[Python-ideas] exception based conditional expression, similar to if-else conditional expression

[Jeff McAninch]

I would like to propose an expression, similar to the if-else expression,
that responds to exceptions.

I had originally posted this (probably mistakenly) on py-dev.  This 
current posting is a cleaned up
version of the idea, based on responses I got on from the earlier posting.

_*Abstract:
*_Proposal for a conditional expression, similar to the if-else 
expression, that responds to exceptions.

_*Motivation:
*_An expression syntax that responds to exceptions, and which reproduces 
the readability and conciseness of the if-else conditional expression, 
would simplify some exception-handling cases, especially within list 
comprehensions.

_*Very Simple Example - type coercion:
*_Current approach:
    try:
        x = float(string)
    except:
        x = float('nan')

Proposed solution using exception-based conditional expression:
    x = float(string) except ValueError: float('nan')


_*Simple Example - type coercion in a list comprehension:
*_Current approach:
    def safe_float(string):
        try:
            x = float(string)
        except ValueError:
            x = float('nan')
        return x
    ...
    xs = (safe(float(string)) for string in strings)

Proposed solution using exception-based conditional expression:
    xs = ((float(string) except ValueError: float('nan')) for string in 
strings)

_*Discussion:
*_In my own python coding, I find I make common use of the if-else 
conditional expression, especially within list comprehensions.  (In one 
of my packages, which has ~5800 lines of code, I found if-else 
expressions in ~1% of the lines.)

Here is a slightly more involved example than the examples presented 
above.  In data processing, I often string together a sequence of 
iterable list comprehensions, corresponding to a sequence of operations 
on a given dataset "ys" to produce a processed dataset "x":
    xs = (operation_A(x) for x in ys)
    xs = (operation_B(x) for x in xs if filter_B(x))
    xs = (operation_C(x) if (some_condition(x)) else operation_D(x) for 
x in xs if filter_C(x))
    # final, explicit list of values
    xs = [ x for x in xs ]
This is often a flexible way for me to define processing and filtering 
sequences which also seems
to have good performance on very large datasets.  One advantage is that 
I can quickly mix-and-match from existing processes like this to make a 
new process.  An exception-based conditional would go nicely
into many of these process sequences, keeping them both robust and flexible.
    xs = (operation_N(x) except exceptionN: operation_Nprime(x) for x in xs)

I also often have object classes which have some common method or 
attribute.  For instance, some of my objects have scope-dependent values:
    x = y.evaluate(scope))
where scope is typically locals(), globals(), or some other 
dictionary-like container.  But, to keep my code modular, I want to 
handle, in the same lines of code, objects which do not have some 
particular method, which leads me to lines of code like:
    x = y.evaluate(locals()) if ('evaluate' in y.__dict__) else y
This seems not very "Pythonic", similar to using type-testing instead of 
try-except.  (My impression was that there was a long-standing trend in 
the evolution of Python to remove tests like this, and I thought that 
was the original motivation for the try-except syntax.)

I would much rather write:
    x = y.evaluate(locals()) except AttributeError: y
or, in the list comprehension example:
    xs = (y.evaluate(locals()) except AttributeError: y for y in ys)

Clearly this can be handled in several ways with the language as it is.  
One way is to define a new function, as in the second simple example above:
    def safe_evaluate(y,scope):
       try:
          x = y.evaluate(scope)
       except AttributeError:
          x = y
       return x
    ...
    xs = (safe_evaluate(y,locals()) for y in ys)
but this quickly (in my packages at least) leads to an annoying 
proliferation of "safe_" functions.
Again, this seems not to be in the "Pythonic" spirit, and is also less 
concise, less readable.  (I also suspect, but have not verified, that 
this is in general less efficient than in-line expressions -- wasn't 
that part of the original motivation for list comprehensions?).

In the thread of my previous post to py-dev, there were comments, 
questions, and suggestions concerning the details of the syntax.  Having 
reflected on this for a couple weeks, I am now most strongly supportive 
of what is essentially just an inline compression of the current 
try-except syntax.  So the following examples would be allowed:
    x = expression0 except: default_expression
    x = expression0 except exception1: expression1 except exception2: 
expression2 except: default_expression

Or, more generally:
    x = expression0\
            except exception1: expression1\
            except exception2: expression2\
            ...
            except exceptionI: expressionI\
            ...
            except: default_expression
In this last example, the behaviour would be as follows:
    - evaluate expression0. 
            If no exception is encountered, return the result.
    - if an exception is encountered,
            search for the matching exception in the except clauses.
    - if a matching exception ("exceptionI") is found,
            evaluate the corresponding expression ("expressionI"), and 
return the result.
    - if no matching exception is found, and a default except: clause 
(i.e., one without and exception)
          is given, evaluate default_expression, and return the result.
    - if no matching exception is found, and no default except clause if 
given,
          pass the exception on to the caller.
    - if a new exception is encountered while evaluating an an except 
expression ("expressionI"),
          pass the exception on to the caller.

I hope I have made a convincing case here.  This seems to me to be a 
natural ("Pythonic") addition to the language.

Jeff McAninch

-- 
==========================
Jeffrey E. McAninch, PhD
Physicist, X-2-IFD
Los Alamos National Laboratory
Phone: 505-667-0374
Email: mcaninch at lanl.gov
==========================

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