TypeError is raised if no stderr is registered, but a file is passed as the stderr argument

[tbhartman]

Description

An TypeError is raised at fake_popen.py:271 when calling with a file passed to argument stderr if no stderr was given during registration

>           buffer.write(data_type(data))
E           TypeError: encoding without a string argument

pyvenv\Lib\site-packages\pytest_subprocess\fake_popen.py:271: TypeError

Example

Below is an example demonstrating the error.

def test_fp_bug(fp):
    with NamedTemporaryFile("wb", delete=False) as f:
        fp.register(
            ["git", 'rev-parse', 'HEAD'],
            stdout=["47c77698dd8e0e35af00da56806fe98fcb9a1056"],
            )
        p = subprocess.Popen(('git', 'rev-parse', 'HEAD'), stdout=f.file, stderr=f.file)
        p.wait()
        fp.allow_unregistered(True)
        p = subprocess.Popen(('git', 'rev-parse', 'HEAD'), stdout=f.file, stderr=f.file)
        p.wait()

This raises the type 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
@@ -273,7 +267,7 @@
         )
         if isinstance(data, (list, tuple)):
             buffer.writelines([data_type(line + "\n") for line in data])
-        elif data is not None:
+        else:
             buffer.write(data_type(data))

     def _convert(self, input: Union[str, bytes]) -> Union[str, bytes]:

[aklajnert]

Thanks for reporting an issue. I think I've never consider the stdout/stderr to be files, hence these cases were not tested and are buggy. I'll try to find some time to look into that, but I can't say exactly when it will happen. Feel free to submit a PR if you need it sooner.

[aklajnert]

This was fixed in #196 and released in version 1.5.4.

The root cause was an overly narrow isinstance check in configure():

# old — missed many file-like objects
elif isinstance(stderr, (io.BufferedWriter, io.TextIOWrapper)):
    self._write_to_buffer(self.__stderr, stderr)

When a file-like object didn't match those exact types, the code fell through and eventually called bytes(None) when no stderr was registered, producing the TypeError. The fix switched to duck-typed detection (_get_io_writer()), which works with any object that has a callable .write and a string .mode.

Please upgrade to 1.5.4 or later if you haven't already.