Problem with Python's is None test?

Question

I like using the is None test for empty variables, it's very flexible, easy and useful. It seems to have stopped working now:

>"" is None
False

>[] is None
False

>{} is None
False

What's going on?

I'm using Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40) [GCC 4.4.5], on Debian/Sid i686 GNU/Linux.

Edit: Awesome tip from Sven Marnach of using bool(""). brb, off to edit some code...

This never should have worked...is None isn't for testing empty variables, it's for testing precisely if they are None. — Nick Bastin
– Nick Bastin, Commented Jun 27, 2011 at 18:22

Sven Marnach · Accepted Answer · 2011-06-27 18:31:05Z

7

The test x is None tests if x really is the None object (that is, if the name x references the object None). What you are looking for is truth value testing:

if "":
    print "non-empty"
else:
    print "empty"

if implicitly converts the condition to bool. You can also explicitly do this:

>>> bool("")
False
>>> bool("x")
True

edited Jun 27, 2011 at 18:31

answered Jun 27, 2011 at 18:21

Sven Marnach

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Comments

senderle · Accepted Answer · 2011-06-27 18:22:35Z

2

Um... that never worked. is tests for instance identity -- even {} is {} is false.

>>> {} is {}
False

answered Jun 27, 2011 at 18:22

senderle

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Comments

inspectorG4dget · Accepted Answer · 2011-06-27 18:23:34Z

1

Only None is None. I think what you are looking for is the boolean values of the empty list, dict, and str

For example:

>>> if "":
...     print 'woo'
... else:
...     print 'hoo'
...
hoo

Same for {} and []

answered Jun 27, 2011 at 18:23

inspectorG4dget

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Comments

Lance Roberts · Accepted Answer · 2011-06-27 21:07:22Z

1

It was never working. Only None is None is True.

edited Jun 27, 2011 at 21:07

Lance Roberts

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answered Jun 27, 2011 at 18:21

Roman Bodnarchuk

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Comments

Artsiom Rudzenka · Accepted Answer · 2011-06-27 19:03:25Z

0

If you want to simply check on empty values it is better to use the following:

a = {}
b = []
c = ""

if a:
    print 'non-empty dict'
else:
    print 'empty dict'

if b:
    print 'non-empty list'
else:
    print 'empty list'

if c:
    print 'non-empty string/value'
else:
    print 'empty string/value'

answered Jun 27, 2011 at 19:03

Artsiom Rudzenka

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