Better to 'try' something and catch the exception or test if it's possible first to avoid an exception?

You should prefer try/except over if/else if that results in

• speed-ups (for example by preventing extra lookups)
• cleaner code (fewer lines/easier to read)

Often, these go hand-in-hand.

---

speed-ups

In the case of trying to find an element in a long list by:

try:
    x = my_list[index]
except IndexError:
    x = 'NO_ABC'

the try, except is the best option when the index is probably in the list and the IndexError is usually not raised. This way you avoid the need for an extra lookup by if index < len(my_list).

Python encourages the use of exceptions, which you handle is a phrase from Dive Into Python. Your example not only handles the exception (gracefully), rather than letting it silently pass, also the exception occurs only in the exceptional case of index not being found (hence the word exception!).

---

cleaner code

The official Python Documentation mentions EAFP: Easier to ask for forgiveness than permission and Rob Knight notes that catching errors rather than avoiding them, can result in cleaner, easier to read code. His example says it like this:

Worse (LBYL 'look before you leap'):

#check whether int conversion will raise an error
if not isinstance(s, str) or not s.isdigit():
    return None
elif len(s) > 10:    #too many digits for int conversion
    return None
else:
    return int(s)

Better (EAFP: Easier to ask for forgiveness than permission):

try:
    return int(s)
except (TypeError, ValueError, OverflowError): #int conversion failed
    return None

In this particular case, you should use something else entirely:

x = myDict.get("ABC", "NO_ABC")

In general, though: If you expect the test to fail frequently, use if. If the test is expensive relative to just trying the operation and catching the exception if it fails, use try. If neither one of these conditions applies, go with whatever reads easier.

Using try and except directly rather than inside an if guard should always be done if there is any possibility of a race condition. For example, if you want to ensure that a directory exists, do not do this:

import os, sys
if not os.path.isdir('foo'):
  try:
    os.mkdir('foo')
  except OSError, e
    print e
    sys.exit(1)

If another thread or process creates the directory between isdir and mkdir, you'll exit. Instead, do this:

import os, sys, errno
try:
  os.mkdir('foo')
except OSError, e
  if e.errno != errno.EEXIST:
    print e
    sys.exit(1)

That will only exit if the 'foo' directory can't be created.