catching exceptions from an except: block

[Steven D'Aprano]

On Wed, 07 Mar 2007 10:32:53 -0800, Arnaud Delobelle wrote:

> Hi all,
> 
> Imagine I have three functions a(x), b(x), c(x) that each return
> something or raise an exception.  Imagine I want to define a function
> that returns a(x) if possible, otherwise b(x), otherwise c(x),
> otherwise raise CantDoIt.
> 
> Here are three ways I can think of doing it:
> 
> ----------
> # This one looks ugly
> def nested_first(x):
>     try:
>         return a(x)
>     except:
>         try:
>             return b(x)
>         except:
>             try:
>                 return c(x)
>             except:
>                 raise CantDoIt


Exceptions are great, but sometimes they get in the way. This is one of
those times.

NULL = object() # get a unique value
def result_or_special(func, x):
    """Returns the result of func(x) or NULL."""
    try:
        return func(x)
    except Exception:
        return NULL

def failer(x):
    """Always fail."""
    raise CantDoIt

def function(x):
    funcs = (a, b, c, failer)
    for func in funcs:
        result = func(x)
        if result is not NULL: break
    return result


Or if you prefer:

def function(x):
    NULL = object()
    funcs = (a, b, c)
    for func in funcs:
        try:
            result = func(x)
        except Exception:
            pass
        else:
            break
    else:
        # we didn't break out of the loop
        raise CantDoIt
    # we did break out of the loop
    return result



-- 
Steven D'Aprano