Python try-else
The statements in the else
block are executed if execution falls off the bottom of the try
- if there was no exception. Honestly, I've never found a need.
However, Handling Exceptions notes:
The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try ... except statement.
So, if you have a method that could, for example, throw an IOError
, and you want to catch exceptions it raises, but there's something else you want to do if the first operation succeeds, and you don't want to catch an IOError from that operation, you might write something like this:
try:
operation_that_can_throw_ioerror()
except IOError:
handle_the_exception_somehow()
else:
# we don't want to catch the IOError if it's raised
another_operation_that_can_throw_ioerror()
finally:
something_we_always_need_to_do()
If you just put another_operation_that_can_throw_ioerror()
after operation_that_can_throw_ioerror
, the except
would catch the second call's errors. And if you put it after the whole try
block, it'll always be run, and not until after the finally
. The else
lets you make sure
- the second operation's only run if there's no exception,
- it's run before the
finally
block, and - any
IOError
s it raises aren't caught here
There is one big reason to use else
- style and readability. 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
Note: Answer copied from recently-posted duplicate here, hence all this "AskPassword" stuff.
One use: test some code that should raise an exception.
try:
this_should_raise_TypeError()
except TypeError:
pass
except:
assert False, "Raised the wrong exception type"
else:
assert False, "Didn't raise any exception"
(This code should be abstracted into a more generic test in practice.)