python sys.__stdout__ is equal to None by default-CodePudding

The problem is simple. I copied an example from a python book about output redirection and firstly sys.stdout is changed to a file object and then is restored to sys.__stdout__. However the restoration part doesn't seem to work because the output later in the code doesn't get displayed at all anywhere.

I found out the issue which i'll explain with this shell code sample:

>>> import sys
>>> sys.stdout
<idlelib.run.StdOutputFile object at 0x00000191D0BCFB50>
>>> sys.__stdout__
>>> print(sys.__stdout__)
None
>>>

So: why is sys.__stdout__ equal to None by default?

EDIT: i know about the context manager in contextlib explained in this issue but i still would like to know why this happens.

CodePudding user response：

It's likely that you're using an OS environment where sys.stdout is not normally set with an initial value. Two relevant portions of the documentation are quoted below (bolded emphasis mine):

From the IDLE documentation:

By default, IDLE runs user code in a separate OS process rather than in the user interface process that runs the shell and editor. In the execution process, it replaces sys.stdin, sys.stdout, and sys.stderr with objects that get input from and send output to the Shell window. The original values stored in sys.__stdin__, sys.__stdout__, and sys.__stderr__ are not touched, but may be None.

From the sys module documentation:

Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.

Under these conditions, IDLE is setting a value for sys.stdout when the initial values for sys.stdout and sys.__stdout__ were None.

If this is problematic for the environment you're using, you can always save the initial value of sys.stdout and restore it later.

CodePudding user response：

According to the docs:

Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.

You're using IDLE, which does not bind __stdout__ as it's a GUI app. It overrides sys.stdout to the IDLE default stdout stream.