Re: comparison with None

Steven Howe wrote:
Steven D'Aprano wrote:
On Thu, 19 Apr 2007 08:18:30 -0400, Steve Holden wrote:

Which is why I suggested using the explicit type(x) == types.NoneType as opposed to
x is None


This seems to go entirely against the spirit of the language. It's about as sensible as writing

(3 > 4) == True


Please! For extra certainty, you should write that as:

((int(3) > int(4)) == True) == True

Explicit is better than sensible, yes?

*wink*



Your example, even with the *wink*, is stupid. The language requires 3 to be an integer, 4 to be an integer.

It also requires the reader to have a sense of humor. Where did you misplace yours? Come on, it was clearly meant to be a light-hearted poke, don't take life so seriously.

The point I was show is with respect to a returned variable (like from a function or method? *wink* *wink*).
For example, if you expect an open file handle, but get a NoneType because you didn't really open a file (having given a bad name or maybe didn't have permission to open a file), then it would be best to test the type of return object before using it. Then you program could handle the error gracefully (*wink* *wink* *wink*).

Here you are actually arguing for the "is" test without realizing it. Since "is" actually checks for object identity ("do my two operands occupy the same memory?") it has no need to check the types of its operands.

If there's any possibility that you might have a NoneType (of which, as I appear to have to keep reminding you, there is precisely ONE instance, so why not just say "None", since it's the ONLY NoneType object in the known universe) the correct paradigm is to guard the code with

if f:
...

I am aware that this opens the gates to the hordes who will now pile in pointing out that this will technique will fail if f is zero (integer, real or complex) or one of the many other items that evaluates to false in a Boolean context. I merely point out that none of those things are a file, and that to allow f to be *either* a file *or* one of those other things is inviting trouble in a very big way.

However, if the hordes *insist* (as the hordes tend to, especially when partial to salty snails), the very *most* you need is

if f is None:
...

and anything else is overkill that harms the readability of your program. It's that poor readability that Steven D'Aprano was making fun of in his remarks above, which in truth only echoes what I felt like but refrained from writing when I pointed out the redundancy in the expression (3 > 4) == True.

As much as I love Python, it's ability to morph an object type can be a pain. Testing before using can prevent a program from Error-ing out.

You still don't convince me that there is there is *any* value in checking the type of an object to determine whether the object is None. If its type is types.NoneType it *must* be None. If it isn't None then no harm can be done by an identity comparison with None, since the type of the operands is irrelevant to "is".

regards
Steve

PS: Revision question: How many objects of type NoneType are there?
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