tmpdir, tmpdir_factory and tmp_path_factory rely on getpass.getuser() for determining the basetemp directory. I found that the user name returned by getpass.getuser() may return characters that are not allowed for directory names. This may lead to errors while creating the temporary directory.

The situation in which I reproduced this issue was while being logged in through an ssh connection into my Windows 10 x64 Enterprise version (1909) using an OpenSSH_for_Windows_7.7p1 server. In this configuration the command python -c "import getpass; print(getpass.getuser())" returns my domain username e.g. contoso\john_doe instead of john_doe as when logged in regularly using a local session.

When trying to create a temp directory in pytest through e.g. tmpdir_factory.mktemp('foobar') this fails with the following error message:

self = WindowsPath('C:/Users/john_doe/AppData/Local/Temp/pytest-of-contoso/john_doe')
mode = 511, parents = False, exist_ok = True

    def mkdir(self, mode=0o777, parents=False, exist_ok=False):
        """
        Create a new directory at this given path.
        """
        if self._closed:
            self._raise_closed()
        try:
>           self._accessor.mkdir(self, mode)
E           FileNotFoundError: [WinError 3] The system cannot find the path specified: 'C:\\Users\\john_doe\\AppData\\Local\\Temp\\pytest-of-contoso\\john_doe'

C:\Python38\lib\pathlib.py:1266: FileNotFoundError

I could also reproduce this without the complicated ssh/windows setup with pytest 6.2.2 using the following commands from a cmd:

echo def test_tmpdir(tmpdir):>test_tmp.py
echo   pass>>test_tmp.py
set LOGNAME=contoso\john_doe
py.test test_tmp.py

Thanks for having a look at this!