[Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)

ilya ilya.nikokoshev at gmail.com
Fri Aug 7 13:06:11 CEST 2009

I believe people now discuss this both on python-dev and python-ideas,
though since I'm new to both lists, I can't really tell where this

I played a little with this syntax, my try_ function and @catch
decorator (which are at http://mit.edu/~unknot/www/try_cond.py):

    #   x = float(string) except float('nan') if ValueError
    x = try_(float, string, except_ = float('nan'), if_ = ValueError)

    @catch(ValueError = float('nan'))
    def x1(): return float(string)

    #   y = float(string) except ValueError: float('nan')
    y = try_(float, string, { ValueError: float('nan') })

    @catch({ValueError: float('nan')})
    def y1(): return float(string)

    #   try:
    #       z = open(string, 'r')
    #   except IOError as e:
    #       if e.errno == 2:
    #               z = 'not_exist'
    #       else:
    #               raise
    z = try_(open, string, 'r', iocatcher({2: 'no file!'}))

    @catch(iocatcher({2: 'nothing!'}))
    def z1(): return open(string, 'r')

Here are my overall feelings:

(1) it would be interesting to come up with syntax for except/if
clause, but it's not obvious how to make one and this fact itself may
kill the idea.
(2) the more reasonable approach to things like this is by defining a
separate block and then performing a "catch" operation with it.
Unfortunately, this looks very clumsy as currently this can only be
done by defining a separate function. I think code blocks are a good
direction to explore.

2009/8/7 Kristján Valur Jónsson <kristjan at ccpgames.com>:
> Unless I am very much mistaken, this is the approach Ruby takes.
> Everything is an expression.  For example, the value of a block is the value of
> The last expression in the block.
> I've never understood the need to have a distinction betwen statements and expressions, not when expressions can have side effects.  It's like that differentce between procedures and functions in pascal that only serves to confuse
> K
>> -----Original Message-----
>> From: python-dev-bounces+kristjan=ccpgames.com at python.org
>> [mailto:python-dev-bounces+kristjan=ccpgames.com at python.org] On Behalf
>> Of Xavier Morel
>> Sent: 6. ágúst 2009 10:25
>> To: python-dev at python.org
>> Subject: Re: [Python-Dev] (try-except) conditional expression similar
>> to (if-else) conditional (PEP 308)
>> Wouldn't it be smarter to fix the issue once and for all by looking
>> into making Python's compound statements (or even all statements
>> without restrictions) expressions that can return values in the first
>> place? Now I don't know if it's actually possible, but if it is the
>> problem becomes solved not just for try:except: (and twice so for
>> if:else:) but also for while:, for: (though that one's already served
>> pretty well by comprehensions) and with:.
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