sys standard streams can be None

[mondeja]

Bug Report

File descriptors could be None at Python initialization.

• Note that the print() function implementation handles the case of stdout being None: https://github.com/python/cpython/blob/6078f2033ea15a16cf52fe8d644a95a3be72d2e3/Python/bltinmodule.c#L2060
• Note that the base Exception implementation handles the case of stderr being None: https://github.com/python/cpython/blob/6078f2033ea15a16cf52fe8d644a95a3be72d2e3/Python/errors.c#L1525
• There are other cases in CPython, check https://github.com/search?q=repo%3Apython%2Fcpython+%22file+%3D%3D+Py_None%22&type=code

To Reproduce (unix)

$ python3 -c 'import sys;print(sys.stdin)' 0>&- 
None
$ python3 -c 'import sys;print(sys.stderr)' 2>&-
None
$ python3 -c 'import sys;print(sys.stdout, file=sys.stderr)' 1>&-
None
$ python3 -c 'import sys;print(sys.__stdin__)' 0>&-
None
$ python3 -c 'import sys;print(sys.__stderr__)' 2>&-
None
$ python3 -c 'import sys;print(sys.__stdout__, file=sys.stderr)' 1>&-
None

Expected Behavior

I want the real possible types of the previous sys.std* variables.

Actual Behavior

https://github.com/python/mypy/blob/df35dcf020b3b03a8e3280edf8ada8c6ad8e0da5/mypy/typeshed/stdlib/sys/__init__.pyi#L71-L80

Your Environment

N/A

[Akuli]

There is also another, perhaps more common way for sys.stdin, sys.stdout and sys.stderr to be None. On Windows, if you run a file with pythonw.exe (by naming it foo.pyw and double-clicking it, for example), it will run without a command prompt window, and with sys.std* files set to None.

This is how GUI apps usually run on Windows. For code that should work in these GUI apps, you need print(foo, file=sys.stderr) instead of sys.stderr.write(foo), or explicit None checks.