On Fri, 24 Sep 2010 05:32:55 am Alex Hall wrote:
> Hi all,
> A general coding question: is it better to use return(False) (or 0,
> or -1, or whatever) or to raise whateverError("oops")? Are there
> cases for each?
Yes.
There is absolutely no point whatsoever raising an exception, or
returning a "magic" sentinel value, if an ordinary value would do. For
example, this would be silly:
def is_spam(string):
"""Return True if string equals "spam", otherwise
raise an exception."""
if string.lower().strip() == "spam":
return True
raise ValueError("not spam")
If is_spam can only ever return True, there's no need to look at the
return result. So why bother to return anything? This would be better:
def verify_spam(string):
"""Raise ValueError unless string is spam."""
if string.lower().strip() != "spam":
raise ValueError("not spam")
(Technically, this will return None, but that's just because Python
doesn't have procedures, only functions.)
This is appropriate if your use-case is something like this:
def prepare_meal(meat, condiments):
"""Prepare a yummy meal from the only meat-like substance
worth eating."""
verify_spam(meat)
if "pickle" in condiments:
# I don't like pickle
condiments.remove("pickle")
# but I do like spam
condiments.append("mashed spam")
return "spam" + spread(condiments, meat) + "spam spam spam"
On the other hand, returning True or False would be better if your
use-case was like this:
def prepare_meal(meat, condiments):
"""Prepare a yummy meal."""
if is_spam(meat):
vikings.sing("spam spam spam LOVELY SPAM!!!")
if "pickle" in condiments:
condiments.remove("pickle")
return "bread" + spread(condiments, meat) + "bread"
A good guide is to look at what functions in the standard library do.
You'll find examples of functions that return a magic value or raise an
exception:
"ethel the aardvark".find('spam')
=> -1
"ethel the aardvark".index('spam')
=> raises exception
Of course, functions should generally raise exceptions for errors:
"ethel the aardvark".find(42)
=> raises exception
There are very few places in Python that suppress and hide arbitrary
errors from the user. This should be done with care, if at all.
--
Steven D'Aprano
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