I/O operation on closed file

[serhiy-storchaka]

Running Black 20.8b1 with the --diff option on a tree (> 100 files) produces errors like

error: cannot format tests/api/test_core.py: I/O operation on closed file

Running black --diff with a single file does not produce this error.

Linux/Python 3.7.8+

[jpgrayson]

As another data point from another code base, I observed this issue when 17 of 188 files would be changed by black. Once those files were changed to satisfy black, the issue went away.

When only 5 of 188 files do not satisfy black, the problem occurs sporadically for me.

So it seems like the problem manifests when there are a sufficient number of diffs being generated.

[jpgrayson]

The problematic behavior only happens when the optional colorama package is installed.

When wrapping the output stream with colorama, it is assumed that colorama returns a wrapped stream without its own detach() method, but a wrapped stream is only returned on Windows platforms. The problem is that when no wrapper is affected, the code still unconditionally reassigns the original stream's detach() method to be a no-op.

I'll post a PR for this shortly.