Re: [Python-ideas] suggestion for try/except program flow

On Sat, Mar 28, 2009 at 9:54 AM, Mark Donald wrote:

Unless something has changed in Python 3+, I believe Nick's idiom requires the generic handler code to be copied into a second except clause to achieve identical behaviour, as follows...

except BaseException as e:    if isinstance(e, Cheese):        # handle cheese    # handle all manner of stuff, including cheese except:    # handle all manner of OTHER stuff in the same way

In 3.0, the second except clause is unreachable because all exceptions inherit from BaseException.

...which makes the nested try block more semantic. This can be tested by putting > raise "this is deprecated..."  into the try block, which generates an error that is NOT caught by "except BaseException" however IS caught by "except".

I realise that "recatch" may seem like a trivial suggestion, but I believe it would be a slight improvement, which means it's not trivial. Try/except statements are supposedly preferential to "untried" if statements that handle isolated cases, but if statements are a lot easier to imagine, so people use them all them time - style be damned. Each slight improvement in try/except is liable to make it more attractive to coders, the end result being better code (for example, the implementation of PEP 341 increased my team's use of try/except significantly, causing a huge improvement to code readability).

Imagine if you could do this:

   runny = True    runnier_than_you_like_it = False    raise Cheese except Cheese, e    # handle cheese    if runny: recatch as Camembert except Camembert, e    # handle camembert    if runnier_than_you_like_it:        recatch    else:        uncatch # ie, else clause will be effective... except:    # not much of a cheese shop, really is it? else:    # negotiate vending of cheesy comestibles finally:    # sally forth

And, I'm not trying to be belligerent here, but before somebody says...

Cheese(Exception) Camembert(Cheese)

...just please have a little think about it.

Mark

2009/3/28 Guido van Rossum :

...

On Sat, Mar 28, 2009 at 5:06 AM, Mark Donald wrote:

...

In this situation, which happens to me fairly frequently...

try:  try:     raise Cheese  except Cheese, e:     # handle cheese     raise except:  # handle all manner of stuff, including cheese

...it would be nice (& more readable) if one were able to recatch a named exception with the generic (catch-all) except clause of its own try, something like this:

try:  raise Cheese except Cheese, e:  # handle cheese  recatch except:  # handle all manner of stuff, including cheese

I'm not sure recatch is all that more reasonable -- it's another fairly obscure control flow verb. I think the current situation isn't so bad. Nick already pointed out an idiom for doing this without two try clauses:

except BaseException as e:  if isinstance(e, Cheese):    # handle cheese  # handle all manner of stuff, including cheese

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

-- --Guido van Rossum (home page: http://www.python.org/~guido/)