Python != operation vs "is not"
Solution 1
==
is an equality test. It checks whether the right hand side and the left hand side are equal objects (according to their __eq__
or __cmp__
methods.)
is
is an identity test. It checks whether the right hand side and the left hand side are the very same object. No methodcalls are done, objects can't influence the is
operation.
You use is
(and is not
) for singletons, like None
, where you don't care about objects that might want to pretend to be None
or where you want to protect against objects breaking when being compared against None
.
Solution 2
First, let me go over a few terms. If you just want your question answered, scroll down to "Answering your question".
Definitions
Object identity: When you create an object, you can assign it to a variable. You can then also assign it to another variable. And another.
>>> button = Button()
>>> cancel = button
>>> close = button
>>> dismiss = button
>>> print(cancel is close)
True
In this case, cancel
, close
, and dismiss
all refer to the same object in memory. You only created one Button
object, and all three variables refer to this one object. We say that cancel
, close
, and dismiss
all refer to identical objects; that is, they refer to one single object.
Object equality: When you compare two objects, you usually don't care that it refers to the exact same object in memory. With object equality, you can define your own rules for how two objects compare. When you write if a == b:
, you are essentially saying if a.__eq__(b):
. This lets you define a __eq__
method on a
so that you can use your own comparison logic.
Rationale for equality comparisons
Rationale: Two objects have the exact same data, but are not identical. (They are not the same object in memory.) Example: Strings
>>> greeting = "It's a beautiful day in the neighbourhood."
>>> a = unicode(greeting)
>>> b = unicode(greeting)
>>> a is b
False
>>> a == b
True
Note: I use unicode strings here because Python is smart enough to reuse regular strings without creating new ones in memory.
Here, I have two unicode strings, a
and b
. They have the exact same content, but they are not the same object in memory. However, when we compare them, we want them to compare equal. What's happening here is that the unicode object has implemented the __eq__
method.
class unicode(object):
# ...
def __eq__(self, other):
if len(self) != len(other):
return False
for i, j in zip(self, other):
if i != j:
return False
return True
Note: __eq__
on unicode
is definitely implemented more efficiently than this.
Rationale: Two objects have different data, but are considered the same object if some key data is the same. Example: Most types of model data
>>> import datetime
>>> a = Monitor()
>>> a.make = "Dell"
>>> a.model = "E770s"
>>> a.owner = "Bob Jones"
>>> a.warranty_expiration = datetime.date(2030, 12, 31)
>>> b = Monitor()
>>> b.make = "Dell"
>>> b.model = "E770s"
>>> b.owner = "Sam Johnson"
>>> b.warranty_expiration = datetime.date(2005, 8, 22)
>>> a is b
False
>>> a == b
True
Here, I have two Dell monitors, a
and b
. They have the same make and model. However, they neither have the same data nor are the same object in memory. However, when we compare them, we want them to compare equal. What's happening here is that the Monitor object implemented the __eq__
method.
class Monitor(object):
# ...
def __eq__(self, other):
return self.make == other.make and self.model == other.model
Answering your question
When comparing to None
, always use is not
. None is a singleton in Python - there is only ever one instance of it in memory.
By comparing identity, this can be performed very quickly. Python checks whether the object you're referring to has the same memory address as the global None object - a very, very fast comparison of two numbers.
By comparing equality, Python has to look up whether your object has an __eq__
method. If it does not, it examines each superclass looking for an __eq__
method. If it finds one, Python calls it. This is especially bad if the __eq__
method is slow and doesn't immediately return when it notices that the other object is None
.
Did you not implement __eq__
? Then Python will probably find the __eq__
method on object
and use that instead - which just checks for object identity anyway.
When comparing most other things in Python, you will be using !=
.
Solution 3
Consider the following:
class Bad(object):
def __eq__(self, other):
return True
c = Bad()
c is None # False, equivalent to id(c) == id(None)
c == None # True, equivalent to c.__eq__(None)
Solution 4
None
is a singleton, therefore identity comparison will always work, whereas an object can fake the equality comparison via .__eq__()
.
Solution 5
>>> () is () True >>> 1 is 1 True >>> (1,) == (1,) True >>> (1,) is (1,) False >>> a = (1,) >>> b = a >>> a is b True
Some objects are singletons, and thus is
with them is equivalent to ==
. Most are not.
viksit
The most common mistake people make when designing something completely foolproof, is underestimating the ingenuity of complete fools. - Douglas Adams, Mostly Harmless
Updated on June 23, 2020Comments
-
viksit almost 4 years
In a comment on this question, I saw a statement that recommended using
result is not None
vs
result != None
I was wondering what the difference is, and why one might be recommended over the other?
-
SilentGhost about 14 years
-
viksit about 14 yearsHmm. While the answer to both questions is the same concept, I think the upvotes and detailed answers here do contribute independently to the concept of identity and equality testing.
-
Michael Wang almost 5 years
-
-
viksit about 14 yearsAh interesting! In which situations might one want to fake the equality comparison btw? I'm guessing this has security implications in some way.
-
Thomas Wouters about 14 yearsIt's not about faking equality, it's about implementing equality. There are lots of reasons to want to define how an object compares to another.
-
Greg Hewgill about 14 yearsI would say it's more confusion implications than security implications.
-
viksit about 14 yearsThanks for the answer - could you elaborate on situations when an object can break, being compared to None?
-
Ignacio Vazquez-Abrams about 14 yearsI have not come up against a reason to fake equality against
None
, but incorrect behavior regardingNone
could occur as a side effect of implementing equality against other types. It's not so much security implications as it is just correctness implications. -
user1066101 about 14 years@viksit.
None
has few methods and almost no attributes. If your__eq__
test expected a method or attribute, it might break.def __eq__( self, other ): return self.size == other.size
. For example, will break ifother
happens to beNone
. -
Mike Graham about 14 yearsMost of these only work by coincidence/implementation detail.
()
and1
are not inherently singletons. -
ephemient about 14 yearsIn the CPython implementation, the small integers (
-NSMALLNEGINTS <= n <= NSMALLPOSINTS
) and empty tuples are singletons. Indeed it's not documented nor guaranteed, but it's unlikely to change. -
Tyler about 14 yearsMy favorite way to comprehend this is: Python's
is
is like Java's==
. Python's==
is like Java's.equals()
. Of course this only helps if you know Java. -
Mike Graham about 14 yearsIt is how it is implemented, but it isn't meaningful or useful or educational.
-
me_and over 10 yearsAnd in particular, CPython is not the only Python implementation. Relying on behaviour that can vary across Python implementations would seem to generally be a Bad Idea™ to me.
-
Orwellophile about 7 years@MatrixFrog: In PHP or JavaScript we would say that
is
is like===
(very equal), and converselyis not
is like!==
(not exactly equal). -
Asad Moosvi over 6 yearsIs
is not
a single operator or is it just negating the result ofis
internally likenot foo is bar
? -
FriskySaga over 2 yearsGreat answer! To add on, an example of an object that would be break when being compared to None via an equality test would be the
dateutil.relativedelta.relativedelta
object. When compared to!= None
, it would return False, and when compared to== None
, it would also return False. Weird, I know.