Why use Python's “else” clause in try/except b

问题:

This question already has answers here:

Closed 7 years ago.

Possible Duplicate:
Python try-else

I'm not seeing the benefit of it, at least based on the example I just read in Dive Into Python:

try:
    from EasyDialogs import AskPassword
except ImportError:
    getpass = default_getpass
else:
    getpass = AskPassword

(http://www.diveintopython.net/file_handling/index.html)

Why couldn't you achieve the same effect with the shorter/simpler:

try:
    from EasyDialogs import AskPassword
    getpass = AskPassword
except ImportError:
    getpass = default_getpass

What am I missing?

回答1:

There isn't an advantage in the example, except possibly for style. It's generally a good idea to keep code that can cause exceptions near the code that deals with them. For example, compare these:

try:
    from EasyDialogs import AskPassword
    # 20 other lines
    getpass = AskPassword
except ImportError:
    getpass = default_getpass

and

try:
    from EasyDialogs import AskPassword
except ImportError:
    getpass = default_getpass
else:
    # 20 other lines
    getpass = AskPassword

The second one is good when the except can't return early, or re-throw the exception. If possible, I would have written:

try:
    from EasyDialogs import AskPassword
except ImportError:
    getpass = default_getpass
    return False // or throw Exception('something more descriptive')

# 20 other lines
getpass = AskPassword

回答2:

I personally find it clearer in some situations. Naturally the greater deal of code should be ran when an exception does not occur. So in a way you are saying:

try:
    this_very_dangerous_call()
except ValueError:
    # if it breaks
    handle_value_error()
else:
    continue_with_my_code()
    # more code

Thus you are visually separating the exception handling code from the rest of the code. It's like saying: "Try this, if it breaks do something, if it doesn't [insert long explanation here]"