如何重定向输出<None>在python中将对象输入到文本文件？

Question

I am struck in one problem.I am trying to redirect the output stream to text file but the problem that is coming is the object through which i am redirecting is of type None . After redirecting to file, the only output i am able to see is None .

from contextlib import contextmanager


@contextmanager
def stdout_redirected(new_stdout):
   save_stdout = sys.stdout
   sys.stdout = new_stdout
   try:
       yield None
   finally:
       sys.stdout = save_stdout

    with open('output3.txt', "w") as f:
        with stdout_redirected(f):
            XYZ = hid.core.show_hids(target_vid=vendor, target_pid=product)
            print XYZ

How to redirect the output from none type object ?

Answer 1

You can't redirect the output from a None type object. Such objects don't have any output.

Nothing you can do in your code is going to make this line

XYZ = hid.core.show_hids(target_vid=des, target_pid=bes)

work the way you want it to. That is because your code is assigning the return value of the function to XYZ and show_hids() returns the value None . You may think that is not how it should work, but that is how it actually does work. If a function does return None , or (as in this case) lacks a return statement, then None is what you are going to get back and you can't change that without rewriting the function.

You can see output, so show_hids() is clearly sending that output to the console itself. If you want it to go somewhere else you have to get show_hids() to send the output where you want it.

You were advised in the comments to consult the documentation on this subject. Understand that small third-party libraries maintained by only one or two people are likely to have documentation that is less comprehensive than you would like. That in turn means it may take a little more work to consult. In this case, all you needed to do was go to the project page on GitHub and look at the examples folder , which contains sample code called show_hids.py and that contains this sample Python 2 call:

output = codecs.getwriter('mbcs')(sys.stdout)
hid.core.show_hids(output = output)

This makes it very clear that you are expected to pass an open file object as the parameter output to the function if you want the output to go somewhere other than sys.stdout .

But that may only get you so far. Some USB devices have Unicode characters in the device names, and when I run the the sample code under Python 2 it chokes on the name Natural® Ergonomic Keyboard 4000 because of the trademark sign. And fiddling with codecs.getwriter() won't change that because the problem isn't happening at the output stage.

I've posted it as an issue on GitHub but if it were up to me I would flag it Will not fix because it is a pure Python 2 problem, and support for Python 2 ends this year.

So if you run into this problem I recommend that you switch to Python 3. Teaching yourself Python 2 is a dead end.

Then all you need to do is this:

with open(r"C:\Users\xxxxx\55561448.txt", "w", encoding="UTF-8") as f:
    hid.core.show_hids(output=f)

When I suggested in the comment that you try reassigning sys.stdout as a last resort, this is what I meant:

default_stdout = sys.stdout
sys.stdout = open('help.txt', 'w')
hid.core.show_hids(target_vid=des, target_pid=bes)
sys.stdout = default_stdout

That temporarily monkeypatches sys.stdout so that show_hids() will do what you want. But there is no need to do that in this case because the show_hids() itself provides a means to send the output where you want.