How to properly mock subprocess.check_output()?


This Content is from Stack Overflow. Question asked by Thops

I’m utesting float(self) that is supposed to get downloadable file size from yt-dlp.

To call yt-dlp I’m using subprocess.check_output().
I’d like to mock it (subprocess.check_output) but can’t find the way how.

Please help.

Tried approaches:

First one doesn’t work at all. It always reads 0.0.
Second one brings this traceback (Attribute Error):

Traceback (most recent call last):
  File "/usr/lib/python3.10/unittest/", line 1369, in patched
    return func(*newargs, **newkeywargs)
  File "/home/elphael/PycharmProjects/hoarder/core/", line 61, in test__float__handlesSpecialCode
    self.assertAlmostEqual(15.83, float(obj))  # verify it returns value
  File "/home/elphael/PycharmProjects/hoarder/core/", line 40, in __float__
    bytesize = subprocess.check_output(f"yt-dlp -O filesize_approx {self.url}".split(),
  File "/usr/lib/python3.10/", line 420, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.10/", line 501, in run
    with Popen(*popenargs, **kwargs) as process:
AttributeError: __enter__

I’ve tried to mock __enter __ but to no avail.


  1. grab self.url and poll yt-dlp
  2. convert bytes to Mb (auxillary.bytesto())
  3. assign result to self.length and return
    def __float__(self):

            bytesize = subprocess.check_output(f"yt-dlp -O filesize_approx {self.url}".split(),
            megabytes = auxillary.bytesto(bytesize, 'm')  # I only need approx size
            self.length = round(megabytes, 2)
        except subprocess.CalledProcessError:
            return -2.0
        return self.length


  1. init a class that calls float(self)
  2. verify self.length = expected
    def test__float__handlesSpecialCode(self, mock_subproc_popen):
            -1.0 length is meant to show that length is yet to be calculated
        process_mock = Mock()
        attrs = {"communicate.return_value": ("output", "error")}
        mock_subproc_popen.return_value = process_mock
        obj = TFile("", -1.0)
        self.assertAlmostEqual(15.83, float(obj))  # verify it returns value
        self.assertNotEqual(-1, obj.length)  # verify self.length preserves that value

Any input is deeply appreciated.


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