Terry J. Reedy added the comment:
If sys.stderr is set to None, then "if file is None: file = sys.stderr" will
have no effect and 'file.write' raises "AttributeError: 'NoneType' object has
no attribute 'write'" (actual quote). Do you propose to catch that? If not,
it is not much of an improvement. Multiple people have reported on python-list
that they are mystified by messages like this when they never used a None
object as an argument.
I am sympathetic to not overlooking bugs. However, a traceback is normally
lost in this situation anyway. If sys.stderr is closed, trying to write a
ValueError traceback (or any other traceback) to stderr generates another
ValueError. Python apparently tries to write a double traceback to
sys.__stderr__ as a backup (see below). If that is also unusable (perhaps
because it *is* sys.stderr), then the traceback disappears anyway. This is
what happens (on Windows, at least) with python started from either an icon or
in Command Prompt.
>>> import sys; sys.stderr.close()
>>> from warnings import warn; warn('foo')
>>> sys.stderr is sys.__stderr__
True
When Idle starts an icon, the result is the same except that the user process
is restarted. Also, two streams are not the same; sys.__stderr__ is None.
If Idle is run with python from a console, there is still no message in the
Idle shell but since sys.__stderr__ points back to the console, the double
traceback appears there: first the warn ValueError, then 'while
proccessing...', the traceback ValueError. Idle replaces showwarning. I
believe it now only changes the formatting, but I could have it use
sys.__stderr__ when not None to print the warning instead of or in addition to
the tracebacks.
If the code above is placed in a file and run in batch mode, python quits with
no output.
C:\Programs\Python34>python temop.py
C:\Programs\Python34>
When run from the idle editor running under pythonw, the user process does the
same (exit without output).
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<http://bugs.python.org/issue22298>
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