Question

Should I test if something is valid or just try to do it and catch the exception?

• Is there any solid documentation saying that one way is preferred?
• Is one way more pythonic?

For example, should I:

if len(my_list) >= 4:
    x = my_list[3]
else:
    x = 'NO_ABC'

Or:

try:
    x = my_list[3]
except IndexError:
    x = 'NO_ABC'

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Some thoughts…
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?

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I’m not referring to situations where you can only do things 1 way; for example:

try:
    import foo
except ImportError:
    import baz

Answer 1

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.

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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!).

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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