Having problem with Shared Reference in case of Lists

Question

a=b=[1,2,3]
print (a is b) #True

But

a=[1,2,3]
print (a is [1,2,3]) #False  

Why does the second part print False ?

Answer 1

Multiple assignment in Python creates two names that point to the same object. For example,

>>> a=b=[1,2,3]
>>> a[0] = 10
>>> b
[10, 2, 3]

---

is can be used to check whether two names (a and b) hold the reference to the same memory location (object). Therefore,

a=b=[1,2,3]  # a and b hold the same reference
print (a is b) # True

---

Now in this example,

a = [1,2,3]
print (a is [1,2,3]) # False

a does not hold the same reference to the object [1, 2, 3], even though a and [1, 2, 3] are lists with identical elements.

In case you want to compare whether two lists contain the same elements, you can use ==:

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

Answer 2

Your first one explicitly makes a and b references to the object created by the list display [1,2,3].

In your second code, both uses of the list display [1,2,3] necessarily create new list objects, because lists are mutable and you don't want to implicitly share references to them.

Consider a simpler example:

a = []
b = []
a.append(1)

Do you want b to be modified as well?

For immutable values, like ints, the language implementation may cause literals to reuse references to existing objects, but it's not something that can be relied on.

Answer 3

the problem is the logic operator you are using. You are asking are these identical object with is and not if they are the equal (same data). One is a reference to a object and the other is the object so even though they are equal the are not the same.

Why your results

When you are setting a and b as the same list you are saying that a and b should be linked and should reference the same data so they are identical to each other but a and b are not the object [1,2,3] they are a reference to a list that is the same.

In summary

== - equal to (same).

is - identical to.

So if you want to check if they are equal(same) use:

>>> a=[1,2,3]
>>> print (a == [1,2,3])
True

Similar question worth reading: Is there a difference between "==" and "is"?

Hope this helps, Harry.