rethrowing python exception. Which to catch?

Try it and see:

def failure():
    raise ValueError, "Real error"

def apologize():
    raise TypeError, "Apology error"

try:
    failure()
except ValueError:
    apologize()
    raise

The result:

Traceback (most recent call last):
  File "<pyshell#14>", line 10, in <module>
    apologize()
  File "<pyshell#14>", line 5, in apologize
    raise TypeError, "Apology error"
TypeError: Apology error

The reason: the “real” error from the original function was already caught by the except. apologize raises a new error before the raise is reached. Therefore, the raise in the except clause is never executed, and only the apology’s error propagates upward. If apologize raises an error, Python has no way of knowing that you were going to raise a different exception after apologize.

Note that in Python 3, the traceback will mention both exceptions, with a message explaining how the second one arose:

Traceback (most recent call last):
  File "./prog.py", line 9, in <module>
  File "./prog.py", line 2, in failure
ValueError: Real error

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./prog.py", line 11, in <module>
  File "./prog.py", line 5, in apologize
TypeError: Apology error

However, the second exception (the “apology” exception) is still the only one that propagates outward and can be caught by a higher-level except clause. The original exception is mentioned in the traceback but is subsumed in the later one and can no longer be caught.