Pytest failing on file open command string assert - what's the best way to test this? - python-requests

Question

I am constructing a command to pass to requests library to Post an attachment - as in
files= attachment = {"attachment": ("image.png", open("C:\tmp\sensor.png", "rb"), "image/png")}
The code is working but I cannot get PyTest to test it as -is because of the open command which is executed when evaluated. Here is simplified code of the problem
import pytest
def openfile():
cmd = {"cmd": open(r"C:\tmp\sensor.png")}
return cmd
def test_openfile():
cmd = openfile()
#assert str(cmd) == str({"cmd": open(r"C:\tmp\sensor.png")}) # this works
assert cmd == {"cmd": open(r"C:\tmp\sensor.png")} # this does not
PyTest complains that the two side are different but then confirms they are the same in the diff panel!
Expected :{'cmd': <_io.TextIOWrapper name='C:\tmp\sensor.png' mode='r' encoding='cp1252'>}
Actual :{'cmd': <_io.TextIOWrapper name='C:\tmp\sensor.png' mode='r' encoding='cp1252'>}
'Click to see difference' - Opening diff panel reports 'Contents are identical'!
I can just stick with comparing the generated string with expected string but am wondering if there is a better way to do this.
Ideas?

Answer 1

You need to test the properties of the actual file buffer that is returned by the open call, instead of the references to that buffer, for example:
def test_openfile():
cmd = openfile()
expected_filename = r"C:\tmp\sensor.png"
assert "cmd" in cmd
file_cmd = cmd["cmd"]
assert file_cmd.name == expected_filename
with open(expected_filename) as f:
contents = f.read()
assert file_cmd.read() == contents
Note that in a test you may not have the file contents, or have them in another place like a fixture, so testing the file contents may have to be adapted, or may not be needed, depending on what you want to test.

Answer 2

After talking this through with a friend I think my original approach is perfectly valid. For anyone that trips over this question here's why:
I am trying to pytest building of an executable parameter to pass to another library for execution. The execution of the parameter is not relevant, just that it is correctly formatted. The test is to compare what is generated with the expected parameter ( as if I typed it) .
Therefore casting to string or json and comparing is appropriate since that is what a human does to manually check the code!