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

Answer 1

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