AssertionError if stderr == subprocess.STDOUT and stdout is a file

[tbhartman]

Description

An AssertionError is raised at fake_popen.py:188when calling with argument stderr=subprocess.STDOUT if stdout is a file (stdout is expected to be None:

>           assert self.stdout is not None
E           AssertionError

pyvenv\Lib\site-packages\pytest_subprocess\fake_popen.py:188: AssertionError

subprocess.Popen handles this correctly; stderr is simply written to the file at stdout.

Example

Below is an example demonstrating the error. I have GIT_TRACE set so that git rev-parse gives a small amount of stdout and stderr:

$ git rev-parse HEAD
06:05:28.502294 exec-cmd.c:244          trace: resolved executable dir: C:/Program Files/Git/mingw64/bin
06:05:28.537291 git.c:463               trace: built-in: git rev-parse HEAD
47c77698dd8e0e35af00da56806fe98fcb9a1056

I define a function that runs the subprocess (so that I can call the same thing, using either the fake or real Popen:

def run_git_revparse():
    import os
    os.environ['GIT_TRACE'] = '1'
    with NamedTemporaryFile("w+b", delete=False) as f:
        path = pathlib.Path(f.name)
        # NOTE: I pass f.file because for some reason:
        #           isinstance(f, io.BufferedWriter) == False
        #       so pytest_subprocess isinstance checks would fail.  I am
        #       hesitant to change those tests, though you could simply check
        #       for a few key methods:
        #           hasattr(f.mode)
        #           isinstance(f.mode, str)
        #           hasattr(f.write)
        #           isinstance(f.write, types.FunctionType)
        p = subprocess.Popen(('git', 'rev-parse', 'HEAD'), stdout=f.file, stderr=subprocess.STDOUT)
        f.close()
        assert p.wait() == 0
        assert path.exists()
        output = path.read_text()

        # print to sys.stdout so I can see with `pytest --capture=no`
        print('-'*50)
        print(output)
        print('-'*50)

        output = output.splitlines()
        assert len(output) == 3

        # account for stderr coming before or after stdout
        if 'exec-cmd.c' in output[1]:
            output[2], output[0], output[1] = output
        # first two lines should be stderr
        assert 'exec-cmd.c' in output[0]
        assert 'git.c' in output[1]
        # last line should be a hex string
        assert len(set(output[2]) - set('0123456789abcdef')) == 0

Then define my test function, which first runs the real git, then the fake "git":

def test_fp(fp):
    print() # for --capture=no
    fp.allow_unregistered(True)
    run_git_revparse()
    fp.register(
        ["git", 'rev-parse', 'HEAD'],
        stdout=["47c77698dd8e0e35af00da56806fe98fcb9a1056"],
        stderr=[
            '05:17:17.982923 exec-cmd.c:244          trace: resolved executable dir: C:/Program Files/Git/mingw64/bin',
            '05:17:18.000760 git.c:463               trace: built-in: git rev-parse HEAD'
            ])
    run_git_revparse()

This raises the assertion error. I can correct this:

--- fake_popen.py   2024-02-10 06:09:01.897909800 -0500
+++ fake_popen.py       2024-02-10 06:08:49.610943500 -0500
@@ -181,22 +181,16 @@
         stdout = kwargs.get("stdout")
         if stdout == subprocess.PIPE:
             self.stdout = self._prepare_buffer(self.__stdout)
-        elif self.__is_io_writer(stdout):
+        elif isinstance(stdout, (io.BufferedWriter, io.TextIOWrapper)):
             self._write_to_buffer(self.__stdout, stdout)
         stderr = kwargs.get("stderr")
         if stderr == subprocess.STDOUT and self.__stderr:
-            if self.__is_io_writer(stdout):
-                self._write_to_buffer(self.__stderr, stdout)
-            else:
-                assert self.stdout is not None
-                self.stdout = self._prepare_buffer(self.__stderr, self.stdout)
+            assert self.stdout is not None
+            self.stdout = self._prepare_buffer(self.__stderr, self.stdout)
         elif stderr == subprocess.PIPE:
             self.stderr = self._prepare_buffer(self.__stderr)
-        elif self.__is_io_writer(stderr):
+        elif isinstance(stderr, (io.BufferedWriter, io.TextIOWrapper)):
             self._write_to_buffer(self.__stderr, stderr)
-
-    def __is_io_writer(self, o):
-        return isinstance(o, (io.BufferedWriter, io.TextIOWrapper, io.BufferedRandom))

     def _prepare_buffer(
         self,

Note:

• I pulled out the isinstance testing.
• I added io.BufferedRandom to the group, as that is what I had, and I would expect it should be included...my test still won't work without it.

[aklajnert]

Thank you for reporting that problem. Right now I don't really have time to work on that project. Feel free to submit a PR if you need it sooner, otherwise I'll work on it in the future, but can't specify exactly when.