Is there any difference between "foo is None" and "foo == None"?

Joe_Shaw_156 0
Viewed 31586

Is there any difference between:

if foo is None: pass

and

if foo == None: pass

The convention that I've seen in most Python code (and the code I myself write) is the former, but I recently came across code which uses the latter. None is an instance (and the only instance, IIRC) of NoneType, so it shouldn't matter, right? Are there any circumstances in which it might?

python
12 Answers

is always returns True if it compares the same object instance

Whereas == is ultimately determined by the __eq__() method

i.e.


>>> class Foo(object):
       def __eq__(self, other):
           return True

>>> f = Foo()
>>> f == None
True
>>> f is None
False
user_1990

You may want to read this object identity and equivalence.

The statement 'is' is used for object identity, it checks if objects refer to the same instance (same address in memory).

And the '==' statement refers to equality (same value).

borrego_27720

A word of caution: 

if foo:
  # do something

Is not exactly the same as:

if x is not None:
  # do something

The former is a boolean value test and can evaluate to false in different contexts. There are a number of things that represent false in a boolean value tests for example empty containers, boolean values. None also evaluates to false in this situation but other things do too.

user_29790

(ob1 is ob2) equal to (id(ob1) == id(ob2))

user_698850

For None there shouldn't be a difference between equality (==) and identity (is). The NoneType probably returns identity for equality. Since None is the only instance you can make of NoneType (I think this is true), the two operations are the same. In the case of other types this is not always the case. For example:

list1 = [1, 2, 3]
list2 = [1, 2, 3]
if list1==list2: print "Equal"
if list1 is list2: print "Same"

This would print "Equal" since lists have a comparison operation that is not the default returning of identity.

user_3600

@Jason:

I recommend using something more along the lines of

if foo:
    #foo isn't None
else:
    #foo is None

I don't like using "if foo:" unless foo truly represents a boolean value (i.e. 0 or 1). If foo is a string or an object or something else, "if foo:" may work, but it looks like a lazy shortcut to me. If you're checking to see if x is None, say "if x is None:".

user_18210 
a is b # returns true if they a and b are true alias
a == b # returns true if they are true alias or they have values that are deemed equivalence 


a = [1,3,4]
b = a[:] #creating copy of list
a is b # if gives false
False
a == b # gives true
True
user_44170900
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