[FIXED] Why does my revectored sys.stdout.write() method get called after I've unlinked it?

Issue

I wrote a tee() class, which redirects stdout to both the terminal and a file, based on Triptych's answer at https://stackoverflow.com/questions/616645/how-do-i-duplicate-sys-stdout-to-a-log-file-in-python.

It works the first couple of times my program runs, but the 3rd time I get:

  File "C:\Users\Dave\data\Code\Python\lib\nerdlib.py", line 351, in write
    with open(self.name, "a",  encoding="utf-8") as f:

TypeError: expected str, bytes or os.PathLike object, not NoneType

It seems that even after I do sys.stdout = self.old_stdout (restoring the execution vector to the place it started from), somehow my write method is still getting called (in which case the error is expected).

But why is my method still getting called?

This error occurs in the Spyder development environment - not sure if that's related.

Code is below - it's simple enough.

# based on Triptych's answer at https://stackoverflow.com/questions/616645/how-do-i-duplicate-sys-stdout-to-a-log-file-in-python
class tee():
    ''' Tees stdout to both a log file and the terminal.

        if append is True (default), appends to existing file

        if stderr is True (default), tees stederr to same file

        Usage:
            t = tee("filename.txt")
            ...
            print(1, 2, 3)
            ...
            t.__del__() # returns sys.stdout to original state
    '''

    def __init__(self, filepath, append=True, stderr=True):

        self.old_stdout = sys.stdout
        self.old_stderr = sys.stderr
        self.name = filepath

        if (not append) or (not os.path.exists(self.name)):
            makeEmptyFile(self.name)

        sys.stdout = self

        if stderr:
            sys.stderr = self

    def write(self, text):

        self.old_stdout.write(text)

        # open file, write, then close it again (slow, but safe - permits other processes to write to same file)
        with open(self.name, "a",  encoding="utf-8") as f:
           f.write(text)

    def flush(self):
        pass

    def stop(self):
        sys.stdout = self.old_stdout
        sys.stdout = self.old_stderr
        self.name = None

    def __del__(self):
        self.stop()

Solution

The danger here is that setting sys.stdout = sys.old_stdout stops code from getting a new handle on the local replacement, but it doesn't stop any old handle from working.

As a contrived example:

class MyThread(threading.Thread):
    def __init__(self, log_dest=sys.stdout):
        self.log_dest = log_dest
    def run(self):
        # ...do something-or-other, and occasionally...
        self.log_dest.write('Hey, something happened')

Because log_dest gets assigned a value when MyThread was initialized, changing sys.stdout back to its old value doesn't prevent it from still trying to use your now-invalidated object.

Answered By - Charles Duffy