Python – difference between “==” and “is”

equalitypythonreferencesemantics

My Google-fu has failed me.

In Python, are the following two tests for equality equivalent?

n = 5
# Test one.
if n == 5:
    print 'Yay!'

# Test two.
if n is 5:
    print 'Yay!'

Does this hold true for objects where you would be comparing instances (a list say)?

Okay, so this kind of answers my question:

L = []
L.append(1)
if L == [1]:
    print 'Yay!'
# Holds true, but...

if L is [1]:
    print 'Yay!'
# Doesn't.

So == tests value where is tests to see if they are the same object?

Best Solution

is will return True if two variables point to the same object, == if the objects referred to by the variables are equal.

>>> a = [1, 2, 3]
>>> b = a
>>> b is a 
True
>>> b == a
True

# Make a new copy of list `a` via the slice operator, 
# and assign it to variable `b`
>>> b = a[:] 
>>> b is a
False
>>> b == a
True

In your case, the second test only works because Python caches small integer objects, which is an implementation detail. For larger integers, this does not work:

>>> 1000 is 10**3
False
>>> 1000 == 10**3
True

The same holds true for string literals:

>>> "a" is "a"
True
>>> "aa" is "a" * 2
True
>>> x = "a"
>>> "aa" is x * 2
False
>>> "aa" is intern(x*2)
True

Please see this question as well.