Should I test
if something is valid or just
try to do it and catch the exception?
For example, should I:
if len(my_list) >= 4: x = my_list else: x = "NO_ABC"
try: x = my_list except IndexError: x = "NO_ABC"
PEP 20 says:
Errors should never pass silently.
Unless explicitly silenced.
Should using a
try instead of an
if be interpreted as an error passing silently? And if so, are you explicitly silencing it by using it in this way, therefore making it OK?
I"m not referring to situations where you can only do things 1 way; for example:
try: import foo except ImportError: import baz
You should prefer
if/else if that results in
Often, these go hand-in-hand.
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!).
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