Why is Exception output not captured by contextlib.redirect_stderr context manager?

In the following code, I would have expected the exception output and traceback to be written to test.txt file, but that does not happen.

import contextlib

with open("test.txt", "a") as f:
    with contextlib.redirect_stderr(f):
        raise Exception("Hello")

Why is it not working as expected and how can I correctly redirect the exception output to a file?

Solution

Because redirecting sys.stderr doesn't catch exceptions in python (it would be bad if it did, right? Because exceptions are only supposed to be caught by try/except blocks.) so the exception handling code interrupts your running code (and the redirect).

Proof:

import contextlib
from sys import stderr

with open("test.txt", "a") as f:
    with contextlib.redirect_stderr(f):
        sys.stderr.write("hello")
        sys.stderr.flush()

writes correctly.

To redirect the output from all exceptions to a file you have two options: catch all exceptions globally in a try/except block and do the writing yourself, or use shell redirection to send the script's stderr to a file, with something like

python my_script.py 2>errors.txt

If you want to do it yourself, see this question for discussion of getting the traceback.