[Python-ideas] try-else without except or finally

[Matt Joiner]

Your examples of else with loops are missing the extra behaviour on break?
This is where the real value is in this usage.
On Nov 11, 2011 9:11 AM, "Terry Reedy" <tjreedy at udel.edu> wrote:

> On Thu, Nov 10, 2011 at 10:53 AM, Guido van Rossum<guido at python.org>
>>  wrote:
>>
>>> On Thu, Nov 10, 2011 at 5:35 AM, Rob Cliffe<rob.cliffe at btinternet.**com<rob.cliffe at btinternet.com>>
>>>  wrote:
>>>
>>>> "except: raise" makes it explicit that an exception is to be propogated.
>>>>  Far from being pointless it makes the code much more readable.
>>>>  "Explicit
>>>> is better than implicit."
>>>>
>>>
>>> +1. Nobody's going to understand try...else... on first reading.
>>>
>>
> Perhaps because an else: in this context, without being subordinate to
> anything, would be irrelevant, equivalent to 'pass' or, more exactly, 'if
> True:'.
>
> Else is always subordinate to some conditional clause$. The 'if' clause
> for try statements is the first except clause, not the try header: 'except
> x:' means something like 'if raised(x):' (since the corresponding try setup
> statement). The else clause is actually an elif clause: 'else:' means 'elif
> nothing_raised():'.
>
> To illustrate, the C equivalent of
>
> try:
>  a = b/c
> except ZeroDivisionError:
>  print('bah')
> else:
>  print('hooray')
>
> would be (minus the punctuation that I have forgotten) something like
>
> errno = 0  /* setup error detection mechanism, just as try: does */
> a = b/c
> if errno == EZERODIV  /* or whatever it is */
>  printf('bah')
> elif errno == 0  /* ie, no error since setup */
>  printf('hooray')
>
>  I suspect Python *could* have been defined to require 'except
> NOEXCEPTION:' instead of 'else:' by initializing an internal exception
> variable (equivalent to C's errno) with NOEXCEPTION. Howver, 'else:' is
> definitely nicer (just as normal 'else:' is equivalent to and nicer than
> 'elif True:').
>
>
> $ A for loop is equivalent to a while loop with condition z"iterator
> (initially iter(iterable)) is not empty". Break and continue have the same
> meaning as in while loops.
>
> Any while loop, such as
>
> while c:
>  x
> else:
>  y
>
> is equivalent to an if statement with goto. In
> 'Python-with-label-and-goto' the above would be the same as
>
> label start_loop
> if c:
>  x
>  goto start_loop
> else:
>  y
>
> So: in Python (as elsewhere), 'else' clauses always follow one or more
> headers that specify conditional execution and only trigger when the
> previous condition or conditions are false. Their use in various statement
> types is consistent and logically correct.
>
> --
> Terry Jan Reedy
>
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