Python “is” statement: what is happening? [duplicate]

Answer 1

is means is same instance. It evaluates to true if the variables on either side of the operator point to the same object and false otherwise.

Reference, near the bottom.

Answer 2

The best way to describe WHY that happens is this:

Here is your example

>>> x = []
>>> y = []
>>> print(x is y)
... False

x and y are actually two different lists, so if you add something to x, it does not appear in y

>>> x.append(1)
>>> print(x)
... [1]
>>> print(y)
... []

So how do we make (x is y) evaluate true?

>>> x = []
>>> y = x
>>> print(x is y)
... True

>>> x.append(10)

>>> print(x)
... [10]
>>> print(y)
... [10]

>>> print(x is y)
... True

if you want to see if two lists have the same contents...

>>> x = []
>>> y = []
>>> print(x == y)
... True

>>> x.append(21)

>>> print(x)
... [21]
>>> print(y)
... []

>>> print(x == y)
... False

>>> y = [21]
>>> print(x == y)
... True

Answer 3

a is not b is a special operator which is equivalent to not a is b.

The operator a is b returns True if a and b are bound to the same object, otherwise False. When you create two empty lists you get two different objects, so is returns False (and therefore is not returns True).

Answer 4

is is the identity comparison.

== is the equality comparison.

Your statement is making two different lists and checking if they are the same instance, which they are not. If you use == it will return true and because they are both empty lists.

Answer 5

@Jiaaro is right. Using is with immutable data types is dangerous because it is not predictable because of Pythons interpreter optimization.

See this example:

10 * "a" is 10 * "a"    # True
100 * "a" is 100 * "a"  # False

In the second line it is faster to create a new object with a new id for the interpreter. So use the is operator only with mutable types.

Answer 6

I know I am posting to a pretty old post. however, this might help someone stumble upon this like me.

"is" checks, whether a memory address is same or not, while "==" check if the value is same or not. Would be much clear from the following example

let's first talk about immutable objects, as it's easy to understand

# could be any immutable object
immutable_a = 10
immutable_b = 10

# prints address of a and b variable
print "address of a is %s" % id(immutable_a)
print "address of a is %s" % id(immutable_b)

# as both addresses is same, following shall be true
print immutable_a is immutable_b

# as the values are also same, following shall be true as well
print immutable_a == immutable_b

now let's talk about mutable objects

# could be any mutable object
mutable_a = [10]
mutable_b = [10]

# prints address of a and b variable
print "address of mutable_a is %s" % id(mutable_a)
print "address of mutable_b is %s" % id(mutable_b)

# as addresses are not same, following shall be false
print mutable_a is mutable_b

# as the values are same, following shall be true
print mutable_a == mutable_b