what is difference between [None] and [] in python? [duplicate]

Answer 1

[] is an empty list

[None] is a list with one element. That one element is None

is checks for reference equality. If both objects refer to the same object by reference then is will return true.

a = []
b = a
a is [] #false
a is b  #true

Answer 2

Question 1:

None is an object. It is of type "NoneType".
This can be seen by doing something like this in the terminal:

>>> type(None)
<type 'NoneType'>

So, when you put this object in a list, the list has one element.

Question 2:

The assignment operator in Python, =, is used to attach a name to an object. In the case of immutable objects, like integers, multiple names can be attached to the same object. That is what you are doing with a and b. So, when you test their identity, using the is operator, you see that the two names point to the identical object.

Alternatively, when you attach a name to a newly created list (which you created with the [] operator) it is a different list each time.

Answer 3

You want to use None to imply that there is no valid object. You want to use [] to imply an object that is of type list and has no elements.

[None] is a list with one element which is None

>>>c= []  # This is a new list object
>>>d= []  # This is another new list object

In Python , x is y is used to check whether x and y are the same objects. Here, c and d point to different list objects. so,

>>>print c is d
False

is expected.

On the other hand,

>>>c= []  # This is a new list object
>>>d = c  # This is the same object as c
>>>print c is d
True

Here, a and b are primitives, not objects

>>>a= 1
>>>b=1

So, this is expected:

print a is b
True

Answer 4

None is a valid element, but you can treat it like a stub or placeholder. So it counts as an element inside the list even if there is only a None.

For (equality) comparisons you shouldn't use is. Use ==!

Because is can lead to strange behaviour if you don't know exactly when and how to use it. For example:

>>> 1900 is 1900
True

>>> a = 1900
>>> b = 1900
>>> a is b
False

>>> a, b = 1900, 1900
>>> a is b
True

This rather strange behaviour is explained for example in this question: Why does Python handle '1 is 1**2' differently from '1000 is 10**3'?

This won't happen when you use ==:

>>> a == b
True
>>> 1900 == 1900
True

like one would expect.

Answer 5

[None] does not mean that there is nothing in the list. None is a keyword in python which has a special meaning. It is like NIL or NULL in other languages.

When you say [None], you are saying "I would like to have a list which contains the special object called None". This is different than saying "I would like a list which contains no elements" (by typing []).