Vorticity Vorticity - 2 years ago 151
Python Question

Use of True, False, and None as return values in python functions

I think that I fully understand this, but I just want to make sure since I keep seeing people say to NEVER EVER test against

, or
. They suggest that routines should raise an error rather than return False or None. Anyway, I have many situations where I simply want to know if a flag is set or not so my function returns True or False. There are other situations where I have a function return None if there was no useful result. From my thinking, neither is problematic so long as I realize that I should never use:

if foo == True
if foo == False
if foo == None

and should instead use:

if foo is True
if foo is False
if foo is None

since True, False, and None are all singletons and will always evaluate the way I expect when using "is" rather than "==". Am I wrong here?

Along the same lines, would it be more pythonic to modify the functions that sometimes return None so that they raise an error instead? Say I have an instance method called "get_attr()" that retrieves an attribute from some file. In the case where it finds that the attribute I requested does not exist is it appropriate to return None? Would it be better to have them raise an error and catch it later?

Ben Ben
Answer Source

The advice isn't that you should never use True, False, or None. It's just that you shouldn't use if x == True.

if x == True is silly because == is just a binary operator! It has a return value of either True or False, depending on whether its arguments are equal or not. And if condition will proceed if condition is true. So when you write if x == True Python is going to first evaluate x == True, which will become True if x was True and False otherwise, and then proceed if the result of that is true. But if you're expecting x to be either True or False, why not just use if x directly!

Likewise, x == False can usually be replaced by not x.

There are some circumstances where you might want to use x == True. This is because an if statement condition is "evaluated in boolean context" to see if it is "truthy" rather than testing exactly against True. For example, non-empty strings, lists, and dictionaries are all considered truthy by an if statement, as well as non-zero numeric values, but none of those are equal to True. So if you want to test whether an arbitrary value is exactly the value True, not just whether it is truthy, when you would use if x == True. But I almost never see a use for that. It's so rare that if you do ever need to write that, it's worth adding a comment so future developers (including possibly yourself) don't just assume the == True is superfluous and remove it.

Using x is True instead is actually worse. You should never use is with basic built-in immutable types like booleans (True, False), numbers, and strings. The reason is that for these types we care about values, not identity. == tests that values are the same for these types, while is always tests identities.

Testing identities rather than values is bad because an implementation could theoretically construct new boolean values rather than go find existing ones, leading to you having two True values that have the same value but are stored in different placed in memory and have different identities. In practice I'm pretty sure True and False are always re-used by the Python interpreter so this won't happen, but that's really an implementation detail. This issue trips people up all the time with strings, because short strings and literal strings that appear directly in the program source are recycled by Python so 'foo' is 'foo' always returns True. But it's easy to construct the same string 2 different ways and have Python give them different identities. Observe the following:

>>> stars1 = ''.join('*' for _ in xrange(100))
>>> stars2 = '*' * 100
>>> stars1 is stars2
>>> stars1 == stars2

EDIT: So it turns out that Python's equality on booleans is a little unexpected (at least to me):

>>> True is 1
>>> True == 1
>>> True == 2
>>> False is 0
>>> False == 0
>>> False == 0.0

The rationale for this, as explained in the notes when bools were introduced in Python 2.3.5, is that the old behaviour of using integers 1 and 0 to represent True and False was good, but we just wanted more descriptive names for numbers we intended to represent truth values.

One way to achieve that would have been to simply have True = 1 and False = 0 in the builtins; then 1 and True really would be indistinguishable (including by is). But that would also mean a function returning True would show 1 in the interactive interpreter, so what's been done instead is to create bool as a subtype of int. The only thing that's different about bool is str and repr; bool instances still have the same data as int instances, and still compare equality the same way, so True == 1.

So it's wrong to use x is True when x might have been set by some code that expects that "True is just another way to spell 1", because there are lots of ways to construct values that are equal to True but do not have the same identity as it:

>>> a = 1L
>>> b = 1L
>>> c = 1
>>> d = 1.0
>>> a == True, b == True, c == True, d == True
(True, True, True, True)
>>> a is b, a is c, a is d, c is d
(False, False, False, False)

And it's wrong to use x == True when x could be an arbitrary Python value and you only want to know whether it is the boolean value True. The only certainty we have is that just using x is best when you just want to test "truthiness". Thankfully that is usually all that is required, at least in the code I write!

A more sure way would be x == True and type(x) is bool. But that's getting pretty verbose for a pretty obscure case. It also doesn't look very Pythonic by doing explicit type checking... but that really is what you're doing when you're trying to test precisely True rather than truthy; the duck typing way would be to accept truthy values and allow any user-defined class to declare itself to be truthy.

If you're dealing with this extremely precise notion of truth where you not only don't consider non-empty collections to be true but also don't consider 1 to be true, then just using x is True is probably okay, because presumably then you know that x didn't come from code that considers 1 to be true. I don't think there's any pure-python way to come up with another True that lives at a different memory address (although you could probably do it from C), so this shouldn't ever break despite being theoretically the "wrong" thing to do.

And I used to think booleans were simple!

End Edit

In the case of None, however, the idiom is to use if x is None. In many circumstances you can use if not x, because None is a "falsey" value to an if statement. But it's best to only do this if you're wanting to treat all falsey values (zero-valued numeric types, empty collections, and None) the same way. If you are dealing with a value that is either some possible other value or None to indicate "no value" (such as when a function returns None on failure), then it's much better to use if x is None so that you don't accidentally assume the function failed when it just happened to return an empty list, or the number 0.

My arguments for using == rather than is for immutable value types would suggest that you should use if x == None rather than if x is None. However, in the case of None Python does explicitly guarantee that there is exactly one None in the entire universe, and normal idiomatic Python code uses is.

Regarding whether to return None or raise an exception, it depends on the context.

For something like your get_attr example I would expect it to raise an exception, because I'm going to be calling it like do_something_with(get_attr(file)). The normal expectation of the callers is that they'll get the attribute value, and having them get None and assume that was the attribute value is a much worse danger than forgetting to handle the exception when you can actually continue if the attribute can't be found. Plus, returning None to indicate failure means that None is not a valid value for the attribute. This can be a problem in some cases.

For an imaginary function like see_if_matching_file_exists, that we provide a pattern to and it checks several places to see if there's a match, it could return a match if it finds one or None if it doesn't. But alternatively it could return a list of matches; then no match is just the empty list (which is also "falsey"; this is one of those situations where I'd just use if x to see if I got anything back).

So when choosing between exceptions and None to indicate failure, you have to decide whether None is an expected non-failure value, and then look at the expectations of code calling the function. If the "normal" expectation is that there will be a valid value returned, and only occasionally will a caller be able to work fine whether or not a valid value is returned, then you should use exceptions to indicate failure. If it will be quite common for there to be no valid value, so callers will be expecting to handle both possibilities, then you can use None.

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