Repair colorama wrapping on non-Windows platforms (#1670)

* Repair colorama wrapping on non-Windows platforms

The wrap_stream_for_windows() function calls
colorama.initialise.wrap_stream() function to apply colorama's magic to
wrapper to the output stream. Except this wrapper is only applied on
Windows platforms that need it, otherwise the original stream is
returned as-is.

The colorama wrapped stream lacks a detach() method, so a no-op lambda
was being assigned to the wrapped stream.

The problem is that the no-op lambda was being assigned unconditionally
whether or not colorama actually returns a wrapped stream, thus
replacing the original TextIOWrapper's detach() method. Replacing the
detach() method with a no-op lambda is the root cause of the problem
observed in #1664.

The solution is to only assign the no-op detach() method if the stream
lacks its own detach() method.

Repairs #1664

CHANGES.md

@@ -15,6 +15,8 @@
 
 - fixed a crash when PWD=/ on POSIX (#1631)
 
+- fixed "I/O operation on closed file" when using --diff (#1664)
+
 - Prevent coloured diff output being interleaved with multiple files (#1673)
 
 - Added support for PEP 614 relaxed decorator syntax on python 3.9 (#1711)

README.md

@@ -636,6 +636,7 @@ Multiple contributions by:
 - [Paul Ganssle](mailto:p.ganssle@gmail.com)
 - [Paul Meinhardt](mailto:mnhrdt@gmail.com)
 - [Peter Bengtsson](mailto:mail@peterbe.com)
+- [Peter Grayson](mailto:pete@jpgrayson.net)
 - [Peter Stensmyr](mailto:peter.stensmyr@gmail.com)
 - pmacosta
 - [Quentin Pradet](mailto:quentin@pradet.me)

src/black/__init__.py

@@ -871,30 +871,22 @@ def color_diff(contents: str) -> str:
 
 def wrap_stream_for_windows(
     f: io.TextIOWrapper,
-) -> Union[io.TextIOWrapper, "colorama.AnsiToWin32.AnsiToWin32"]:
+) -> Union[io.TextIOWrapper, "colorama.AnsiToWin32"]:
     """
-    Wrap the stream in colorama's wrap_stream so colors are shown on Windows.
+    Wrap stream with colorama's wrap_stream so colors are shown on Windows.
 
-    If `colorama` is not found, then no change is made. If `colorama` does
-    exist, then it handles the logic to determine whether or not to change
-    things.
+    If `colorama` is unavailable, the original stream is returned unmodified.
+    Otherwise, the `wrap_stream()` function determines whether the stream needs
+    to be wrapped for a Windows environment and will accordingly either return
+    an `AnsiToWin32` wrapper or the original stream.
     """
     try:
-        from colorama import initialise
-
-        # We set `strip=False` so that we can don't have to modify
-        # test_express_diff_with_color.
-        f = initialise.wrap_stream(
-            f, convert=None, strip=False, autoreset=False, wrap=True
-        )
-
-        # wrap_stream returns a `colorama.AnsiToWin32.AnsiToWin32` object
-        # which does not have a `detach()` method. So we fake one.
-        f.detach = lambda *args, **kwargs: None  # type: ignore
+        from colorama.initialise import wrap_stream
     except ImportError:
-        pass
-
-    return f
+        return f
+    else:
+        # Set `strip=False` to avoid needing to modify test_express_diff_with_color.
+        return wrap_stream(f, convert=None, strip=False, autoreset=False, wrap=True)
 
 
 def format_stdin_to_stdout(