Mailman 3 (try-except) conditional expression similar to (if-else) conditional (PEP 308) - Python-Dev

6 Aug 2009

      I took a look at the options 1 and 2:

    x = float(string) except float('nan') if ValueError
    y = float(string) except ValueError: float('nan')

and I think this can be done just as easily with existing syntax:

    x = try_1(float, string, except_ = float('nan'), if_ = ValueError)
    y = try_2(float, string, { ValueError: float('nan') })

Here's the full example:

----- example starts -----

def try_1(func, *args, except_ = None, if_ = None):
    try:
        return func(*args)
    except if_ as e:
        return except_

def try_2(func, *args):
    'The last argument is a dictionary {exception type: return value}.'
    dic = args[-1]
    try:
        return func(*args[:-1])
    except Exception as e:
        for k,v in dic.items():
            if isinstance(e, k):
                return v
        raise

for string in ['5', 'five']:
    #   x = float(string) except float('nan') if ValueError
    x = try_1(float, string, except_ = float('nan'), if_ = ValueError)
    #   y = float(string) except ValueError: float('nan')
    y = try_2(float, string, { ValueError: float('nan') })
    print(x, y)

----- example ends -----

As a side note, if I just subscribed to python-dev, is it possible to
quote an old email? Below is my manual cut-and-paste quote:

---------- my quote --------------

Nick Coghlan wrote:
...
P.J. Eby wrote:
...
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
...
[Jeffrey E. McAninch, PhD]
...
I very often want something like a try-except conditional expression
similar
to the if-else conditional.
An example of the proposed syntax might be:
   x = float(string) except float('nan')
or possibly
   x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a different
meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not "as"
contradictory, IMO...  ;-)
(We're probably well into python-ideas territory at this point, but I'll
keep things where the thread started for now)
The basic idea appears sound to me as well. I suspect finding an
acceptable syntax is going to be the sticking point.
Breaking the problem down, we have three things we want to separate:
1. The expression that may raise the exception
2. The expression defining the exceptions to be caught
3. The expression to be used if the exception actually is caught
...
From there it is possible to come up with all sorts of variants.
Option 1:
Change the relative order of the clauses by putting the exception
definition last:
x = float(string) except float('nan') if ValueError
  op(float(string) except float('nan') if ValueError)
I actually like this one (that's why I listed it first). It gets the
clauses out of order relative to the statement, but the meaning still
seems pretty obvious to me.
A further extension (if we need it):

     result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan')

The 'else' part handles any other exceptions (not necessarily a good idea!).

or:

     result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan') if ValueError

Handles a number of different exceptions.
...
Option 2:
Follow the lamba model and allow a colon inside this form of expression:
x = float(string) except ValueError: float('nan')
  op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but
embedding colons inside expressions is somewhat ugly. Yes, lambda
already does it, but lambda can hardly be put forward as a paragon of
beauty.
A colon is also used in a dict literal.
...
Option 3a/3b:
Raymond's except-else suggestion:
x = float(string) except ValueError else float('nan')
  op(float(string) except ValueError else float('nan'))
[snip]
-1

(try-except) conditional expression similar to (if-else) conditional (PEP 308)

ilya

Eric Pruitt

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