attribute** of `Popen` return object
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* The will of not using a different mocking framework than default `unittest`
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So the final workaround I've come up with is :
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* Mocking the FIFO with a temporary file
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* Writing to this file the mocked content
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* Rewinding to the beginning of the file
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* Make the `Popen` mock returning this file as `stdout` attribute
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* Running concerned tests
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* Deleting the temporary file
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Everything is more or less explained in this Gist :
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<script src="https://gist.github.com/HorlogeSkynet/5b1378752669cce829d110fd48c4a5c8.js"></script>
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Any comment or improvement would be welcome :ok_hand:
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