comparison with type of None variables

Question

I know that comparison of types is not recommended, but I have some code that does this in an if elif series. However, I am confused as to how None values work.

def foo(object) otype = type(object) #if otype is None: # this doesn't work if object is None: # this works fine print("yep") elif otype is int: elif ...

How come I can compare just fine with is int and so forth, but not with is None? types.NoneType seems to be gone in Python 3.2, so I can't use that...

The following

i = 1 print(i) print(type(i)) print(i is None) print(type(i) is int)

prints

1 <class 'int'> False True

whereas

i = None print(i) print(type(i)) print(i is None) print(type(i) is None)

prints

None <class 'NoneType'> True False

I guess None is special, but what gives? Does NoneType actually exist, or is Python lying to me?

Silas Ray · Accepted Answer · 2012-11-05 19:12:42Z

None is a special-case singleton provided by Python. NoneType is the type of the singleton object. type(i) is None is False, but type(i) is type(None) should be true.

Andrew Clark · Accepted Answer · 2012-11-05 20:41:14Z

7

You should never need to compare to NoneType, because None is a singleton. If you have some object obj that might be None, just is obj is None or obj is not None.

The reason your comparison does not work is that None is not a type, it is a value. It would similar to trying type(1) is 1 in your integer example. If you really wanted to do a NoneType check, you could use type(obj) is type(None), or better yet isinstance(obj, type(None)).

edited Nov 5, 2012 at 20:41

answered Nov 5, 2012 at 19:12

Andrew Clark

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4 Comments

user395760 Over a year ago

Very true and important, but the other parts of the question deserve an answer too.

Silas Ray Over a year ago

Which makes checking None-ness a special case in any type-checking code. Sometimes, it is cleaner to check for NoneType than to check x is None or isinstance(x, (footype, bartype)).

Sven Marnach Over a year ago

Object identity is actually the right thing for checking for a type: type(obj) is SomeType is what I recommend for checking for exact type matches (a very rare requirement). In the common case, use isinstance().

Andrew Clark Over a year ago

@SvenMarnach Good to know! For some reason I thought type checking called for a value comparison.

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