为什么Exception（str（））抛出异常？

Question

In a CLI application that may or may not be run with a debug parameter, I am catching an exception and selectively rethrowing it:

try:
    doSomething()
except Exception as e:
    if debug==True:
        raise Exception(str(e))

Interestingly, the raise Exception() code itself is throwing this:

Traceback (most recent call last):
  File "./app.py", line 570, in getSomething
    raise Exception(str(e))
Exception: expected string or buffer

Would not str(e) return a string? I could only imagine that perhaps it is returning None so I tried a general Exception (as seen in the code) hoping that it would never be None. Why might e not be castable to string ?

Answer 1

I think you are misunderstanding the Exception message.

In your doSomething , an exception raised, the exception is expected string or buffer . Then you use that string to re-throw an exception. And you do not catch this exception. So, the interpreter stops and print the message.

>>> try:
...     raise Exception('expected string or buffer')
... except Exception as e:
...     raise Exception(str(e))
... 
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
Exception: expected string or buffer
>>> 

Answer 2

As a side note, if you want to re-throw an exception, a stand-alone raise statement will do it, which raises the last raised exception. This will give you the actual error as well, rather than just passing the message to Exception .

try:
    doSomething()
except:  # except by itself catches any exception
         # better to except a specific error though
    if debug:  # use implicit truth check of `if`
        raise  # re-raise the caught exception

Also, note that everything can be converted to a string (unless you explicitly say it can't ).