Generic Exception Handling in Python the “Right Way”

Question

Sometimes I find myself in the situation where I want to execute several sequential commands like such:

try:
    foo(a, b)
except Exception, e:
    baz(e)
tr         


        

Answer 1

If they're simple one-line commands, you can wrap them in lambdas:

for cmd in [
    (lambda: foo (a, b)),
    (lambda: bar (c, d)),
]:
    try:
        cmd ()
    except StandardError, e:
        baz (e)

You could wrap that whole thing up in a function, so it looked like this:

ignore_errors (baz, [
    (lambda: foo (a, b)),
    (lambda: bar (c, d)),
])

Answer 2

The best approach I have found, is to define a function like such:

def handle_exception(function, reaction, *args, **kwargs):
    try:
        result = function(*args, **kwargs)
    except Exception, e:
        result = reaction(e)
    return result

But that just doesn't feel or look right in practice:

handle_exception(foo, baz, a, b)
handle_exception(bar, baz, c, d)

Answer 3

In your specific case, you can do this:

try:
    foo(a, b)
    bar(c, d)
except Exception, e:
    baz(e)

Or, you can catch the exception one step above:

try:
    foo_bar() # This function can throw at several places
except Exception, e:
    baz(e)

Answer 4

You could use the with statement if you have python 2.5 or above:

from __future__ import with_statement
import contextlib

@contextlib.contextmanager
def handler():
    try:
        yield
    except Exception, e:
        baz(e)

Your example now becomes:

with handler():
    foo(a, b)
with handler():
    bar(c, d)

Answer 5

If this is always, always the behaviour you want when a particular function raises an exception, you could use a decorator:

def handle_exception(handler):
    def decorate(func):
        def call_function(*args, **kwargs):
            try:
                func(*args, **kwargs)
            except Exception, e:
                handler(e)
        return call_function
    return decorate

def baz(e):
    print(e)

@handle_exception(baz)
def foo(a, b):
    return a + b

@handle_exception(baz)
def bar(c, d):
    return c.index(d)

Usage:

>>> foo(1, '2')
unsupported operand type(s) for +: 'int' and 'str'
>>> bar('steve', 'cheese')
substring not found

Answer 6

You could try something like this. This is vaguely C macro-like.

class TryOrBaz( object ):
    def __init__( self, that ):
        self.that= that
    def __call__( self, *args ):
        try:
            return self.that( *args )
        except Exception, e:
            baz( e )

TryOrBaz( foo )( a, b )
TryOrBaz( bar )( c, d )