Exception traceback is hidden if not re-raised immediately

Question

I've got a piece of code similar to this:

import sys

def func1():
    func2()

def func2():
    raise Exception('test error')

def main():
    err = None

    try:
        func1()
    except:
        err = sys.exc_info()[1]
        pass

    # some extra processing, involving checking err details (if err is not None)

    # need to re-raise err so caller can do its own handling
    if err:
        raise err

if __name__ == '__main__':
    main()

When func2 raises an exception I receive the following traceback:

Traceback (most recent call last):
  File "err_test.py", line 25, in <module>
    main()
  File "err_test.py", line 22, in main
    raise err
Exception: test error

From here I don't see where the exception is coming from. The original traceback is lost.

How can I preserve original traceback and re-raise it? I want to see something similar to this:

Traceback (most recent call last):
  File "err_test.py", line 26, in <module>
    main()
  File "err_test.py", line 13, in main
    func1()
  File "err_test.py", line 4, in func1
    func2()
  File "err_test.py", line 7, in func2
    raise Exception('test error')
Exception: test error

Answer 1

A blank raise raises the last exception.

# need to re-raise err so caller can do its own handling
if err:
    raise

If you use raise something Python has no way of knowing if something was an exception just caught before, or a new exception with a new stack trace. That's why there is the blank raise that preserves the stack trace.

Reference here