Why use Python's "else" clause in try/except block?

Question

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?

What if the line `getpass = AskPassword` could raise an `ImportError` itself? — mgilson, Jan 29 '13 at 18:55
Dup? [Python try-else](http://stackoverflow.com/questions/855759/python-try-else) — Izkata, Jan 29 '13 at 18:57
The problem with this is that you end up using your exceptions for controlflow instead of for error handling. — Randall Hunt, Jan 29 '13 at 18:57
@Izkata I think you can hit the close button on this one without asking for second opinions :P... — Maarten Bodewes, Jan 29 '13 at 18:59
@owlstead I don't have close-vote privilege yet on this site — Izkata, Jan 29 '13 at 19:01
@Izkata whoops, ok, good catch in that case... hitting close... — Maarten Bodewes, Jan 29 '13 at 19:02
@owlstead I may have been too fast anyway: that question's answers are largely about functionality; my answer is about style — Izkata, Jan 29 '13 at 19:03
In this case the else clause verbose because you can do `from EasyDialogs import AskPassword as getpass` — Steven Rumbalski, Jan 29 '13 at 19:06
Considering that this is python, its probably just a matter of readability and logical flow. It's more pythonic because it breaks into blocks rather than relying on the manner in which rasing an error redirects flow to dictate your logic. — ApproachingDarknessFish, Jan 29 '13 at 19:08
Izkata -- Thanks for finding the same question elsewhere; it had good answers. — odigity, Jan 29 '13 at 19:11

score 5 · Accepted Answer · edited Mar 07 '18 at 17:51

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

It's still a dupe I think, but as for code style I'll vote up. Better add it to the duplicates answers though. — Maarten Bodewes, Jan 29 '13 at 19:10

score 2 · Answer 2 · answered Jan 29 '13 at 19:04

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]"

answered Jan 29 '13 at 19:04

dmg

7,438
2
24
33

Problem is I don't like that ordering. I think the intended/optimistic code path should be prominent, and exception handling afterwards. This pattern puts it in the middle, between the two halves of the primary paths. Doesn't sit well with me. – odigity Jan 29 '13 at 19:13
Well it can be used to distinguish the parts of the code that can and can't throw an exception. But this is also in a way similar to the situation where you have a function which is something like "if a then b else c" and the question being whether the "common case" (which is a lot longer and has more logic in it) should be the "then" block or the "else" block? – dmg Jan 29 '13 at 19:18
Also have a look at http://stackoverflow.com/questions/855759/python-try-else – dmg Jan 29 '13 at 19:27

Why use Python's "else" clause in try/except block?

2 Answers2

Linked